synthesis of related literature in research

How to Synthesize Written Information from Multiple Sources

Shona McCombes

Content Manager

B.A., English Literature, University of Glasgow

Shona McCombes is the content manager at Scribbr, Netherlands.

Learn about our Editorial Process

Saul Mcleod, PhD

Editor-in-Chief for Simply Psychology

BSc (Hons) Psychology, MRes, PhD, University of Manchester

Saul Mcleod, PhD., is a qualified psychology teacher with over 18 years of experience in further and higher education. He has been published in peer-reviewed journals, including the Journal of Clinical Psychology.

On This Page:

When you write a literature review or essay, you have to go beyond just summarizing the articles you’ve read – you need to synthesize the literature to show how it all fits together (and how your own research fits in).

Synthesizing simply means combining. Instead of summarizing the main points of each source in turn, you put together the ideas and findings of multiple sources in order to make an overall point.

At the most basic level, this involves looking for similarities and differences between your sources. Your synthesis should show the reader where the sources overlap and where they diverge.

Unsynthesized Example

Franz (2008) studied undergraduate online students. He looked at 17 females and 18 males and found that none of them liked APA. According to Franz, the evidence suggested that all students are reluctant to learn citations style. Perez (2010) also studies undergraduate students. She looked at 42 females and 50 males and found that males were significantly more inclined to use citation software ( p < .05). Findings suggest that females might graduate sooner. Goldstein (2012) looked at British undergraduates. Among a sample of 50, all females, all confident in their abilities to cite and were eager to write their dissertations.

Synthesized Example

Studies of undergraduate students reveal conflicting conclusions regarding relationships between advanced scholarly study and citation efficacy. Although Franz (2008) found that no participants enjoyed learning citation style, Goldstein (2012) determined in a larger study that all participants watched felt comfortable citing sources, suggesting that variables among participant and control group populations must be examined more closely. Although Perez (2010) expanded on Franz’s original study with a larger, more diverse sample…

Step 1: Organize your sources

After collecting the relevant literature, you’ve got a lot of information to work through, and no clear idea of how it all fits together.

Before you can start writing, you need to organize your notes in a way that allows you to see the relationships between sources.

One way to begin synthesizing the literature is to put your notes into a table. Depending on your topic and the type of literature you’re dealing with, there are a couple of different ways you can organize this.

Summary table

A summary table collates the key points of each source under consistent headings. This is a good approach if your sources tend to have a similar structure – for instance, if they’re all empirical papers.

Each row in the table lists one source, and each column identifies a specific part of the source. You can decide which headings to include based on what’s most relevant to the literature you’re dealing with.

For example, you might include columns for things like aims, methods, variables, population, sample size, and conclusion.

For each study, you briefly summarize each of these aspects. You can also include columns for your own evaluation and analysis.

The summary table gives you a quick overview of the key points of each source. This allows you to group sources by relevant similarities, as well as noticing important differences or contradictions in their findings.

Synthesis matrix

A synthesis matrix is useful when your sources are more varied in their purpose and structure – for example, when you’re dealing with books and essays making various different arguments about a topic.

Each column in the table lists one source. Each row is labeled with a specific concept, topic or theme that recurs across all or most of the sources.

Then, for each source, you summarize the main points or arguments related to the theme.

The purposes of the table is to identify the common points that connect the sources, as well as identifying points where they diverge or disagree.

Step 2: Outline your structure

Now you should have a clear overview of the main connections and differences between the sources you’ve read. Next, you need to decide how you’ll group them together and the order in which you’ll discuss them.

For shorter papers, your outline can just identify the focus of each paragraph; for longer papers, you might want to divide it into sections with headings.

There are a few different approaches you can take to help you structure your synthesis.

If your sources cover a broad time period, and you found patterns in how researchers approached the topic over time, you can organize your discussion chronologically .

That doesn’t mean you just summarize each paper in chronological order; instead, you should group articles into time periods and identify what they have in common, as well as signalling important turning points or developments in the literature.

If the literature covers various different topics, you can organize it thematically .

That means that each paragraph or section focuses on a specific theme and explains how that theme is approached in the literature.

Source Used with Permission: The Chicago School

If you’re drawing on literature from various different fields or they use a wide variety of research methods, you can organize your sources methodologically .

That means grouping together studies based on the type of research they did and discussing the findings that emerged from each method.

If your topic involves a debate between different schools of thought, you can organize it theoretically .

That means comparing the different theories that have been developed and grouping together papers based on the position or perspective they take on the topic, as well as evaluating which arguments are most convincing.

Step 3: Write paragraphs with topic sentences

What sets a synthesis apart from a summary is that it combines various sources. The easiest way to think about this is that each paragraph should discuss a few different sources, and you should be able to condense the overall point of the paragraph into one sentence.

This is called a topic sentence , and it usually appears at the start of the paragraph. The topic sentence signals what the whole paragraph is about; every sentence in the paragraph should be clearly related to it.

A topic sentence can be a simple summary of the paragraph’s content:

“Early research on [x] focused heavily on [y].”

For an effective synthesis, you can use topic sentences to link back to the previous paragraph, highlighting a point of debate or critique:

“Several scholars have pointed out the flaws in this approach.” “While recent research has attempted to address the problem, many of these studies have methodological flaws that limit their validity.”

By using topic sentences, you can ensure that your paragraphs are coherent and clearly show the connections between the articles you are discussing.

As you write your paragraphs, avoid quoting directly from sources: use your own words to explain the commonalities and differences that you found in the literature.

Don’t try to cover every single point from every single source – the key to synthesizing is to extract the most important and relevant information and combine it to give your reader an overall picture of the state of knowledge on your topic.

Step 4: Revise, edit and proofread

Like any other piece of academic writing, synthesizing literature doesn’t happen all in one go – it involves redrafting, revising, editing and proofreading your work.

Checklist for Synthesis

•   Do I introduce the paragraph with a clear, focused topic sentence?
•   Do I discuss more than one source in the paragraph?
•   Do I mention only the most relevant findings, rather than describing every part of the studies?
•   Do I discuss the similarities or differences between the sources, rather than summarizing each source in turn?
•   Do I put the findings or arguments of the sources in my own words?
•   Is the paragraph organized around a single idea?
•   Is the paragraph directly relevant to my research question or topic?
•   Is there a logical transition from this paragraph to the next one?

Further Information

How to Synthesise: a Step-by-Step Approach

Help…I”ve Been Asked to Synthesize!

Learn how to Synthesise (combine information from sources)

How to write a Psychology Essay

• University of Oregon Libraries
• Research Guides

How to Write a Literature Review

• 6. Synthesize
• Literature Reviews: A Recap
• Reading Journal Articles
• Does it Describe a Literature Review?
• 1. Identify the Question
• 2. Review Discipline Styles
• Searching Article Databases
• Finding Full-Text of an Article
• Citation Chaining
• When to Stop Searching
• 4. Manage Your References
• 5. Critically Analyze and Evaluate

Synthesis Visualization

Synthesis matrix example.

• 7. Write a Literature Review

• Synthesis Worksheet

About Synthesis

Approaches to synthesis.

You can sort the literature in various ways, for example:

How to Begin?

Read your sources carefully and find the main idea(s) of each source

Look for similarities in your sources – which sources are talking about the same main ideas? (for example, sources that discuss the historical background on your topic)

Use the worksheet (above) or synthesis matrix (below) to get organized

This work can be messy. Don't worry if you have to go through a few iterations of the worksheet or matrix as you work on your lit review!

Four Examples of Student Writing

In the four examples below, only ONE shows a good example of synthesis: the fourth column, or  Student D . For a web accessible version, click the link below the image.

Long description of "Four Examples of Student Writing" for web accessibility

• Download a copy of the "Four Examples of Student Writing" chart

Click on the example to view the pdf.

From Jennifer Lim

• << Previous: 5. Critically Analyze and Evaluate
• Next: 7. Write a Literature Review >>
• Last Updated: May 3, 2024 5:17 PM
• URL: https://researchguides.uoregon.edu/litreview

Contact Us Library Accessibility UO Libraries Privacy Notices and Procedures

1501 Kincaid Street Eugene, OR 97403 P: 541-346-3053 F: 541-346-3485

• Visit us on Facebook
• Visit us on Twitter
• Visit us on Youtube
• Visit us on Instagram
• Report a Concern
• Nondiscrimination and Title IX
• Accessibility
• Privacy Policy
• Find People

Literature Syntheis 101

How To Synthesise The Existing Research (With Examples)

By: Derek Jansen (MBA) | Expert Reviewer: Eunice Rautenbach (DTech) | August 2023

One of the most common mistakes that students make when writing a literature review is that they err on the side of describing the existing literature rather than providing a critical synthesis of it. In this post, we’ll unpack what exactly synthesis means and show you how to craft a strong literature synthesis using practical examples.

This post is based on our popular online course, Literature Review Bootcamp . In the course, we walk you through the full process of developing a literature review, step by step. If it’s your first time writing a literature review, you definitely want to use this link to get 50% off the course (limited-time offer).

Overview: Literature Synthesis

• What exactly does “synthesis” mean?
• Aspect 1: Agreement
• Aspect 2: Disagreement
• Aspect 3: Key theories
• Aspect 4: Contexts
• Aspect 5: Methodologies
• Bringing it all together

What does “synthesis” actually mean?

As a starting point, let’s quickly define what exactly we mean when we use the term “synthesis” within the context of a literature review.

Simply put, literature synthesis means going beyond just describing what everyone has said and found. Instead, synthesis is about bringing together all the information from various sources to present a cohesive assessment of the current state of knowledge in relation to your study’s research aims and questions .

Put another way, a good synthesis tells the reader exactly where the current research is “at” in terms of the topic you’re interested in – specifically, what’s known , what’s not , and where there’s a need for more research .

So, how do you go about doing this?

Well, there’s no “one right way” when it comes to literature synthesis, but we’ve found that it’s particularly useful to ask yourself five key questions when you’re working on your literature review. Having done so,  you can then address them more articulately within your actual write up. So, let’s take a look at each of these questions.

1. Points Of Agreement

The first question that you need to ask yourself is: “Overall, what things seem to be agreed upon by the vast majority of the literature?”

For example, if your research aim is to identify which factors contribute toward job satisfaction, you’ll need to identify which factors are broadly agreed upon and “settled” within the literature. Naturally, there may at times be some lone contrarian that has a radical viewpoint , but, provided that the vast majority of researchers are in agreement, you can put these random outliers to the side. That is, of course, unless your research aims to explore a contrarian viewpoint and there’s a clear justification for doing so. 

Identifying what’s broadly agreed upon is an essential starting point for synthesising the literature, because you generally don’t want (or need) to reinvent the wheel or run down a road investigating something that is already well established . So, addressing this question first lays a foundation of “settled” knowledge.

Need a helping hand?

2. Points Of Disagreement

Related to the previous point, but on the other end of the spectrum, is the equally important question: “Where do the disagreements lie?” .

In other words, which things are not well agreed upon by current researchers? It’s important to clarify here that by disagreement, we don’t mean that researchers are (necessarily) fighting over it – just that there are relatively mixed findings within the empirical research , with no firm consensus amongst researchers.

This is a really important question to address as these “disagreements” will often set the stage for the research gap(s). In other words, they provide clues regarding potential opportunities for further research, which your study can then (hopefully) contribute toward filling. If you’re not familiar with the concept of a research gap, be sure to check out our explainer video covering exactly that .

3. Key Theories

The next question you need to ask yourself is: “Which key theories seem to be coming up repeatedly?” .

Within most research spaces, you’ll find that you keep running into a handful of key theories that are referred to over and over again. Apart from identifying these theories, you’ll also need to think about how they’re connected to each other. Specifically, you need to ask yourself:

• Are they all covering the same ground or do they have different focal points  or underlying assumptions ?
• Do some of them feed into each other and if so, is there an opportunity to integrate them into a more cohesive theory?
• Do some of them pull in different directions ? If so, why might this be?
• Do all of the theories define the key concepts and variables in the same way, or is there some disconnect? If so, what’s the impact of this ?

Simply put, you’ll need to pay careful attention to the key theories in your research area, as they will need to feature within your theoretical framework , which will form a critical component within your final literature review. This will set the foundation for your entire study, so it’s essential that you be critical in this area of your literature synthesis.

If this sounds a bit fluffy, don’t worry. We deep dive into the theoretical framework (as well as the conceptual framework) and look at practical examples in Literature Review Bootcamp . If you’d like to learn more, take advantage of our limited-time offer to get 60% off the standard price.

4. Contexts

The next question that you need to address in your literature synthesis is an important one, and that is: “Which contexts have (and have not) been covered by the existing research?” .

For example, sticking with our earlier hypothetical topic (factors that impact job satisfaction), you may find that most of the research has focused on white-collar , management-level staff within a primarily Western context, but little has been done on blue-collar workers in an Eastern context. Given the significant socio-cultural differences between these two groups, this is an important observation, as it could present a contextual research gap .

In practical terms, this means that you’ll need to carefully assess the context of each piece of literature that you’re engaging with, especially the empirical research (i.e., studies that have collected and analysed real-world data). Ideally, you should keep notes regarding the context of each study in some sort of catalogue or sheet, so that you can easily make sense of this before you start the writing phase. If you’d like, our free literature catalogue worksheet is a great tool for this task.

5. Methodological Approaches

Last but certainly not least, you need to ask yourself the question: “What types of research methodologies have (and haven’t) been used?”

For example, you might find that most studies have approached the topic using qualitative methods such as interviews and thematic analysis. Alternatively, you might find that most studies have used quantitative methods such as online surveys and statistical analysis.

But why does this matter?

Well, it can run in one of two potential directions . If you find that the vast majority of studies use a specific methodological approach, this could provide you with a firm foundation on which to base your own study’s methodology . In other words, you can use the methodologies of similar studies to inform (and justify) your own study’s research design .

On the other hand, you might argue that the lack of diverse methodological approaches presents a research gap , and therefore your study could contribute toward filling that gap by taking a different approach. For example, taking a qualitative approach to a research area that is typically approached quantitatively. Of course, if you’re going to go against the methodological grain, you’ll need to provide a strong justification for why your proposed approach makes sense. Nevertheless, it is something worth at least considering.

Regardless of which route you opt for, you need to pay careful attention to the methodologies used in the relevant studies and provide at least some discussion about this in your write-up. Again, it’s useful to keep track of this on some sort of spreadsheet or catalogue as you digest each article, so consider grabbing a copy of our free literature catalogue if you don’t have anything in place.

Bringing It All Together

Alright, so we’ve looked at five important questions that you need to ask (and answer) to help you develop a strong synthesis within your literature review.  To recap, these are:

• Which things are broadly agreed upon within the current research?
• Which things are the subject of disagreement (or at least, present mixed findings)?
• Which theories seem to be central to your research topic and how do they relate or compare to each other?
• Which contexts have (and haven’t) been covered?
• Which methodological approaches are most common?

Importantly, you’re not just asking yourself these questions for the sake of asking them – they’re not just a reflection exercise. You need to weave your answers to them into your actual literature review when you write it up. How exactly you do this will vary from project to project depending on the structure you opt for, but you’ll still need to address them within your literature review, whichever route you go.

The best approach is to spend some time actually writing out your answers to these questions, as opposed to just thinking about them in your head. Putting your thoughts onto paper really helps you flesh out your thinking . As you do this, don’t just write down the answers – instead, think about what they mean in terms of the research gap you’ll present , as well as the methodological approach you’ll take . Your literature synthesis needs to lay the groundwork for these two things, so it’s essential that you link all of it together in your mind, and of course, on paper.

Psst… there’s more!

This post is an extract from our bestselling short course, Literature Review Bootcamp . If you want to work smart, you don't want to miss this .

You Might Also Like:

excellent , thank you

Submit a Comment Cancel reply

Your email address will not be published. Required fields are marked *

Save my name, email, and website in this browser for the next time I comment.

• Print Friendly

The Sheridan Libraries

• Write a Literature Review
• Sheridan Libraries
• Find This link opens in a new window
• Evaluate This link opens in a new window

Get Organized

• Lit Review Prep Use this template to help you evaluate your sources, create article summaries for an annotated bibliography, and a synthesis matrix for your lit review outline.

Synthesize your Information

Synthesize: combine separate elements to form a whole.

Synthesis Matrix

A synthesis matrix helps you record the main points of each source and document how sources relate to each other.

After summarizing and evaluating your sources, arrange them in a matrix or use a citation manager to help you see how they relate to each other and apply to each of your themes or variables.  

By arranging your sources by theme or variable, you can see how your sources relate to each other, and can start thinking about how you weave them together to create a narrative.

• Step-by-Step Approach
• Example Matrix from NSCU
• Matrix Template
• << Previous: Summarize
• Next: Integrate >>
• Last Updated: Sep 26, 2023 10:25 AM
• URL: https://guides.library.jhu.edu/lit-review

Literature Review How To

• Things To Consider
• Synthesizing Sources
• Video Tutorials
• Books On Literature Reviews

What is Synthesis

What is Synthesis? Synthesis writing is a form of analysis related to comparison and contrast, classification and division. On a basic level, synthesis requires the writer to pull together two or more summaries, looking for themes in each text. In synthesis, you search for the links between various materials in order to make your point. Most advanced academic writing, including literature reviews, relies heavily on synthesis. (Temple University Writing Center)  

How To Synthesize Sources in a Literature Review

Literature reviews synthesize large amounts of information and present it in a coherent, organized fashion. In a literature review you will be combining material from several texts to create a new text – your literature review.

You will use common points among the sources you have gathered to help you synthesize the material. This will help ensure that your literature review is organized by subtopic, not by source. This means various authors' names can appear and reappear throughout the literature review, and each paragraph will mention several different authors. 

When you shift from writing summaries of the content of a source to synthesizing content from sources, there is a number things you must keep in mind: 

• Look for specific connections and or links between your sources and how those relate to your thesis or question.
• When writing and organizing your literature review be aware that your readers need to understand how and why the information from the different sources overlap.
• Organize your literature review by the themes you find within your sources or themes you have identified. 
• << Previous: Things To Consider
• Next: Video Tutorials >>
• Last Updated: Nov 30, 2018 4:51 PM
• URL: https://libguides.csun.edu/literature-review

Report ADA Problems with Library Services and Resources

Write a Literature Review

• Find This link opens in a new window

Get Organized

• Lit Review Prep Use this template to help you evaluate your sources, create article summaries for an annotated bibliography, and a synthesis matrix for your lit review outline.

Synthesize your Information

Synthesize: combine separate elements to form a whole.

Synthesis Matrix

A synthesis matrix helps you record the main points of each source and document how sources relate to each other.  After summarizing and evaluating your sources, arrange them in a matrix to help you see how they relate to each other, and apply to each of your themes or variables.  By arranging your sources in a matrix by theme or variable, you can see how your sources relate to each other, and can start thinking about how you weave them together to create a narrative.

• Step-by-Step Approach
• Example Matrix from NSCU
• Matrix Template
• << Previous: Summarize
• Next: Integrate >>
• Last Updated: Oct 11, 2023 12:20 PM
• URL: https://libraryguides.goucher.edu/literature-review

Library Guides

Literature reviews: synthesis.

• Criticality

Synthesise Information

So, how can you create paragraphs within your literature review that demonstrates your knowledge of the scholarship that has been done in your field of study?  

You will need to present a synthesis of the texts you read.  

Doug Specht, Senior Lecturer at the Westminster School of Media and Communication, explains synthesis for us in the following video:  

Synthesising Texts  

What is synthesis? 

Synthesis is an important element of academic writing, demonstrating comprehension, analysis, evaluation and original creation.  

With synthesis you extract content from different sources to create an original text. While paraphrase and summary maintain the structure of the given source(s), with synthesis you create a new structure.  

The sources will provide different perspectives and evidence on a topic. They will be put together when agreeing, contrasted when disagreeing. The sources must be referenced.  

Perfect your synthesis by showing the flow of your reasoning, expressing critical evaluation of the sources and drawing conclusions.  

When you synthesise think of "using strategic thinking to resolve a problem requiring the integration of diverse pieces of information around a structuring theme" (Mateos and Sole 2009, p448). 

Synthesis is a complex activity, which requires a high degree of comprehension and active engagement with the subject. As you progress in higher education, so increase the expectations on your abilities to synthesise. 

How to synthesise in a literature review: 

Identify themes/issues you'd like to discuss in the literature review. Think of an outline.  

Read the literature and identify these themes/issues.  

Critically analyse the texts asking: how does the text I'm reading relate to the other texts I've read on the same topic? Is it in agreement? Does it differ in its perspective? Is it stronger or weaker? How does it differ (could be scope, methods, year of publication etc.). Draw your conclusions on the state of the literature on the topic.  

Start writing your literature review, structuring it according to the outline you planned.  

Put together sources stating the same point; contrast sources presenting counter-arguments or different points.  

Present your critical analysis.  

Always provide the references. 

The best synthesis requires a "recursive process" whereby you read the source texts, identify relevant parts, take notes, produce drafts, re-read the source texts, revise your text, re-write... (Mateos and Sole, 2009). 

What is good synthesis?  

The quality of your synthesis can be assessed considering the following (Mateos and Sole, 2009, p439):  

Integration and connection of the information from the source texts around a structuring theme. 

Selection of ideas necessary for producing the synthesis. 

Appropriateness of the interpretation.  

Elaboration of the content.  

Example of Synthesis

Original texts (fictitious): 

Synthesis: 

Animal experimentation is a subject of heated debate. Some argue that painful experiments should be banned. Indeed it has been demonstrated that such experiments make animals suffer physically and psychologically (Chowdhury 2012; Panatta and Hudson 2016). On the other hand, it has been argued that animal experimentation can save human lives and reduce harm on humans (Smith 2008). This argument is only valid for toxicological testing, not for tests that, for example, merely improve the efficacy of a cosmetic (Turner 2015). It can be suggested that animal experimentation should be regulated to only allow toxicological risk assessment, and the suffering to the animals should be minimised.   

Bibliography

Mateos, M. and Sole, I. (2009). Synthesising Information from various texts: A Study of Procedures and Products at Different Educational Levels. European Journal of Psychology of Education,  24 (4), 435-451. Available from https://doi.org/10.1007/BF03178760 [Accessed 29 June 2021].

• << Previous: Structure
• Next: Criticality >>
• Last Updated: Nov 18, 2023 10:56 PM
• URL: https://libguides.westminster.ac.uk/literature-reviews

CONNECT WITH US

• USC Libraries
• Research Guides

Organizing Your Social Sciences Research Paper

• 5. The Literature Review
• Purpose of Guide
• Design Flaws to Avoid
• Independent and Dependent Variables
• Glossary of Research Terms
• Reading Research Effectively
• Narrowing a Topic Idea
• Broadening a Topic Idea
• Extending the Timeliness of a Topic Idea
• Academic Writing Style
• Applying Critical Thinking
• Choosing a Title
• Making an Outline
• Paragraph Development
• Research Process Video Series
• Executive Summary
• The C.A.R.S. Model
• Background Information
• The Research Problem/Question
• Theoretical Framework
• Citation Tracking
• Content Alert Services
• Evaluating Sources
• Primary Sources
• Secondary Sources
• Tiertiary Sources
• Scholarly vs. Popular Publications
• Qualitative Methods
• Quantitative Methods
• Insiderness
• Using Non-Textual Elements
• Limitations of the Study
• Common Grammar Mistakes
• Writing Concisely
• Avoiding Plagiarism
• Footnotes or Endnotes?
• Further Readings
• Generative AI and Writing
• USC Libraries Tutorials and Other Guides
• Bibliography

A literature review surveys prior research published in books, scholarly articles, and any other sources relevant to a particular issue, area of research, or theory, and by so doing, provides a description, summary, and critical evaluation of these works in relation to the research problem being investigated. Literature reviews are designed to provide an overview of sources you have used in researching a particular topic and to demonstrate to your readers how your research fits within existing scholarship about the topic.

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . Fourth edition. Thousand Oaks, CA: SAGE, 2014.

Importance of a Good Literature Review

A literature review may consist of simply a summary of key sources, but in the social sciences, a literature review usually has an organizational pattern and combines both summary and synthesis, often within specific conceptual categories . A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information in a way that informs how you are planning to investigate a research problem. The analytical features of a literature review might:

• Give a new interpretation of old material or combine new with old interpretations,
• Trace the intellectual progression of the field, including major debates,
• Depending on the situation, evaluate the sources and advise the reader on the most pertinent or relevant research, or
• Usually in the conclusion of a literature review, identify where gaps exist in how a problem has been researched to date.

Given this, the purpose of a literature review is to:

• Place each work in the context of its contribution to understanding the research problem being studied.
• Describe the relationship of each work to the others under consideration.
• Identify new ways to interpret prior research.
• Reveal any gaps that exist in the literature.
• Resolve conflicts amongst seemingly contradictory previous studies.
• Identify areas of prior scholarship to prevent duplication of effort.
• Point the way in fulfilling a need for additional research.
• Locate your own research within the context of existing literature [very important].

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper. 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . Los Angeles, CA: SAGE, 2011; Knopf, Jeffrey W. "Doing a Literature Review." PS: Political Science and Politics 39 (January 2006): 127-132; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012.

Types of Literature Reviews

It is important to think of knowledge in a given field as consisting of three layers. First, there are the primary studies that researchers conduct and publish. Second are the reviews of those studies that summarize and offer new interpretations built from and often extending beyond the primary studies. Third, there are the perceptions, conclusions, opinion, and interpretations that are shared informally among scholars that become part of the body of epistemological traditions within the field.

In composing a literature review, it is important to note that it is often this third layer of knowledge that is cited as "true" even though it often has only a loose relationship to the primary studies and secondary literature reviews. Given this, while literature reviews are designed to provide an overview and synthesis of pertinent sources you have explored, there are a number of approaches you could adopt depending upon the type of analysis underpinning your study.

Argumentative Review This form examines literature selectively in order to support or refute an argument, deeply embedded assumption, or philosophical problem already established in the literature. The purpose is to develop a body of literature that establishes a contrarian viewpoint. Given the value-laden nature of some social science research [e.g., educational reform; immigration control], argumentative approaches to analyzing the literature can be a legitimate and important form of discourse. However, note that they can also introduce problems of bias when they are used to make summary claims of the sort found in systematic reviews [see below].

Integrative Review Considered a form of research that reviews, critiques, and synthesizes representative literature on a topic in an integrated way such that new frameworks and perspectives on the topic are generated. The body of literature includes all studies that address related or identical hypotheses or research problems. A well-done integrative review meets the same standards as primary research in regard to clarity, rigor, and replication. This is the most common form of review in the social sciences.

Historical Review Few things rest in isolation from historical precedent. Historical literature reviews focus on examining research throughout a period of time, often starting with the first time an issue, concept, theory, phenomena emerged in the literature, then tracing its evolution within the scholarship of a discipline. The purpose is to place research in a historical context to show familiarity with state-of-the-art developments and to identify the likely directions for future research.

Methodological Review A review does not always focus on what someone said [findings], but how they came about saying what they say [method of analysis]. Reviewing methods of analysis provides a framework of understanding at different levels [i.e. those of theory, substantive fields, research approaches, and data collection and analysis techniques], how researchers draw upon a wide variety of knowledge ranging from the conceptual level to practical documents for use in fieldwork in the areas of ontological and epistemological consideration, quantitative and qualitative integration, sampling, interviewing, data collection, and data analysis. This approach helps highlight ethical issues which you should be aware of and consider as you go through your own study.

Systematic Review This form consists of an overview of existing evidence pertinent to a clearly formulated research question, which uses pre-specified and standardized methods to identify and critically appraise relevant research, and to collect, report, and analyze data from the studies that are included in the review. The goal is to deliberately document, critically evaluate, and summarize scientifically all of the research about a clearly defined research problem . Typically it focuses on a very specific empirical question, often posed in a cause-and-effect form, such as "To what extent does A contribute to B?" This type of literature review is primarily applied to examining prior research studies in clinical medicine and allied health fields, but it is increasingly being used in the social sciences.

Theoretical Review The purpose of this form is to examine the corpus of theory that has accumulated in regard to an issue, concept, theory, phenomena. The theoretical literature review helps to establish what theories already exist, the relationships between them, to what degree the existing theories have been investigated, and to develop new hypotheses to be tested. Often this form is used to help establish a lack of appropriate theories or reveal that current theories are inadequate for explaining new or emerging research problems. The unit of analysis can focus on a theoretical concept or a whole theory or framework.

NOTE : Most often the literature review will incorporate some combination of types. For example, a review that examines literature supporting or refuting an argument, assumption, or philosophical problem related to the research problem will also need to include writing supported by sources that establish the history of these arguments in the literature.

Baumeister, Roy F. and Mark R. Leary. "Writing Narrative Literature Reviews."  Review of General Psychology 1 (September 1997): 311-320; Mark R. Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Kennedy, Mary M. "Defining a Literature." Educational Researcher 36 (April 2007): 139-147; Petticrew, Mark and Helen Roberts. Systematic Reviews in the Social Sciences: A Practical Guide . Malden, MA: Blackwell Publishers, 2006; Torracro, Richard. "Writing Integrative Literature Reviews: Guidelines and Examples." Human Resource Development Review 4 (September 2005): 356-367; Rocco, Tonette S. and Maria S. Plakhotnik. "Literature Reviews, Conceptual Frameworks, and Theoretical Frameworks: Terms, Functions, and Distinctions." Human Ressource Development Review 8 (March 2008): 120-130; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

Structure and Writing Style

I.  Thinking About Your Literature Review

The structure of a literature review should include the following in support of understanding the research problem :

• An overview of the subject, issue, or theory under consideration, along with the objectives of the literature review,
• Division of works under review into themes or categories [e.g. works that support a particular position, those against, and those offering alternative approaches entirely],
• An explanation of how each work is similar to and how it varies from the others,
• Conclusions as to which pieces are best considered in their argument, are most convincing of their opinions, and make the greatest contribution to the understanding and development of their area of research.

The critical evaluation of each work should consider :

• Provenance -- what are the author's credentials? Are the author's arguments supported by evidence [e.g. primary historical material, case studies, narratives, statistics, recent scientific findings]?
• Methodology -- were the techniques used to identify, gather, and analyze the data appropriate to addressing the research problem? Was the sample size appropriate? Were the results effectively interpreted and reported?
• Objectivity -- is the author's perspective even-handed or prejudicial? Is contrary data considered or is certain pertinent information ignored to prove the author's point?
• Persuasiveness -- which of the author's theses are most convincing or least convincing?
• Validity -- are the author's arguments and conclusions convincing? Does the work ultimately contribute in any significant way to an understanding of the subject?

II.  Development of the Literature Review

Four Basic Stages of Writing 1.  Problem formulation -- which topic or field is being examined and what are its component issues? 2.  Literature search -- finding materials relevant to the subject being explored. 3.  Data evaluation -- determining which literature makes a significant contribution to the understanding of the topic. 4.  Analysis and interpretation -- discussing the findings and conclusions of pertinent literature.

Consider the following issues before writing the literature review: Clarify If your assignment is not specific about what form your literature review should take, seek clarification from your professor by asking these questions: 1.  Roughly how many sources would be appropriate to include? 2.  What types of sources should I review (books, journal articles, websites; scholarly versus popular sources)? 3.  Should I summarize, synthesize, or critique sources by discussing a common theme or issue? 4.  Should I evaluate the sources in any way beyond evaluating how they relate to understanding the research problem? 5.  Should I provide subheadings and other background information, such as definitions and/or a history? Find Models Use the exercise of reviewing the literature to examine how authors in your discipline or area of interest have composed their literature review sections. Read them to get a sense of the types of themes you might want to look for in your own research or to identify ways to organize your final review. The bibliography or reference section of sources you've already read, such as required readings in the course syllabus, are also excellent entry points into your own research. Narrow the Topic The narrower your topic, the easier it will be to limit the number of sources you need to read in order to obtain a good survey of relevant resources. Your professor will probably not expect you to read everything that's available about the topic, but you'll make the act of reviewing easier if you first limit scope of the research problem. A good strategy is to begin by searching the USC Libraries Catalog for recent books about the topic and review the table of contents for chapters that focuses on specific issues. You can also review the indexes of books to find references to specific issues that can serve as the focus of your research. For example, a book surveying the history of the Israeli-Palestinian conflict may include a chapter on the role Egypt has played in mediating the conflict, or look in the index for the pages where Egypt is mentioned in the text. Consider Whether Your Sources are Current Some disciplines require that you use information that is as current as possible. This is particularly true in disciplines in medicine and the sciences where research conducted becomes obsolete very quickly as new discoveries are made. However, when writing a review in the social sciences, a survey of the history of the literature may be required. In other words, a complete understanding the research problem requires you to deliberately examine how knowledge and perspectives have changed over time. Sort through other current bibliographies or literature reviews in the field to get a sense of what your discipline expects. You can also use this method to explore what is considered by scholars to be a "hot topic" and what is not.

III.  Ways to Organize Your Literature Review

Chronology of Events If your review follows the chronological method, you could write about the materials according to when they were published. This approach should only be followed if a clear path of research building on previous research can be identified and that these trends follow a clear chronological order of development. For example, a literature review that focuses on continuing research about the emergence of German economic power after the fall of the Soviet Union. By Publication Order your sources by publication chronology, then, only if the order demonstrates a more important trend. For instance, you could order a review of literature on environmental studies of brown fields if the progression revealed, for example, a change in the soil collection practices of the researchers who wrote and/or conducted the studies. Thematic [“conceptual categories”] A thematic literature review is the most common approach to summarizing prior research in the social and behavioral sciences. Thematic reviews are organized around a topic or issue, rather than the progression of time, although the progression of time may still be incorporated into a thematic review. For example, a review of the Internet’s impact on American presidential politics could focus on the development of online political satire. While the study focuses on one topic, the Internet’s impact on American presidential politics, it would still be organized chronologically reflecting technological developments in media. The difference in this example between a "chronological" and a "thematic" approach is what is emphasized the most: themes related to the role of the Internet in presidential politics. Note that more authentic thematic reviews tend to break away from chronological order. A review organized in this manner would shift between time periods within each section according to the point being made. Methodological A methodological approach focuses on the methods utilized by the researcher. For the Internet in American presidential politics project, one methodological approach would be to look at cultural differences between the portrayal of American presidents on American, British, and French websites. Or the review might focus on the fundraising impact of the Internet on a particular political party. A methodological scope will influence either the types of documents in the review or the way in which these documents are discussed.

Other Sections of Your Literature Review Once you've decided on the organizational method for your literature review, the sections you need to include in the paper should be easy to figure out because they arise from your organizational strategy. In other words, a chronological review would have subsections for each vital time period; a thematic review would have subtopics based upon factors that relate to the theme or issue. However, sometimes you may need to add additional sections that are necessary for your study, but do not fit in the organizational strategy of the body. What other sections you include in the body is up to you. However, only include what is necessary for the reader to locate your study within the larger scholarship about the research problem.

Here are examples of other sections, usually in the form of a single paragraph, you may need to include depending on the type of review you write:

• Current Situation : Information necessary to understand the current topic or focus of the literature review.
• Sources Used : Describes the methods and resources [e.g., databases] you used to identify the literature you reviewed.
• History : The chronological progression of the field, the research literature, or an idea that is necessary to understand the literature review, if the body of the literature review is not already a chronology.
• Selection Methods : Criteria you used to select (and perhaps exclude) sources in your literature review. For instance, you might explain that your review includes only peer-reviewed [i.e., scholarly] sources.
• Standards : Description of the way in which you present your information.
• Questions for Further Research : What questions about the field has the review sparked? How will you further your research as a result of the review?

IV.  Writing Your Literature Review

Once you've settled on how to organize your literature review, you're ready to write each section. When writing your review, keep in mind these issues.

Use Evidence A literature review section is, in this sense, just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence [citations] that demonstrates that what you are saying is valid. Be Selective Select only the most important points in each source to highlight in the review. The type of information you choose to mention should relate directly to the research problem, whether it is thematic, methodological, or chronological. Related items that provide additional information, but that are not key to understanding the research problem, can be included in a list of further readings . Use Quotes Sparingly Some short quotes are appropriate if you want to emphasize a point, or if what an author stated cannot be easily paraphrased. Sometimes you may need to quote certain terminology that was coined by the author, is not common knowledge, or taken directly from the study. Do not use extensive quotes as a substitute for using your own words in reviewing the literature. Summarize and Synthesize Remember to summarize and synthesize your sources within each thematic paragraph as well as throughout the review. Recapitulate important features of a research study, but then synthesize it by rephrasing the study's significance and relating it to your own work and the work of others. Keep Your Own Voice While the literature review presents others' ideas, your voice [the writer's] should remain front and center. For example, weave references to other sources into what you are writing but maintain your own voice by starting and ending the paragraph with your own ideas and wording. Use Caution When Paraphrasing When paraphrasing a source that is not your own, be sure to represent the author's information or opinions accurately and in your own words. Even when paraphrasing an author’s work, you still must provide a citation to that work.

V.  Common Mistakes to Avoid

These are the most common mistakes made in reviewing social science research literature.

• Sources in your literature review do not clearly relate to the research problem;
• You do not take sufficient time to define and identify the most relevant sources to use in the literature review related to the research problem;
• Relies exclusively on secondary analytical sources rather than including relevant primary research studies or data;
• Uncritically accepts another researcher's findings and interpretations as valid, rather than examining critically all aspects of the research design and analysis;
• Does not describe the search procedures that were used in identifying the literature to review;
• Reports isolated statistical results rather than synthesizing them in chi-squared or meta-analytic methods; and,
• Only includes research that validates assumptions and does not consider contrary findings and alternative interpretations found in the literature.

Cook, Kathleen E. and Elise Murowchick. “Do Literature Review Skills Transfer from One Course to Another?” Psychology Learning and Teaching 13 (March 2014): 3-11; Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . London: SAGE, 2011; Literature Review Handout. Online Writing Center. Liberty University; Literature Reviews. The Writing Center. University of North Carolina; Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: SAGE, 2016; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012; Randolph, Justus J. “A Guide to Writing the Dissertation Literature Review." Practical Assessment, Research, and Evaluation. vol. 14, June 2009; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016; Taylor, Dena. The Literature Review: A Few Tips On Conducting It. University College Writing Centre. University of Toronto; Writing a Literature Review. Academic Skills Centre. University of Canberra.

Writing Tip

Break Out of Your Disciplinary Box!

Thinking interdisciplinarily about a research problem can be a rewarding exercise in applying new ideas, theories, or concepts to an old problem. For example, what might cultural anthropologists say about the continuing conflict in the Middle East? In what ways might geographers view the need for better distribution of social service agencies in large cities than how social workers might study the issue? You don’t want to substitute a thorough review of core research literature in your discipline for studies conducted in other fields of study. However, particularly in the social sciences, thinking about research problems from multiple vectors is a key strategy for finding new solutions to a problem or gaining a new perspective. Consult with a librarian about identifying research databases in other disciplines; almost every field of study has at least one comprehensive database devoted to indexing its research literature.

Frodeman, Robert. The Oxford Handbook of Interdisciplinarity . New York: Oxford University Press, 2010.

Another Writing Tip

Don't Just Review for Content!

While conducting a review of the literature, maximize the time you devote to writing this part of your paper by thinking broadly about what you should be looking for and evaluating. Review not just what scholars are saying, but how are they saying it. Some questions to ask:

• How are they organizing their ideas?
• What methods have they used to study the problem?
• What theories have been used to explain, predict, or understand their research problem?
• What sources have they cited to support their conclusions?
• How have they used non-textual elements [e.g., charts, graphs, figures, etc.] to illustrate key points?

When you begin to write your literature review section, you'll be glad you dug deeper into how the research was designed and constructed because it establishes a means for developing more substantial analysis and interpretation of the research problem.

Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1 998.

Yet Another Writing Tip

When Do I Know I Can Stop Looking and Move On?

Here are several strategies you can utilize to assess whether you've thoroughly reviewed the literature:

• Look for repeating patterns in the research findings . If the same thing is being said, just by different people, then this likely demonstrates that the research problem has hit a conceptual dead end. At this point consider: Does your study extend current research?  Does it forge a new path? Or, does is merely add more of the same thing being said?
• Look at sources the authors cite to in their work . If you begin to see the same researchers cited again and again, then this is often an indication that no new ideas have been generated to address the research problem.
• Search Google Scholar to identify who has subsequently cited leading scholars already identified in your literature review [see next sub-tab]. This is called citation tracking and there are a number of sources that can help you identify who has cited whom, particularly scholars from outside of your discipline. Here again, if the same authors are being cited again and again, this may indicate no new literature has been written on the topic.

Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: Sage, 2016; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

• << Previous: Theoretical Framework
• Next: Citation Tracking >>
• Last Updated: May 2, 2024 4:39 PM
• URL: https://libguides.usc.edu/writingguide

Literature Review Basics

• What is a Literature Review?
• Synthesizing Research
• Using Research & Synthesis Tables
• Additional Resources

About the Research and Synthesis Tables

Research Tables and Synthesis Tables are useful tools for organizing and analyzing your research as you assemble your literature review. They represent two different parts of the review process: assembling relevant information and synthesizing it. Use a Research table to compile the main info you need about the items you find in your research -- it's a great thing to have on hand as you take notes on what you read! Then, once you've assembled your research, use the Synthesis table to start charting the similarities/differences and major themes among your collected items.

We've included an Excel file with templates for you to use below; the examples pictured on this page are snapshots from that file.

• Research and Synthesis Table Templates This Excel workbook includes simple templates for creating research tables and synthesis tables. Feel free to download and use!

Using the Research Table

This is an example of a  research table,  in which you provide a basic description of the most important features of the studies, articles, and other items you discover in your research. The table identifies each item according to its author/date of publication, its purpose or thesis, what type of work it is (systematic review, clinical trial, etc.), the level of evidence it represents (which tells you a lot about its impact on the field of study), and its major findings. Your job, when you assemble this information, is to develop a snapshot of what the research shows about the topic of your research question and assess its value (both for the purpose of your work and for general knowledge in the field).

Think of your work on the research table as the foundational step for your analysis of the literature, in which you assemble the information you'll be analyzing and lay the groundwork for thinking about what it means and how it can be used.

Using the Synthesis Table

This is an example of a  synthesis table  or  synthesis matrix , in which you organize and analyze your research by listing each source and indicating whether a given finding or result occurred in a particular study or article ( each row lists an individual source, and each finding has its own column, in which X = yes, blank = no). You can also add or alter the columns to look for shared study populations, sort by level of evidence or source type, etc. The key here is to use the table to provide a simple representation of what the research has found (or not found, as the case may be). Think of a synthesis table as a tool for making comparisons, identifying trends, and locating gaps in the literature.

How do I know which findings to use, or how many to include?  Your research question tells you which findings are of interest in your research, so work from your research question to decide what needs to go in each Finding header, and how many findings are necessary. The number is up to you; again, you can alter this table by adding or deleting columns to match what you're actually looking for in your analysis. You should also, of course, be guided by what's actually present in the material your research turns up!

• << Previous: Synthesizing Research
• Next: Additional Resources >>
• Last Updated: Sep 26, 2023 12:06 PM
• URL: https://usi.libguides.com/literature-review-basics

Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design (2020)

Chapter: chapter 2 - literature review and synthesis.

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

4 Literature Review and Synthesis Literature Review Purpose of Literature Review Performance-based seismic design (PBSD) for infrastructure in the United States is a developing field, with new research, design, and repair technologies; definitions; and method- ologies being advanced every year. A synthesis report, NCHRP Synthesis 440: Performance- Based Seismic Bridge Design (Marsh and Stringer 2013), was created to capture PBSD understanding up to that point. This synthesis report described the background, objec- tives, and research up until 2011 to 2012 and synthesized the information, including areas where knowledge gaps existed. The literature review in this research report focuses on new infor mation developed after the efforts of NCHRP Synthesis 440. The intention is that this research report will fuel the next challenge: developing a methodology to implement PBSD for bridge design. Literature Review Process Marsh and Stringer (2013) performed an in-depth bridge practice review by sending a questionnaire to all 50 states, with particular attention to regions with higher seismic hazards. The survey received responses from a majority of those agencies. This process was continued in the current project with a request for new information or research that the state depart- ment of transportation (DOT) offices have participated in or are aware of through other organizations. The research team reached out to the list of states and researchers in Table 1. An X within a box is placed in front of their names if they responded. The team also examined the websites of the state DOTs that participated to investigate whether something was studied locally, especially work being developed in California. The research team made an additional effort to perform a practice review of bridge designs, research, and other design industries, specifically in the building industry. The building industry has been developing PBSD for more than 20 years, and some of their developments are appli- cable to bridge design. These combined efforts have allowed the research team to assemble an overview of the state of PBSD engineering details and deployment since Marsh and Stringer’s (2013) report was published. NCHRP Synthesis 440 primarily dealt with the effects of strong ground motion shaking. Secondary effects such as tsunami/seiche, ground failure (surface rupture, liquefaction, or slope failure), fire, and flood were outside the scope of this study. Regardless, their impact on bridges may be substantial, and investigation into their effects is undoubtedly important. C H A P T E R 2

Literature Review and Synthesis 5 The following e-mail was sent to the owners and researchers. Dear (individual): We are assisting Modjeski & Masters with the development of proposed guidelines for Performance- Based Seismic Bridge Design, as part of NCHRP [Project] 12-106. Lee Marsh and our Team at BergerABAM are continuing our efforts from NCHRP Synthesis 440, which included a literature review up to December of 2011. From this timeframe forward, we are looking for published research, contractual language, or owner documents that deal with the following categories: 1. Seismic Hazards (seismic hazard levels, hazard curves, return periods, geo-mean vs. maximum direc- tion, probabilistic vs. deterministic ground motions, conditional mean spectrum, etc.) 2. Structure Response (engineering design parameters, materials and novel columns, isolation bearings, modeling techniques, etc.) 3. Damage Limit States (performance descriptions, displacement ductility, drift ratios, strain limits, rotation curvature, etc.) 4. Potential for Loss (damage descriptions, repairs, risk of collapse, economical loss, serviceability loss, etc.) 5. Performance Design Techniques (relating hazard to design to performance to risk, and how to assess [these] levels together) If you are aware of this type of resource, please provide a contact that we can work with to get this information or provide a published reference we can gather. Your assistance is appreciated. We want to minimize your time, and ask that you respond by Wednesday, 8 February 2017. Thank you again, Research Team Synthesis of PBSD (2012–2016) Objectives of NCHRP Synthesis 440 The synthesis gathered data from a number of different but related areas. Marsh and Stringer (2013), herein referred to as NCHRP Synthesis 440, set the basis for this effort. The research report outline follows what has been added to the NCHRP Synthesis 440 effort since 2012. The information gathered that supplements NCHRP Synthesis 440 includes, but is not limited to, the following topics. • Public and engineering expectations of seismic design and the associated regulatory framework Participation State Alaska DOT Arkansas DOT California DOT (Caltrans) Illinois DOT Indiana DOT Missouri DOT Montana DOT Nevada DOT Oregon DOT South Carolina DOT Utah DOT Washington State DOT Table 1. List of state DOT offices and their participation.

6 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design • Seismic hazard analysis • Structural analysis and design • Damage analysis • Loss analysis • Organization-specific criteria for bridges • Project-specific criteria Where new or updated information is available for these areas, a summary is included. Marsh and Stringer (2013) also identified gaps in the knowledge base of PBSD, current as of 2012, that need to be closed. Knowledge gaps certainly exist in all facets of PBSD; however, key knowledge gaps that should be closed in order to implement PBSD are covered. • Gaps related to seismic hazard prediction • Gaps related to structural analysis • Gaps related to damage prediction • Gaps related to performance • Gaps related to loss prediction • Gaps related to regulatory oversight and training • Gaps related to decision making These knowledge gaps have been filled in somewhat in this research report but, for the most part, these areas are still the key concepts that require additional development to further the development of a PBSD guide specification. Public and Engineering Expectations of Seismic Design and the Associated Regulatory Framework The public expectation of a structure, including a bridge, is that it will withstand an earthquake, but there is a limited understanding of what that actually means. Decision makers struggle to understand how a bridge meeting the current requirements of the AASHTO Guide Specifications for LRFD Seismic Bridge Design (2011), herein referred to as AASHTO guide specifications, will perform after either the expected (design) or a higher level earthquake. Decision makers understand the basis of life safety, wherein the expectation is that no one will perish from a structure collapsing, but often mistakenly believe that the structure will also be usable after the event. In higher level earthquakes, even in some lower level events, this is not true without repair, retrofit, or replacement. In the past decade, there has been an increased awareness by owners and decision makers as to the basis of seismic design. As a result, a need has developed for performance criteria so that economic and social impacts can be interwoven with seismic design into the decision processes (see Figure 1). Several states, including California, Oregon, and the State of Washington, are working toward resiliency plans, although these are developed under different titles or programs within the states. Resiliency has been defined in several ways: (1) amount of damage from an event measured in fatalities, structural replacement cost, and recovery time and (2) the time to resto- ration of lifelines, reoccupation of homes and structures, and, in the short term, resumption of normal living routines. The California DOT Caltrans has generated risk models and is in the process of developing a new seismic design specification to address PBSD in bridge design. The risk models and specifications are not published yet, but the use in PBSD is discussed in greater detail later in this chapter.

Literature Review and Synthesis 7 The State of Washington The State of Washington’s resiliency plan, outlined in Washington State Emergency Management Council–Seismic Safety Committee (2012), works to identify actions and policies before, during, and after an earthquake event that can leverage existing policies, plans, and initiatives to realize disaster resilience within a 50-year life cycle. The hazard level used for trans- portation planning is the 1000 year event. The goals for transportation systems vary depending on the type of service a route provides, as shown in following components of the plan. For major corridors such as Interstates 5, 90, and 405 and floating bridges SR 520, I-90, and Hood Canal, the target timeframe for response and recovery is between 1 to 3 days and 1 to 3 months, depending on location. The current anticipated timeframe based on current capacity and without modifications is between 3 months to 1 year and 1 to 3 years, depending on location. The actual response and recovery time will depend on a number of factors. For example: 1. The number of Washington State DOT personnel who are able to report to work may be limited by a variety of circumstances, including where personnel were at the time of the earthquake and whether they sustained injuries. 2. Bridges and roadways in earthquake-affected areas must be inspected. How long this takes will depend on the number and accessibility of the structures and the availability of qualified inspectors. 3. Some bridges and segments of road may be rendered unusable or only partially usable as a result of the earthquake or secondary effects. The response and recovery timeframe will depend on the number, the location, and the extent of the damage. 4. Certain earthquake scenarios could result in damage to the Ballard Locks and cause the water level in Lake Washington to drop below the level required to operate the floating bridges. 5. Depending on the scenario and local conditions, liquefaction and slope failure could damage both interstates and planned detours. During the first 3 days after the event, the Washington State Department of Transportation (Washington State DOT) will inspect bridges and begin repairs as needed. Washington State DOT’s first priority will be to open key routes for emergency response vehicles. Subsequent phases of recovery will include setting up detours where necessary and regulating the type and Figure 1. PBSD decision-making process (Guidelines Figure 2.0-1). References to guidelines figures and tables within parentheses indicate the proposed AASHTO guidelines.

8 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design volume of traffic, to give the public as much access as possible while damaged roads and bridges are repaired. For major and minor arterials, which encompass arterial roadways (including bridges) other than the interstates (so therefore includes state highways and many city and county roads), the target timeframe for response and recovery is between 0 to 24 hours and 3 months to 1 year, depending on location; the percentage of roadways that are open for use will increase over this period. Anticipated timeframe based on current capacity is between 1 week to 1 month and 1 to 3 years, depending on location; the percentage of roadways that are open for use will increase over this period. The goal of Washington State Emergency Management Council’s resiliency plan is to establish a means to coordinate agencies, public–private partnerships, and standards toward these resiliency goals. The plan outlines goals for recovery times for transportation systems in terms of hours, days, weeks, months, and years, with targets to achieve different levels of recovery (see Table 2) as follows. Similar recovery timeframe processes were established for service sectors (e.g., hospitals, law enforcement, and education); utilities; ferries, airports, ports, and navigable waterways; mass transit; and housing. The overall resiliency plan also discusses the degree to which the recovery of one component or sector would depend on the restoration of another. The key interdependencies that the participants identified include information and communication technologies, transportation, electricity, fuel, domestic water supplies, wastewater systems, finance and banking, and planning and community development. It appears that the implementation of the Washington State Emergency Management Council’s initiative, originally assumed to take 2.5 to 3 years in 2012, has not seen significant development since then. However, the State’s initiative to develop a more resilient community has been extended down to the county level, with King County’s efforts referenced in Rahman et al. (2014) and, at the city level, with the City of Seattle referenced in CEMP (2015). This reflects the commitment needed not only by the legislature and the state departments but also by other agencies (e.g., county, city, or utilities) and the public to take an interest in, and provide funding for, the development of a resiliency plan. The recovery continuum is presented graphically in Figure 2. Developing this relationship with other agency plans is an iterative process that will take time, as shown in Figure 3. Identifying the critical sectors of the agency is necessary to develop a resiliency model and determine how to approach a disaster recovery framework. King County worked from Washington State’s initiative to develop Figure 4. The Oregon DOT Oregon DOT has developed a variation of the approach identified by the State of Wash- ington; further discussion is found later in this chapter. Other Resilience Documents The building industry has recently seen the development of two additional documents that address PBSD in terms of expectations and process. The REDi Rating System from REDi (2013) sets an example for incorporating resilience- based design into the PBSD process. This document outlines structural resilience objectives for organizational resilience, building resilience, loss assessment, and ambient resilience to evaluate and rate the decision making and design methodology using PBSD for a specific project.

Literature Review and Synthesis 9 The document is one of the only references that addresses a system to develop probabilistic methods to estimate downtime. The overall intent is to provide a roadmap to resilience. This roadmap is intended to allow owners to resume business operation and to provide livable conditions quickly after an earthquake. The Los Angeles Tall Buildings Structural Design Council (LATBSDC 2014) created an alter- native procedure specific to their location. Design specification criteria are identified and modi- fications are described as appropriate for the PBSD approach to tall buildings in this localized Minimal (A minimum level of service is restored, primarily for the use of emergency responders, repair crews, and vehicles transporting food and other critical supplies.) Functional (Although service is not yet restored to full capacity, it is sufficient to get the economy moving again—for example, some truck/freight traffic can be accommodated. There may be fewer lanes in use, some weight restrictions, and lower speed limits.) Operational (Restoration is up to 80 to 90 percent of capacity: A full level of service has been restored and is sufficient to allow people to commute to school and to work.) Time needed for recovery to 80 to 90 percent operational given current conditions. Source: Washington State Emergency Management Council–Seismic Safety Committee (2012). Table 2. Washington State’s targets of recovery.

10 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design Source: Adapted from FHWA by CEMP (2015). Figure 2. Recovery continuum process. Source: CEMP (2015). Figure 3. Relationship of disaster recovery framework to other city plans. region. This procedure is a good example of how PBSD criteria and methodology need to be established locally, with a knowledge of risk, resources, and performance needs in order to set the criteria for true PBSD. Seismic Hazard Prediction As outlined in NCHRP Synthesis 440, the seismic hazard includes the regional tectonics and the local site characteristics from either a deterministic or probabilistic viewpoint. The deterministic form allows the assessment of shaking at a site as a function of the controlling earthquake that can occur on all the identified faults or sources. The probabilistic approach

Literature Review and Synthesis 11 defines an acceleration used in design that would be exceeded during a given window of time (e.g., a 7% chance of exceedance in 75 years). The following subsections provide a summary of procedures currently used within AASHTO, as well as new issues that should be eventually addressed in light of approaches used by the building industry. AASHTO Probabilistic Approach As summarized in the AASHTO guide specifications, the current approach used by AASHTO involves the use of a probabilistic hazard model with a nominal return period of 1000 years. Baker (2013) noted that the probabilistic seismic hazard analysis involves the following five steps: 1. Identify all earthquake sources capable of producing damaging ground motions. 2. Characterize the distribution of earthquake magnitudes (the rates at which earthquakes of various magnitudes are expected to occur). 3. Characterize the distribution of source-to-site distances associated with potential earthquakes. 4. Predict the resulting distribution of ground motion intensity as a function of earthquake magnitude, distance, and so forth. 5. Combine uncertainties in earthquake size, location, and ground motion intensity, using a calculation known as the total probability theorem. While implementation of the five steps in the probabilistic approach is beyond what most practicing bridge engineers can easily perform, AASHTO, working through the U.S. Geological Survey, developed a website hazard tool that allows implementation of the probabilistic proce- dure based on the latitude and longitude of a bridge site. The product of the website includes peak ground acceleration (PGA), spectral acceleration at 0.2 s (Ss), and spectral acceleration at 1 s (S1). These values are for a reference-site condition comprising soft rock/stiff soil, having a time-averaged shear wave velocity (Vs) over the upper 100 feet of soil profile equal to 2500 feet per second (fps). The Geological Survey website can also correct for local site conditions following procedures in the AASHTO Guide Specifications for LRFD Seismic Bridge Design. One of the limitations of the current U.S. Geological Survey hazard website is that it is based on a seismic hazard model developed in 2002. The Geological Survey updated its seismic model in 2008 and then in 2014; however, these updates are currently not implemented within the AASHTO hazard model on the Geological Survey’s website. Oregon and the State of Washington have updated the seismic hazard map used by the Oregon DOT and the Washington State Source: Rahman et al. (2014). Figure 4. Resilient King County critical sectors and corresponding subsectors.

12 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design DOT to include the 2014 U.S. Geological Survey hazard model; however, most state DOTs are still using the out-of-date hazard model. Use of the outdated hazard model introduces some inconsistencies in ground motion prediction, relative to the current Geological Survey hazard website tool at some locations. Discussions are ongoing between NCHRP and the U.S. Geological Survey to update the 2002 website tool. Another issue associated with the current AASHTO probabilistic method is that it is based on the geomean of the ground motion. In other words, the ground motion prediction equations in the hazard model are based on the geomean of recorded earthquake motions. These motions are not necessarily the largest motion. The building industry recognized that the maximum direction could result in larger ground motions and introduced maximum direction corrections. These corrections increase spectral acceleration by a factor of 1.1 and S1 by a factor of 1.3. The relevance of this correction to bridges is discussed in the next subsection of this review. The building industry also introduced a risk-of-collapse correction to the hazard model results. This correction is made to Ss and S1. The size of the correction varies from approximately 0.8 to 1.2 within the continental United States. It theoretically adjusts the hazard curves to provide a 1% risk of collapse in 50 years. The risk-of-collapse corrections were developed by the U.S. Geological Survey for a range of building structures located throughout the United States. Although no similar corrections have been developed for bridges, the rationale for the adjust- ment needs to be further evaluated to determine if the rationale should be applied to bridge structures. As a final point within this discussion of probabilistic methods within the AASHTO guide specifications, there are several other areas of seismic response that need to be considered. These include near-fault and basin effects on ground motions, as well as a long-period transition factor. The near-fault and basin adjustments correct the Ss and S1 spectral accelerations for locations near active faults and at the edge of basins, respectively. These adjustments typically increase spectral accelerations at longer periods (> 1 s) by 10% to 20%, depending on specifics of the site. The long-period transition identifies the point at which response spectral ordinates are no longer proportional to the 1/T decay with increasing period. These near-fault, basin, and long-period adjustments have been quantified within the building industry guidance documents but remain, for the most part, undefined within the AASHTO guide specifications. As bridge discussions and research move closer to true probabilistic format for PBSD, these issues need to be addressed as part of a future implementation process. Correction for Maximum Direction of Motion Over the last decade, a debate has been under way within the building industry regarding the appropriate definition of design response spectra (Stewart et al. 2011). The essence of the argument relates to the representation of bidirectional motion via response spectra. In both the AASHTO LRFD Bridge Design Specifications (2014), as well as the AASHTO Guide Specifications for LRFD Seismic Bridge Design (SGS), response spectra are established by defining spectral ordinates at two or three different periods from design maps developed by the U.S. Geological Survey for a return period of 1000 years. The resulting spectra are then adjusted for local site conditions, resulting in the final design spectra. In establishing the design maps for parameters such as Ss and S1, the U.S. Geological Survey has traditionally relied upon probabilistic seismic hazard analysis, which utilizes ground motion prediction equations (GMPEs) defined by the geometric mean of the two principal directions of recorded motion. In 2006, Boore introduced a new rotation independent geometric mean definition termed GMRotI50 (Boore et al. 2006). Then, in 2010, Boore developed a new defini- tion that does not rely upon the geometric mean termed RotD50 spectra, which can be generi- cally expressed as RotDNN spectra, where NN represents the percentile of response (i.e., 50 is

Literature Review and Synthesis 13 consistent with the median, 0 is the minimum, and 100 is the maximum). The NGA–West2 project GMPEs utilized RotD50 spectra for the ground motion models; however, the 2009 National Earthquake Hazards Reduction Program (NEHRP) provisions adopted a factor to modify the median response, RotD50, to the maximum possible response, RotD100 as the spectra for the design maps (Stewart et al. 2011). Introducing RotD100 resulted in a 10% to 30% increase in spectral ordinates results relative to the geometric mean, which has traditionally been used as a basis of seismic design. In order to appreciate the impact of these choices, a brief discussion of RotDNN spectra is warranted. As described in Boore (2010), for a given recording station, the two orthogonal- component time series are combined into a single time series corresponding to different rotation angles, as shown in Equation 1: aROT(t ; θ) = a1(t)cos(θ) + a2(t)sin(θ) (1) where a1(t ) and a2(t ) are the orthogonal horizontal component acceleration time series and θ is the rotation angle. For example, consider the two orthogonal horizontal component time series, H1 and H2, shown in Figure 5. The single time series corresponding to the rotation angle θ is created by combining the Direction 1 and Direction 2 time series. Then, the response spectrum for that single time series can be obtained, as shown in the figure. The process is repeated for a range of azimuths from 0° to one rotation-angle increment less than 180°. If the rotation-angle increment is θ, then there will be 180/θ single time series, as well as 180/θ corresponding response spectra. For example, if θ = 30°, then there will be six single time series (the original two, as well as four generated time series), as well as six response spectra, as shown in Figure 6. Once the response spectra for all rotation angles are obtained, then the nth percentile of the spectral amplitude over all rotation angles for each period is computed (e.g., RotD50 is the median value and RotD100 is the largest value for all rotation angles). For example, at a given period of 1 s, the response spectra values for all rotation angles are sorted, and the RotD100 value would be the largest value from all rotation angles while RotD50 would be the median. This is repeated for all periods, with potentially different rotation angles, producing the largest Source: Palma (2019). Figure 5. Combination of time series to generate rotation dependent spectra.

14 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design response at any given period (period-dependent rotation angle.) Figure 7 shows an example of the two orthogonal horizontal components, as well as the RotD50 and RotD100 spectra for the as-recorded ground motion from the 2011 Christchurch, New Zealand, earthquake at Kaiapoi North School station. As can be seen in the sample spectra (see Figure 7), the RotD100 spectrum represents a sub- stantial increase in demand when compared with the RotD50 spectrum. The main question facing the bridge community from this point onward is the appropriate selection of response spectra definition. This question can only be answered by developing sample designs to both the RotD50 and RotD100 spectra, which would then be evaluated via no-linear time history analysis. Such a study will require multiple bridge configurations and multiple ground motions. As an example of the potential impact, Figure 8 shows the results of a single-degree-of- freedom bridge column designed according to both RotD50 and RotD100 spectra, along with the resulting nonlinear time history analysis. The column was designed using direct displacement- based design to achieve a target displacement of 45 cm. It is clear from the results in Figure 8d that the nonlinear response of the column designed to the RotD100 spectrum matches the target Source: Palma (2019). Figure 6. Example of time series rotations with an angle increment (p) of 30ç. Source: Palma (2019). Figure 7. Sample spectra for a recorded ground motion pair.

Literature Review and Synthesis 15 reasonably well, while designing to the RotD50 spectrum results in displacements that are much greater than expected. This is, of course, only one result of an axisymmetric system. In the future (and outside the scope of this project), a systematic study could be conducted for both single degree of freedom and multiple degrees of freedom systems. The literature on this topic can be divided into two categories: (1) response spectra definitions and (2) impact on seismic response. The majority of the literature addresses the former. For example, Boore et al. (2006) and Boore (2010) introduced orientation-independent measures of seismic intensity from two horizontal ground motions. Boore et al. (2006) proposed two measures of the geometric mean of the seismic intensity, which are independent of the in-situ orientations of the sensors. One measure uses period-dependent rotation angles to quantify the spectral intensity, denoted GMRotDnn. The other measure is the GMRotInn, where I stands for period-independent. The ground motion prediction equations of Abrahamson and Silva (1997), Figure 8. Single bridge column designed according to both RotD50 and RotD100 spectra (Tabas EQ = Tabas earthquake and displ. = displacement).

16 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design Boore et al. (1997), Campbell and Bozorgnia (2003), and Sadigh et al. (1997) have been updated using GMRotI50 as the dependent variable. Since more users within the building industry expressed the desire to use the maximum spec- tral response over all the rotation angles without geometric means, Boore (2010) introduced the measures of ground-shaking intensity irrespective of the sensor orientation. The measures are RotDnn and RotInn, whose computation is similar to GMRotDnn and GMRotInn without computing the geometric means. With regard to impact on seismic response, the opinion paper by Stewart et al. (2011) and the work by Mackie et al. (2011) on the impact of incidence angle on bridge response are relevant. Specifically, Stewart et al. (2011) noted the importance of computational analysis of structures (which had not been done as of 2011) in proposing appropriate spectra definitions. Other Methodologies for Addressing Seismic Ground Motion Hazards There are several other reports that address the question of the methodology that may be utilized in developing the seismic hazard. These recent studies endeavored to create a method- ology that is easier for engineers, as users, to understand how to tie the seismic hazard to the performance expectation. The variability of these approaches also demonstrates the broad range of options and therefore a limited understanding by practitioners in the bridge design industry. Following are some examples that apply to PBSD. Wang et al. (2016) performed a probabilistic seismic risk analysis (SRA) based on a single ground motion parameter (GMP). For structures whose responses can be better predicted using multiple GMPs, a vector-valued SRA (VSRA) gives accurate estimates of risk. A simplified approach to VSRA, which can substantially improve computational efficiency without losing accuracy, and a new seismic hazard de-aggregation procedure are proposed. This approach and the new seismic hazard de-aggregation procedure would allow an engineer to determine a set of controlling earthquakes in terms of magnitude, source–site distance, and occurrence rate for the site of interest. Wang et al. presented two numerical examples to validate the effectiveness and accuracy of the simplified approach. Factors affecting the approximations in the simplified approach were discussed. Kwong and Chopra (2015) investigated the issue of selecting and scaling ground motions as input excitations for response history analyses of buildings in performance-based earthquake engineering. Many ground motion selection and modification procedures have been developed to select ground motions for a variety of objectives. This report focuses on the selection and scaling of single, horizontal components of ground motion for estimating seismic demand hazard curves of multistory frames at a given site. Worden et al. (2012) used a database of approximately 200,000 modified Mercalli intensity (MMI) observations of California earthquakes collected from U.S. Geological Survey reports, along with a comparable number of peak ground motion amplitudes from California seismic networks, to develop probabilistic relationships between MMI and peak ground velocity (PGV), PGA, and 0.3-s, 1-s, and 3-s 5% damped pseudo-spectral acceleration. After associating each ground motion observation with an MMI computed from all the seismic responses within 2 kilometers of the observation, a joint probability distribution between MMI and ground motion was derived. A reversible relationship was then derived between MMI and each ground motion parameter by using a total least squares regression to fit a bilinear function to the median of the stacked probability distributions. Among the relationships, the fit-to-peak ground velocity has the smallest errors, although linear combinations of PGA and PGV give nominally better results. The magnitude and distance terms also reduce the overall residuals and are justifiable on an information theoretical basis.

Literature Review and Synthesis 17 Another approach to developing the appropriate seismic hazard comes out of Europe. Delavaud et al. (2012) presented a strategy to build a logic tree for ground motion prediction in European countries. Ground motion prediction equations and weights have been determined so that the logic tree captures epistemic uncertainty in ground motion prediction for six different tectonic regions in Europe. This includes selecting candidate GMPEs and simultaneously running them through a panel of six experts to generate independent logic trees and rank the GMPEs on available test data. The collaboration of this information is used to set a weight to the GMPEs and create a consensus logic tree. This output then is run through a sensitivity analysis of the proposed weights on the seismic hazard before setting a final logic tree for the GMPEs. Tehrani and Mitchell (2014) used updated seismic hazard maps for Montreal, Canada to develop a uniform hazard spectra for Site Class C and a seismic hazard curve to analyze bridges in the localized area. Kramer and Greenfield (2016) evaluated three case studies following the 2011 Tohoku earthquake to better understand and design for liquefaction. Existing case history databases are incomplete with respect to many conditions for which geotechnical engineers are often required to evaluate liquefaction potential. These include liquefaction at depth, liquefaction of relatively dense soils, and liquefaction of gravelly soils. Kramer and Greenfield’s investigation of the three case histories will add to the sparse existing data for those conditions, and their interpretations will aid in the validation and development of predictive procedures for liquefaction potential evaluation. Structural Analysis and Design Predicting the structural response to the earthquake ground motions is critical for the PBSD process. NCHRP Synthesis 440 outlined several analysis methods that can be used to accomplish this task. The multimodal linear dynamic procedures are outlined in AASHTO LRFD Bridge Design Specifications (AASHTO 2014) and AASHTO Guide Specifications for LRFD Seismic Bridge Design (AASHTO 2011), although the Guide Specifications also include the parameters for performing a model pushover analysis in addition to prescriptive detail practices to ensure energy-dissipating systems behave as intended and other elements are capacity-protected. Other methods of analysis may be better suited for PBSD, but the initial PBSD approach will likely follow the procedures of the AASHTO guide specifications, with multi-level hazards and performance expectations. Limited research and code development have been accomplished since NCHRP Synthesis 440, but one new analysis method, outlined in Babazadeh et al. (2015), includes a three-dimensional finite element model simulation that is used to efficiently predict intermediate damage limit states in a consistent manner, with the experimental observations extracted from the actual tested columns. Other recent articles of structural analysis identified areas of improvement in the current design methodology that may be beneficial to PBSD. Huff and Pezeshk (2016) compared the substitute structure method methodology for isolated bearings with the displacement-based design methodology for ordinary bridges and showed that these two methodologies vary in estimating inelastic displacements. Huff (2016a) identified issues that are generally simplified or ignored in current practice of predicting inelastic behavior of bridges during earthquakes, both on the capacity (in the section of the element type and geometric nonlinearities) and demand (issues related to viscous dampening levels) sides of the process. The current SGS methodology for nonlinear static procedures were compared in Hajihashemi et al. (2017) with recent methodologies for multimodal pushover procedures that take into account all significant modes of the structure and with modified equivalent linearization procedures developed for

18 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design FEMA-440 (FEMA 2005). All of these analysis articles identify areas of current discussion on how to improve the analytical procedures proposed in the SGS. NCHRP Synthesis 440 focused primarily on new analysis methods, but a recent increased focus, in both academia and industry, has to do with new materials and systems and their impacts on PBSD. The evolution of enhanced seismic performance has been wrapped into several research topics, such as accelerated bridge construction (ABC), novel columns, and PBSD. The following are several aspects, though not all-encompassing, which have been improved upon in the last 6 years or so. Improving Structural Analysis Through Better Material Data The analysis and performance of a bridge are controlled with material property parameters incorporated into the seismic analysis models, specifically for the push-over analysis method. AASHTO Guide Specifications for LRFD Seismic Bridge Design (AASHTO 2011) specifies the strain limits to use for ASTM A706 (Grade 60) and ASTM A615 Grade 60 reinforcement. These strain limits come from Caltrans study of 1,100 mill certificates for ASTM A706 Grade 60 in the mid-1990s for projects in Caltrans bridge construction. The results were reported as elongation—not strain—at peak stress, so select bar pull tests were performed to correlate elongation to strain at peak stress. This was assumed to be a conservative approach, though it has recently been validated with a new ASTM A706 Grade 80 study at North Carolina State University by Overby et al. (2015a), which showed Caltrans numbers, by comparison, for Grade 60 are reasonable and conservative. Overby et al. (2015b) developed stress strain parameters for ASTM A706 Grade 80 reinforcing steel. Approximately 800 tests were conducted on bars ranging from #4 to #18 from multiple heats from three producing mills. Statistical results were presented for elastic modulus, yield strain and stress, strain-hardening strain, strain at maximum stress, and ultimate stress. Research is currently under way at North Carolina State University that aims to identify strain limit states, plastic hinge lengths, and equivalent viscous damping models for bridge columns constructed from A706 Grade 80 reinforcing steel. Work is also under way at the University of California, San Diego, on applications of Grade 80 rebar for capacity-protected members such as bridge cap beams. Design Using New Materials and Systems Structural analysis and design are fundamentally about structural response to the earthquake ground motion and the analysis methods used to develop this relationship. The complexity of the analysis depends on the geometry of the structure and elements and the extent of inelastic behavior. This is coupled with the damage, or performance criteria but has been broken out for the purposes of this report and NCHRP Synthesis 440. Next generation bridge columns, often referred to as novel columns, are improving as a tool for engineers to control both the structural analysis, as the make-up of the material changes the inelastic behavior, and the element performance of bridges in higher seismic hazards. The energy-dissipating benefits of low damage materials—such as ultrahigh-performance concrete (UHPC), engineered cementi- tious composites (ECC), and shape memory alloy, fiber-reinforced polymer (FRP) wraps and tubes, elastomeric bearings, and post-tensioned strands or bars—can be utilized by engineers to improve seismic performance and life-cycle costs after a significant seismic event. Recent (Saiidi et al. 2017) studies tested various combinations of these materials to determine if there are columns that can be built with these materials that are equivalent to, or better than, conventional reinforced concrete columns (in terms of cost, complexity, and construction duration) but that improve seismic performance, provide greater ductility, reduce damage, and accommodate a quicker recovery time and reduce loss in both the bridge and the economic environment.

Literature Review and Synthesis 19 Accelerated bridge construction is also a fast-developing field in bridge engineering, with draft guide specifications for design and construction currently being developed for adop- tion by AASHTO for AASHTO LRFD Bridge Design Specifications (AASHTO 2014). ABC has economic impacts that go beyond seismic engineering, but research is focusing on details and connections for accelerated construction in higher seismic regions, moving two research paths forward at the same time. Tazarv and Saiidi (2014) incorporated ABC research with novel column research to evaluate combined novel column materials that can be constructed quickly. The research focused on the performance of materials and how to incorporate them into practice. Key mechanical properties of reinforcing SMA were defined as follows: • Observed yield strength (fyo) is the stress at the initiation of nonlinearity on the first cycle of loading to the upper plateau. • Austenite modulus (k1) is the average slope between 15% to 70% of fyo. • Post yield stiffness (k2) is the average slope of curve between 2.5% and 3.5% of strain on the upper plateau of the first cycle of loading to 6% strain. • Austenite yield strength (fy) is the stress at the intersection of line passing through origin with slope of k1 and line passing through stress at 3% strain with slope of k2. • Lower plateau inflection strength (fi) is the stress at the inflection point of lower plateau during unloading from the first cycle to 6% strain. • Lower plateau stress factor, β = 1 – (fi/fy). • Residual strain (eres) is the tensile strain after one cycle to 6% and unloading to 1 ksi (7 MPa). • Recoverable super-elastic strain (er) is maximum strain with at least 90% strain recovery capacity. Using the ASTM standard for tensile testing, er ≤ 6%. • Martensite modulus (k3) is the slope of the curve between 8% to 9% strain, subsequent to one cycle of loading to 6% strain, unloading to 1 ksi (7 MPa) and reloading to the ultimate stress. • Secondary post-yield stiffness ratio, α = k3/k1. • Ultimate strain (eu) is strain at failure. A graphical representation is shown in Figure 9, and minimum and expected mechanical properties are listed in Table 3. Other researchers, such as at the University of Washington, are currently testing grouted bars using conventional grouts and finding that these development lengths can be reduced greatly. However, it is the force transfer of the grouted duct to the reinforcing outside the duct that may Figure 9. NiTi SE SMA nonlinear model.

20 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design require additional length to adequately develop the energy-dissipating or capacity-protecting system that was intended by the designer for performance of the bridge in a high seismic event. Tazarv and Saiidi (2014) identified other material properties such as UHPC and ECC, shown in Tables 4 and 5, respectively. Tazarv and Saiidi (2014) also addressed grouted splice sleeve couplers, self-consolidating concrete (SCC), and other connection types that could be used in ABC and novel column configurations, testing these materials in the laboratory to see if various combinations produced a logical system to be carried forward in research, design, and implementation. Trono et al. (2015) studied a rocking post-tensioned hybrid fiber-reinforced concrete (HyFRC) bridge column that was designed to limit damage and residual drifts and that was tested dynamically under earthquake excitation. The column utilized post-tensioned strands, HyFRC, and a combination of unbonded and headed longitudinal reinforcement. There have been two projects related to the field of novel columns and ABC through the National Cooperative Highway Research Program. One project was NCHRP Project 12-101, which resulted in NCHRP Report 864, 2 volumes (Saiidi et al. 2017), and the other project was NCHRP Project 12-105, which resulted in NCHRP Research Report 935 (Saiidi et al. 2020). NCHRP Project 12-101 identified three novel column systems—specifically, SMA and ECC, ECC and FRP, and hybrid rocking column using post-tensioned strands and fiber-reinforced Parameter Tensile Compressive,ExpectedbExpectedbMinimuma Table 3. Minimum expected reinforcing NiTi SE SMA mechanical properties. Properties Range Poisson’s Ratio 0.2 Creep Coefficient* 0.2 to 0.8 Total Shrinkage** *Depends on curing conditions and age of loading. up to 900x10-6 Equation Compressive Strength (f'UHPC) f'UHPC 20 to 30 ksi, (140 to 200 MPa) Coefficient of Thermal Expansion (5.5 to 8.5)x10 -6/°F, (10 to 15)x10-6/°C Specific Creep* (0.04 to 0.3)x10 -6/psi, (6 to 45)x10-6/MPa A time-dependent equation for UHPC strength is available. Tensile Cracking Strength (ft,UHPC) ft,UHPC = 6.7 (psi) f'UHPCEUHPC = 49000 (psi) 0.9 to 1.5 ksi, (6 to 10 MPa) Modulus of Elasticity (EUHPC) 6000 to 10000 ksi, (40 to 70 GPa) **Combination of drying shrinkage and autogenous shrinkage and depends on curing method. Table 4. UHPC mechanical properties.

Literature Review and Synthesis 21 polymer confinement—and compared them to a conventional reinforced column. The research and properties of the material are provided; incorporating laboratory tests and calibration, design examples are created to help engineers understand how to use these advanced materials in a linear elastic seismic demand model and to determine performance using a pushover analysis. It is worth noting that ductility requirements do not accurately capture the perfor- mance capabilities of these novel columns, and drift ratio limits are being used instead, similar to the building industry. NCHRP Project 12-101 also provided evaluation criteria that can be evaluated and incorporated by AASHTO into a guide specification or into AASHTO Guide Specifications for LRFD Seismic Bridge Design (AASHTO 2011) directly. NCHRP Project 12-105 synthesized research, design codes, specifications, and contract language throughout all 50 states and combined the knowledge base and lessons learned for ABC into proposed guide specifications for both design and construction. This work focused on connections, and most of that information is related to seismic performance of ABC elements and systems. Earthquake resisting elements (ERE) and earthquake resisting systems (ERS) are specifically identified, defined, and prescribed for performance in AASHTO guide specifica- tions (AASHTO 2011) but only implicitly applied in AASHTO LRFD Bridge Design Specifications (AASHTO 2014). Since NCHRP Project 12-105 is applicable to both of these design resources, ERE and ERS are discussed in terms of how to apply performance to the force-based seismic design practice of AASHTO LRFD Bridge Design Specifications (AASHTO 2014). The proposed guide specification language also identifies when performance of materials have to be incor- porated into the design, say in higher seismic hazards, and when it is acceptable to apply ABC connections and detailing practices with prescriptive design methodologies. As the industry’s understanding of performance increases, the engineering industry is accepting the benefits that come from a more user-defined engineering practice that is implemented by identifying material properties; evaluating hazards and soil and structural responses; and verifying performance through strain limits, damage limits states, moment curvature, displacements, and ductility. These tools and advancements in ABC and novel column designs, including other material property performance and analytical methodologies, are allowing PBSD to advance in other areas, such as hazard prediction, loss prediction, and the owner decision-making process. Feng et al. (2014a) studied the application of fiber-based analysis to predict the nonlinear response of reinforced concrete bridge columns. Specifically considered were predictions of overall force-deformation hysteretic response and strain gradients in plastic hinge regions. The authors also discussed the relative merits of force-based and displacement-based fiber elements and proposed a technique for prediction of nonlinear strain distribution based on the modified compression field theory. Fulmer et al. (2013) developed a new steel bridge system that is based upon ABC techniques that employ an external socket to connect a circular steel pier to a cap beam through the use of grout and shear studs. The resulting system develops a plastic hinge in the pipe away from the column-to-cap interface. An advantage of the design is ease of construction, as no field welding Properties Range Flexural Strength 1.5 to 4.5 ksi (10 to 30 MPa) Modulus of Elasticity 2600 to 5000 ksi (18 to 34 GPa) Ultimate Tensile Strain 1 to 8% Ultimate Tensile Strength 0.6 to 1.7 ksi (4 to 12 MPa) First Crack Strength 0.4 to 1.0 ksi (3 to 7 MPa) Compressive Strength 3 to 14 ksi (20 to 95 MPa) Table 5. ECC mechanical properties.

22 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design is required: the two assemblies are placed together and the annular space between the column and cap filled with grout. Figure 10 shows the details of this connection, and Figure 11 shows a test of the system. Another system being investigated is isolation bearings or dampening devices. Xie and Zhiang (2016) investigated the effectiveness and optimal design of protective devices for the seismic protection of highway bridges. Fragility functions are first derived by probabilistic seismic demand analysis, repair cost ratios are then derived using a performance-based methodol- ogy, and the associated component failure probability. Subsequently, the researchers tried to identify the optimal design parameters of protective devices for six design cases with various combinations of isolation bearings and fluid dampers and discussed the outcomes. Damage mitigation through isolation and energy dissipation devices is continually improving based on research, development, and implementation in the field. Recent events within the State of Washington, Alaska, and other state agencies have shown that the benefits of these tools can be compromised if the intended performance cannot be sustained for the 75-year design life of the structure. Mackie and Stojadinovic (2015) outlined performance criteria for fabrica- tion and construction that need to be administered properly, and engineers should consider the effects of moisture, salts, or other corrosive environmental conditions that can affect the performance of the isolation or energy-dissipating system. Another constraint with these systems can be the proprietary nature that occurs as a specific isolation or energy-dissipating system is utilized to develop a specific performance expectation that can only be accomplished with the prescribed system. This proprietary nature of these systems can create issues for certain funding sources that require equal bidding opportunities and the project expense that can accompany a proprietary system. To address this type of design constraint, Illinois DOT has been developing an earthquake-resisting system (ERS) to leverage the displacement capacity available at typical bearings in order to provide seismic protection to substructures of typical bridges. LaFave et al. (2013a) identified the effects and design parameters, Source: Fulmer et al. (2013). 5" 4 at 5" O.C. A A A-A Connection Details 45° UT 100% 3 8" 12 Studs Spaced Around Cross Section 30°Typ. 15° Offset Studs Inside Pipe from Cap Beam CL HSS16x0.500 Pipe 24x0.500 2'-0"2 14 " 4 at 5" O.C. 212"-34 "Ø Shear Studs 1'-11" Pipe Stud Detail Grout Provided By and Placed by NCSU Figure 10. Grouted shear stud bridge system.

Literature Review and Synthesis 23 such as fuse capacity, shear response, and sliding response, which can be used to account for more standard bearing configurations in seismic analysis, especially lower seismic hazard regions. A variation on the use of bearings in order to improve seismic performance of a pier wall configuration was outlined in Bignell et al. (2006). Historically, pinned, rocking, and sliding bearings have been used with interior pier walls and steel girder superstructures. These bearing configurations were compared with replacement elastomeric bearing configurations and details for structural analysis techniques, damage limit states, and structural fragility, and performance through probability distributions were utilized as a PBSD process for determining solutions to seismic isolation and enhanced seismic performance. The foundation conditions, pier wall effects, bearing type, and even embankment effects to structural performance were included in this evaluation. Another approach to enhanced performance is modifications to foundation elements or increased understanding and modeling of soil–structure interaction, specifically where lateral spread or liquefaction design conditions make conventional bridge design and elements imprac- tical. One example of this is the seismic design and performance of bridges constructed with rocking foundations, as evaluated in Antonellis and Panagiotou (2013). This type of rocking goes beyond the loss of contact area currently allowed in the guide specifications. The applica- tion of columns supported on rocking foundations accommodates large deformations, while there is far less damage, and can re-center after large earthquakes. Another approach is to tie a tolerable displacement of an individual deep foundation element to a movement that would cause adverse performance, excessive maintenance issues, or functionality problems with the bridge structure. Roberts et al. (2011) established a performance-based soil–structure–interaction design approach for drilled shafts. Chiou and Tsai (2014) evaluated displacement ductility of an in-ground hinging of a fixed head pile. Assessment formulas were developed for the displacement ductility capacity of a fixed-head pile in cohesion-less soils. The parameters in the formulas included the sectional over-strength ratio and curvature ductility capacity, as well as a modification factor for consider- ing soil nonlinearity. The modification factor is a function of the displacement ratio of the pile’s ultimate displacement to the effective soil yield displacement, which is constructed through a number of numerical pushover analyses. Source: Fulmer et al. (2013). Figure 11. Photograph of completed system before seismic testing showing hinge locations.

24 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design Damage Analysis As stated in NCHRP Synthesis 440, it is a fundamental need for the PBSD methodology to determine the type of damage and the likelihood that such damage will occur in the particular components of the structural system. This determination is of vital importance, as the damage sustained by a structure (and its nonstructural components) is directly relatable to the use or loss of a system after an earthquake. Therefore, there is a need to be able to reliably link structural and nonstructural response (internal forces, deformations, accelerations, and displacements) to damage. This is the realm of damage analyses, where damage is defined as discrete observable damage states (e.g., yield, spalling, longitudinal bar buckling, and bar fracture). Although the primary focus of the discussions is on structural components, similar considerations must be made for nonstructural components as well. NCHRP Synthesis 440 outlined an initial discussion on types of structural damage observed during historic earthquakes and laboratory experiments, prefaced the methods that have been developed to predict damage, identified structural details and concepts that could be used to reduce damage even in strong ground shaking, and reviewed post-event inspection tools. The new materials discussed in previous sections also apply to this discussion but are not repeated herein. Accurate damage prediction relies upon accurate definitions of performance limit states at the material level (i.e., strain limits) and the corresponding relationship between strain and displacement. Examples of recent research follow. Research by Feng et al. (2014b, 2014c) used finite element analysis validated by experimental test results to develop a model for predicting the tension strain corresponding to bar buckling. The model considers the impact of loading history on the boundary conditions of longitudinal bar restraint provided by the transverse steel. Goodnight et al. (2016a) identified strain limits to initiate bar buckling based on experimental results from 30 column tests (Equation 2). Following additional bidirectional tests on 12 columns, Equation 2 was revised to Equation 3. In addition, strain limit state equations were proposed for the compression strain in concrete to cause spiral yielding (Goodnight et al. 2017a). Goodnight et al. (2016b) also developed a new plastic hinge length model based on the data collected during those tests, which accounts for the actual curvature distribution in RC bridge columns. The revised model separates the strain penetration component from the flexural component while also recognizing that the hinge length for compression is smaller than that for tension. Brown et al. (2015) developed strain limit state (Equation 4) (tube wall local buckling) and equivalent viscous damping equations for reinforced concrete filled steel tubes (RCFSTs). The recommendations of the authors were based upon reversed cyclic tests of 12 RCFSTs of variable D/t (diameter to thickness) ratios. 0.03 700 0.1 (2)bucklingbar f E P f A s s yhe s ce g ε = + ρ − ′ 0.032 790 0.14 (3)bucklingbar f E P f A s s yhe s ce g ε = + ρ − ′ 0.021 9100 (4)tension buckling D t yε = − ≥ ε

Literature Review and Synthesis 25 where rs = reinforcement ratio, fyhe = expected yield strength of the steel tube (ksi), Es = elastic modulus of steel (ksi), P = axial load (kip), f ′ce = expected concrete strength (ksi), Ag = gross area of concrete (in.2), D = diameter of tube (in.), t = thickness of tube (in.), and ey = yield strain for steel (in./in.). Loss Analysis The PBSD combines the seismic hazard, structural, and damage analysis into a performance matrix that can be estimated into a loss metric. There are many loss metrics that can be used by, and that are important to, stakeholders and decision makers (discussed in detail in NCHRP Synthesis 440), but all these metrics can be boiled down to three main categories: deaths, dollars, and downtime. Bertero (2014) discussed earthquake lessons, in terms of loss, to be considered in both design and construction of buildings. At the beginning of 2010, two large earthquakes struck the Americas. The January 12, 2010, Haiti earthquake with a magnitude 7.0 produced about 300,000 deaths (second by the number of fatalities in world history after the 1556 Shaanxi, China earthquake). A month later, the February 27, 2010, Maule Chilean earthquake with a magnitude 8.8 (an energy release 500 times bigger than that from the Haiti earthquake) produced 500 deaths, most due to the resulting tsunami. However, the Chilean earthquake caused more than $30 billion of direct damage, left dozens of hospitals and thousands of schools nonoperational, and caused a general blackout for several hours, as well as the loss of service of essential communications facilities, crucial to take control of the chaotic after-earthquake situ- ation. Bertero (2014) compared the severity of both earthquakes and comments on their effects to life and the economy of the affected countries, as well as the features of the seismic codes or the absence of codes. An example of risk analysis with PBSD is utilized in Bensi et al. (2011), with the development of a Bayesian network (BN) methodology for performing infrastructure seismic risk assessment and providing decision support with an emphasis on immediate post-earthquake applications. A BN is a probabilistic graphical model that represents a set of random variables and their probabilistic dependencies. The proposed methodology consists of four major components: (1) a seismic demand model of ground motion intensity as a spatially distributed random field, accounting for multiple sources and including finite fault rupture and directivity effects; (2) a model for seismic performance of point-site and distributed components; (3) models of system performance as a function of component states; and (4) models of post-earthquake decision making for inspection and operation or shutdown of components. The use of the term Bayesian to describe this approach comes from the well-known Bayes rule, attributed to the 18th-century mathematician and philosopher Thomas Bayes: A B AB B B A B A( ) ( )( ) ( ) ( ) ( )= =Pr Pr Pr Pr Pr Pr (5) Pr(AB) is the probability of joint occurrence of Events A and B; Pr(A) is the marginal probability of Event A; Pr(A|B) is the conditional probability of Event A, given that Event B

26 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design has occurred; and Pr(B) is the marginal probability of Event B. The quantity Pr(B | A) is known as the likelihood of the observed Event B. Note that the probability of Event A appears on both sides of Equation 5. The Bayes rule describes how the probability of Event A changes given information gained about the occurrence of Event B. For discrete nodes, a conditional probability table is attached to each node that provides the conditional probability mass function (PMF) of the random variable represented by the node, given each of the mutually exclusive combinations of the states of its parents. For nodes without parents (e.g., X1 and X2 in Figure 12), known as root nodes, a marginal probability table is assigned. The joint PMF of all random variables X in the BN is constructed as the product of the conditional PMFs: (6) 1 p x p x Pa xi ii n∏( ) ( )( )= = Bensi et al. (2011) goes on to introduce BN models further and discusses how to incorporate BN-based seismic demand models into bridge design. The BN methodology is applied to modeling of random fields, construction of an approximate transformation matrix, and numer- ical investigation of approximation methods, including a discussion on the effect of correlation approximations on system reliability. Modeling component performance with BNs to capture seismic fragility of point-site components and distributed components, as well as modeling system performance of BNs with both qualitative and conventional methods, is explained. This reference goes on to identify efficient minimal link set (MLS), minimal cut set (MCS) formulations, optimal ordering of efficient MLS and MCS formulations, and heuristic augmen- tation that can be utilized with the BN methodology. Bensi et al. (2011) continues the PBSD process by addressing the owner decision-making process (see more discussion later in the report) and how to incorporate this model into that process. Two example problems are provided utilizing this methodology, including a California high-speed rail system that incorporates the bridge modeling into the example. Similarly, in Tehrani and Mitchell (2014), the seismic performance of 15 continuous four- span bridges with different arrangements of column heights and diameters was studied using incremental dynamic analysis (IDA). These bridges were designed using the Canadian Highway Bridge Design Code provisions (CSA 2006). The IDA procedure has been adopted by some guidelines to determine the seismic performance, collapse capacity, and fragility of buildings. Similar concepts can be used for the seismic assessment of bridges. Fragility curves can be devel- oped using the IDA results to predict the conditional probability that a certain damage state is exceeded at a given intensity measure value. Assuming that the IDA data are lognormally distributed, it is possible to develop the fragility curves at collapse (or any other damage state) by computing only the median collapse capacity and the logarithmic standard deviation of the IDA results for any given damage state. The fragility curves can then be analytically computed using Equation 7 as follows: ln ln (7)50% TOT P failure S x x S a a C( )( ) ( )= = Φ − β     where function F = cumulative normal distribution function, SCa 50% = median capacity determined from the IDA, and βTOT = total uncertainty caused by record-to-record variability, design requirements, test data, and structural modeling. Figure 12. A simple BN.

Literature Review and Synthesis 27 The seismic risk associated with exceeding different damage states in the columns, includ- ing yielding, cover spalling, bar buckling, and structural collapse (i.e., dynamic instability) was predicted. Some simplified equations were derived for Montreal, Quebec, Canada, to estimate the mean annual probability of exceeding different damage states in the columns using the IDA results. Repair and retrofit procedures are linked to loss predictions, as outlined in the FHWA’s retro- fitting manual (Buckle et al. 2006). Several chapters/articles address analysis, methodologies, effects, analytical tools, and costs for retrofit and repairs to mitigate damage or return a structure to a serviceable condition. Zimmerman et al. (2013) is one example, in which numerical techniques and seismic retrofit solutions for shear-critical reinforced concrete columns was investigated, utilizing test data of a reinforced concrete column with widely spaced transverse reinforcement. The study focused on the analysis method of nonlinear trusses and the retrofit option known as supplemental gravity columns, which is an example of how loss prediction and the analysis process are linked and should be iterated through PBSD. Organization-Specific Criteria for Bridges and Project-Specific Criteria NCHRP Synthesis 440 has two sections of criteria: organization-specific criteria for bridges and project-specific criteria. New information for both of these sections since NCHRP Synthesis 440 published is combined. The California DOT (Caltrans) Caltrans is currently updating their Seismic Design Criteria (SDC) to specify requirements to meet the performance goals for newly designed Ordinary Standard and Recovery Standard con- crete bridges. Nonstandard bridges require Project-Specific Seismic Design Criteria, in addition to the SDC, to address their nonstandard features. For both standard and nonstandard bridges, Caltrans is also categorizing their inventory in terms of Ordinary Bridges, Recovery Bridges, and Important Bridges. Some states have had issues with terms like Important or Essential, as a bridge is considered important to those that utilize each bridge. Caltrans is using these terms to correlate with loss analysis of an owner’s infrastructure and the time to reopen the bridge to support lifeline and recovery corridors. The bridge performance is also evaluated using a dual-seismic hazard; for Caltrans SDC they are listed as a Safety Evaluation Earthquake (SEE) for Ordinary Bridges. Both SEE and Functional Evaluation Earthquake (FEE) for Recovery Bridges are summarized in Table 6. Caltrans SDC revisions will also provide updates to the design parameters in Chapter 3 of the SDC and updates to both the analysis methods and displacement ductility demand values in Chapter 4 of the SDC. The adjustments to the displacement ductility demand values are revised to limit the bridge displacements beyond the initial yielding point of the ERE, specifically if a recovery standard bridge is being designed. The revisions to their SDC is an example of how PBSD is being gradually introduced as a better method of dealing with the hazards, soil–structure interaction, analysis tools, methodologies, material properties, damage states, performance, and loss. Similar revisions are being made to Seismic Design Specifications of Highway Bridges, as detailed in Japan Road Association (JRA) revisions in 2012. A synopsis of the revisions is provided in Kuwabara et al. (2013). The JRA specifications apply to Japanese road bridges and consist of five parts: Part I, Common; Part II, Steel Bridges; Part III, Concrete Bridges; Part IV, Substruc- tures; and Part V, Seismic Design. The revisions are based on improvements in terms of safety,

28 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design serviceability, and durability of bridges. Based on those lessons, design earthquake ground motions corresponding to the subduction-type earthquake were revised, and the requirements for easy and secure maintenance (inspection and repair works) for the bridges were clearly specified. JRA has clarified their performance of ERE conventionally reinforced columns for a dual-level (SPL 2 and SPL 3) seismic performance evaluation, as summarized in Table 7. The JRA 2012 revisions also address connection failures between reinforced concrete steel piles and the pile-supported spread footing to improve structural detailing and performance at the head of the piles. This is similar to research performed by the University of Washington, see Stephens et al. (2015) and Stephens et al. (2016) for both Caltrans and Washington State DOT, respectively, to evaluate capacity protecting this region and even considering the development of plastic hinges at these locations for combined hazard events or large lateral spreading and liquefaction occurrences. Caltrans also funded a study by Saini and Saiidi (2014) to address probabilistic seismic design of bridge columns using a probabilistic damage control approach and reliability analysis. Source: Caltrans. BRIDGE CATEGORY SEISMIC HAZARD EVALUATION LEVEL POST EARTHQUAKE DAMAGE STATE EXPECTED POST EARTHQUAKE SERVICE LEVEL Table 6. Caltrans draft proposed seismic design bridge performance criteria. SPL2 SPL3 Note: SPL1: Fully operational is required. Limit state of bridge is serviceability limit state. Negligible structural damage and nonstructural damage are allowed. Table 7. Seismic performance of bridge and limit states of conventionally reinforced concrete bridge column.

Literature Review and Synthesis 29 The probabilistic damage control approach uses the extent of lateral displacement nonlinearity defined by Damage Index (DI) to measure the performance of bridge columns. DI is a measure of damage from the lower measure of zero damage to the ultimate measure of a collapse mecha- nism for an element that has been subjected to base excitations. The performance objective was defined based on predefined apparent Damage States (DS), and the DS were correlated to DIs based on a previous study at the University of Nevada, Reno (Figure 13) (Vosooghi and Saiidi 2010). A statistical analysis of the demand damage index (DIL) was performed to develop fragility curves (load model) and to determine the reliability index for each DS. The results of the reliability analyses were analyzed, and a direct probabilistic damage control approach was developed to calibrate design DI to obtain a desired reliability index against failure. The calculated reliability indices and fragility curves showed that the proposed method could be effectively used in seismic design of new bridges, as well as in seismic assessment of existing bridges. The DS and DI are summarized with performance levels defined by Caltrans in Table 8, which shows the correlation between DS and DI. Figure 14 shows a fragility curve using lognormal distribution. Figure 15 shows both the fragility curves (upper two graphs) and reliability indices (lower two graphs) for four column bents (FCBs), with 4-foot diameter columns that are 30 feet in length in Site D for both the 1000 year and 2500 year seismic events. Note: O-ST = ordinary standard bridge, O-NST = ordinary nonstandard bridge, Rec. = recovery bridge, Imp. = important bridge, and NA = not applicable. Damage State (DS) Service to Public Service to Emergency Emergency Repair Design Damage Index (DI) Earthquake Levels (Years) Table 8. Design performance levels. DI P (D I { D S) Figure 13. Correlation between DS and DI.

30 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design Figure 14. Fragility curve. 100% 80% 60% 40% 20% 0% 0.00 0.20 0.40 0.60 0.80 1.00 P (D I L ) DIL 4.0 3.0 2.0 1.0 0.0 R el ia bi lit y In de x | D S DS3 DS4 DS5 DS6 Damage State (DS) 6.0 5.0 4.0 3.0 2.0 1.0 0.0 R el ia bi lit y In de x | D S DS3 DS4 DS5 DS6 Damage State (DS) (a) (b) (d)(c) 0.00 0.20 0.40 0.60 0.80 1.00 DIL 100% 80% 60% 40% 20% 0% P (D I L ) Figure 15. Fragility curves and reliability indices for FCBs with 4-foot columns in Site D. The Oregon DOT The Oregon DOT is developing a global plan for addressing resiliency in order to improve recovery for the next Cascadia Earthquake and Tsunami, using PBSD in terms of applying applicable hazards, identifying critical services, developing a comprehensive assessment of structures and systems, and updating public policies. The resilience goals are similar to those discussed at the beginning of this chapter, with the following statement: Oregon citizens will not only be protected from life-threatening physical harm, but because of risk reduction measures and pre-disaster planning, communities will recover more quickly and with less continuing vulnerability following a Cascadia subduction zone earthquake and tsunami.

Literature Review and Synthesis 31 Research has shown that the next great (magnitude 9.0) Cascadia subduction zone earth- quake is pending, as shown in Figure 16. This comparison of historical subduction zone earthquakes in northern California, Oregon, and Washington covers 10000 years of seismic history. The evidence of a pending event has made decision makers and the public take notice and put forth resources to develop strategies revolving around PBSD. Oregon’s performance-based features are modified from NCHRP Synthesis 440 to account for a third hazard condition: Cascadia Subduction Zone Earthquake (CSZE) in Oregon DOT’s Bridge Design and Drafting Manual—Section 1, Design (Oregon DOT 2016a; see also Oregon DOT 2016b). Design of new bridges on and west of US 97 references two levels of perfor- mance criteria: life safety and operational. Design of new bridges east of US 97 requires life safety criteria only. Seismic design criteria for life safety and operational criteria are described as follows. • “Life Safety” Criteria: Design all bridges for a 1,000-year return period earthquake (7 percent prob- ability of exceedance in 75 years) to meet the “Life Safety” criteria using the 2014 USGS Hazard Maps. The probabilistic hazard maps for an average return period of 1,000 years and 500 years are available at ODOT Bridge Section website, but not available on USGS website. To satisfy the “Life Safety” criteria, use Response Modification Factors from LRFD Table 3.10.7.1-1 using an importance category of “other.” • “Operational” Criteria: Design all bridges on and west of US 97 to remain “Operational” after a full rupture of Cascadia Subduction Zone Earthquake (CSZE). The full-rupture CSZE hazard maps are available at the ODOT Bridge Section website. To satisfy the “Operational” criteria, use Response Modification Factors from LRFD Table 3.10.7.1-1 using an importance category of “essential.” When requested in writing by a local agency, the “Operational” criteria for local bridges may be waived. The CSZE is a deterministic event, and a deterministic design response spectrum must be generated. To allow for consistency and efficiency in design for the CSZE, an application for generating the design response spectra has been developed by Portland State University (Nako et al. 2009). AASHTO guide specifications values for Table 3.4.2.3-1 are modified into two tables for (1) values of Site Factor, Fpga, at zero-period on the acceleration spectrum and (2) values of Site Factor, Fa, for short-period range of acceleration spectrum. Table 3.4.2.3-2 is replaced with values of Site Factor, Fv, for long-period range of acceleration spectrum. For seismic retrofit projects, the lower level ground motion is modified to be the CSZE with full rupture, as seen in Table 9. Performance levels, including performance level zero (PL0), are specified based on bridge importance and the anticipated service life (ASL) category required. Source: OSSPAC (2013). Figure 16. Cascadia earthquake timeline.

32 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design The South Carolina DOT South Carolina Department of Transportation (South Carolina DOT) has updated its geo- technical design manual (South Carolina DOT 2019). Chapters 12, 13, and 14 for geo technical seismic analysis, hazard, and design, respectively, have been updated to current practices and research, including incorporation of PBSD hazard prediction. South Carolina DOT is also updating their site coefficients to be more appropriate for South Carolina’s geologic and seismic conditions; see Andrus et al. (2014). Note that with the revisions, South Carolina DOT issued a design memorandum in November 2015 that revised the substructure unit quantitative damage criteria (maximum ductility demand) table (Table 7.1 of the SCDOT Seismic Design Specifications for Highway Bridges). See Table 10. The Utah DOT The Utah DOT and Brigham Young University (see Franke et al. 2014a, 2014b, 2015a, 2015b, 2015c, 2016) are researching the ability for engineers to apply the benefits of the full performance- based probabilistic earthquake analysis without requiring specialized software, training, or education. There is an emphasis on differences between deterministic and performance-based procedures for assessing liquefaction hazards and how the output can vary significantly with these two methodologies, especially in areas of low seismicity. Guidance is provided regarding when to use each of the two methodologies and how to bind the analysis effort. Additionally, a simplified performance-based procedure for assessment of liquefaction triggering using liquefaction loading maps was developed with this research. The components of this tool, as well as step-by-step procedures for the liquefaction initiation and lateral spread displacement models, are provided. The tool incorporates the simplified performance-based procedures determined with this research. National Highway Institute Marsh et al. (2014) referenced a manual for the National Highway Institute’s training course for engineers to understand displacement-based LRFD seismic analysis and design of bridges, which is offered through state agencies and open to industry engineers and geotechnical engi- neers. This course helps designers understand the principles behind both force-based AASHTO (AASHTO 2014) and displacement-based AASHTO (AASHTO 2011) methodologies, including a deeper understanding of what performance means in a seismic event. Other similar courses are also being offered to industry and are improving the understanding of practicing engineers. Federal Emergency Management Agency The Federal Emergency Management Agency (FEMA) has developed a series of design guidelines for seismic performance assessment of buildings and three of the five documents EARTHQUAKE GROUND MOTION BRIDGE IMPORTANCE and SERVICE LIFE CATEGORY Table 9. Modifications to minimum performance levels for retrofitted bridges.

Literature Review and Synthesis 33 are referenced in FEMA (2012a, 2012b, 2012c). A step-by-step methodology and explanation of implementation are provided for an intensity-based assessment and for a time-based assess- ment. The process of identifying and developing appropriate fragility curves is demonstrated. A software program called Performance Assessment Calculation Tool has also been developed with a user manual that is included in the FEMA documents to help engineers apply PBSD to the building industry. Japan Road Association The Japan Road Association (JRA) Design Specifications have been revised based on the performance-based design code concept in response to the international harmonization of design codes and the flexible employment of new structures and new construction methods. Figure 17 shows the code structure for seismic design using the JRA Design Specifications. The performance matrix is based on a two-level ground motion (Earthquakes 1 and 2), with the first one based on an interpolate-type earthquake and magnitude of around 8, and the second one with a magnitude of around 7 with a short distance to the structure. Kuwabara et al. (2013) outlined the incremental revisions from the JRA Design Specif i- cations between 2002 and 2012. These revisions include, but are not limited to, the ductility design method of reinforced concrete bridges, plastic hinge length equation, evaluation of hollow columns, and the introduction of high-strength steel reinforcement. Following the 2016 earthquake in Kumamoto, Japan, a new version of the JRA Design Specifications is in the works. Note: Analysis for FEE is not required for OC III bridges. Source: South Carolina DOT (2015). Design Earthquake Operational Classification (OC)Bridge Systems Table 10. South Carolina DOT substructure unit quantitative damage criteria (maximum ductility demand ld).

34 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design Identification of Knowledge Gaps The resources to develop guide specifications for PBSD are improving with examples such as the upcoming Seismic Design Criteria, Version 2 from Caltrans, which will address aspects of PBSD and the building industry’s efforts to develop practices in PBSD and tools for engineers and owners to collaborate on solutions based on performance criteria and expectations. There is still a perception that the bridge industry could better predict likely performance in large, damaging earthquakes than is being done at the present, and there are still gaps in that knowledge base that need to be closed. Most of the knowledge gaps listed in Marsh and Stringer (2013) are still applicable today; see Table 11. The technology readiness levels represent what has been developed and used; what research is done, ongoing, and being discussed; and what only exists in concept. Knowledge gaps certainly exist in all facets of PBSD; however, other key knowledge gaps beyond those listed in NCHRP Synthesis 440 (Marsh and Stringer 2013) that should be closed in order to improve the implementation of PBSD are covered. Objectives of Codes Mandated Specifications Overall Goals Functional Requirements (Basic Requirements) Performance Requirement Level Verification Methods and Acceptable Solutions Can be Modified or May be Selected with Necessary Verifications Importance, Loads, Design Ground Motion, Limit States Principles of Performance Verification Verifications of Seismic Performances (Static and Dynamic Verifications) Evaluation of Limit States of Members (RC and Steel Columns, Bearings, Foundations and Superstructure) Unseating Prevention Systems Principles of Seismic Design Figure 17. Code structure for seismic design using JRA design specifications. TRL Description 0-25 25-50 50-75 75-100 1 PBSD concept exists 2 Seismic hazard deployable 3 Structural analysis deployable 4 Damage analysis deployable 5 Loss analysis deployable 6 Owners willing and skilled in PBSD 7 Design guidelines 8 Demonstration projects 9 Proven effectiveness in earthquake Technology Readiness Level (TRL) % of Development Complete Table 11. Technology readiness levels for PBSD.

Literature Review and Synthesis 35 Gaps related to structural analysis can include minimum and expected properties for reinforcing greater than Grade 80, stainless steel, and other materials that can improve serviceability and in some conditions performance. Oregon DOT has been using stainless steel in their bridges located along the coastline and other highly corrosive environments to extend the service life of the bridge; however, many of these locations are also prone to large CSZE and the use of these materials in earthquake resisting elements is still being developed. In the State of Washington’s resiliency plan, outlined in Washington State Emergency Management Council–Seismic Safety Committee (2012), what is missing is a link between damage levels and return to service. This is a knowledge gap given what we know structurally and what this report is suggesting as a desired goal for post-earthquake recovery. Gaps related to decision makers can include bridge collapse. It is not intended that the PBSD guide specifications will address tsunami events, but the JRA specifications do address tsunami as well as landslide effects. Figures 18 and 19 are examples of these other types of failure systems and show the collapse of bridges caused by effects other than ground motion (Kuwabara et al. 2013). The decision to combine these types of effects with a seismic hazard, even combining liquefaction, down drag, and lateral spreading effects, needs additional clarification and is currently left up to the owner to assess implications of probability, safety, and cost ramifications. Liang and Lee (2013) summarized that in order to update the extreme event design limit states in the AASHTO 2014, combinations of all nonextreme and extreme loads need to be formulated on the same probability-based platform. Accounting for more than one-time variable load creates a complex situation, in which all of the possible load combinations, even many that are not needed for the purpose of bridge design, have to be determined. A formulation of a criterion to determine if a specific term is necessary to be included or rejected is described, and a comparison of the value of a given failure probability to the total pre-set permissible design failure probability can be chosen as this criterion. Figure 18. Collapse of bridge due to landslide. (Note: Reprinted courtesy of the National Institute of Standards and Technology, U.S. Department of Commerce. Not copyrightable in the United States). Source: Kuwabara et al. (2013).

36 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design While the seismic hazard definition was once thought to be relatively well understood, there is a growing knowledge gap related to the effect of rotation angle on intensity of ground motions and how the use of a geometric mean of the motions, or other methods of including the effect of rotation angle (RotDxx), should be incorporated into seismic design. This issue is not specific to PBSD; like all seismic design methods, PBSD is reliant on a full understanding of the hazard definition for proper implementation. The knowledge gaps identified in NCHRP Synthesis 440 are still applicable. Many of these knowledge gaps will become evident to both engineers and decision makers as the PBSD guidelines are developed. Overall, the baseline information to develop PBSD guide specifica- tions are in place. Industry’s end goal of understanding the relationship between risk-based decision making and design decisions and methodologies to meet performance goals is going to be an iterative process. Figure 19. Collapse of bridge due to tsunami. (Note: Reprinted courtesy of the National Institute of Standards and Technology, U.S. Department of Commerce. Not copyrightable in the United States). Source: Kuwabara et al. (2013).

Performance-based seismic design (PBSD) for infrastructure in the United States is a developing field, with new research, design, and repair technologies; definitions; and methodologies being advanced every year.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 949: Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design presents a methodology to analyze and determine the seismic capacity requirements of bridge elements expressed in terms of service and damage levels of bridges under a seismic hazard. The methodology is presented as proposed AASHTO guidelines for performance-based seismic bridge design with ground motion maps and detailed design examples illustrating the application of the proposed guidelines and maps.

Supplemental materials to the report include an Appendix A - SDOF Column Investigation Sample Calculations and Results and Appendix B - Hazard Comparison.

READ FREE ONLINE

Welcome to OpenBook!

You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

Do you want to take a quick tour of the OpenBook's features?

Show this book's table of contents , where you can jump to any chapter by name.

...or use these buttons to go back to the previous chapter or skip to the next one.

Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

To search the entire text of this book, type in your search term here and press Enter .

Share a link to this book page on your preferred social network or via email.

View our suggested citation for this chapter.

Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

Get Email Updates

Do you enjoy reading reports from the Academies online for free ? Sign up for email notifications and we'll let you know about new publications in your areas of interest when they're released.