northridge earthquake case study

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Northridge Case Study

Northridge: A Case Study of an Urban Earthquake

In the early morning hours of January 17, 1994, most residents of Los Angeles were still asleep. At 4:30 that morning, a major earthquake hit the area. The ground shook violently over a large region. Frightened residents ran into the streets. Soon fire trucks were racing to fires caused by broken gas mains. Rescue teams were struggling to reach people trapped in collapsed buildings. It was a scene of widespread chaos.

The magnitude 6.7 Northridge earthquake-with an epicenter just north of Los Angeles and lasting just 15 seconds-was one of the costliest natural disasters in U.S. history. Around 9000 people were injured, some 1600 of these seriously, and 57 people were killed. Over 22,000 people were displaced from their homes. The region suffered an estimated $40 billion in economic losses. Thousands of buildings were damaged or condemned. Nine hospitals were closed, with 2500 beds lost. Nine parking garages collapsed. Portions of eleven major roads were impassable. Sections of seven freeways collapsed and 170 bridges were damaged to some extent.

This case study of an “urban earthquake,” considers scientific observations related to the earthquake, the consequences of the earthquake, and why variations in earthquake hazard exist within a seismically active region such as Los Angeles, using the tools of a geographic information system (GIS). Students explore not just what scientists know about earthquake hazards but how they know what they know.

Investigations and Learning Goals

The case study has four components and learning foci:

Earthquakes and you

Students reflect on the impact a major earthquake could have on them. What if there were no electricity, water, or heat available for one or more days? How might you plan for it and respond to it? Before the earthquake

As a result of this investigation, students will be able to describe the tectonic setting and origin of seismicity in southern California. Students explore the tectonic setting within which the Northridge earthquake took place and begin to get a feel for why it occurred where it did. Why is southern California a seismically active region? What accounts for the pattern of seismicity? Where have large earthquakes like Northridge occurred historically? And what accounts for the complex network of active faulting that characterizes the region?

After considering these questions within a regional tectonic framework, they examine the concepts of earthquake hazard and associated risk. Then they develop a qualitative analysis of hazard and risk for the Los Angeles region before the Northridge earthquake, based on patterns of active faulting and historical seismicity up to that time. Later, after learning where the earthquake occurred, they examine the extent to which the earthquake’s location was consistent with their analysis.

The Earthquake

In this activity, students use triangulation to determine the location of the earthquake and explore the events that occurred in the hours and days following the earthquake. They examine the pattern of aftershocks and determine the strike and dip of the fault plane.

The Aftermath

The damage from the Northridge earthquake was widespread and in some areas severe. Some areas suffered almost total destruction, yet others had minimal damage. In this investigation, students explore the factors that influence the distribution and degree of damage. Using geologic, liquefaction zone and sedimentary thickness maps, along with peak ground acceleration (PGA), Modified Mercalli Intensity and data on the damage levels for over 25000 buildings, they begin to understand the relationships among PGA, geology and damage. As a closing activity, students use the many datasets they have explored to generate a revised estimate of zones of elevated earthquake hazard and risk and compare it to their original interpretation.

Target Learning Audience

The case study is designed for lower division undergraduate students in a geology, earth science, hazards or environmental science course. It could also be used by geoscience majors taking a structural geology, geophysics or hazards course. It assumes the students have been introduced to plate tectonics, earthquakes and faulting, and basic sedimentary depositional processes. No prior experience with a GIS is required, as all instructions are provided.

Teaching Tips

The case study can be completed in two full lab periods or a mix of homework time and one lab period, depending on how and when it is used in a course. The first activity (Earthquakes and you) could be used in class in a Think-Pair-Share activity at the beginning of a lecture on earthquakes. The complete case study could also be used as a semester project, with the students exploring pieces of the case study over the course of the semester or as a wrap up to the semester. The case study contains full instructions for the user, as we assume neither the instructor nor the student has prior experience with a GIS.

Download the Projects and Data for ArcGIS9 

https://www.dropbox.com/sh/bafs3zfbbasc3km/AABe9lo_ft6Y5LqVHep_EvhEa?dl=0  

The authors would greatly appreciate any feedback users have on how to effectively use the case study in a specific course and input on how to improve the materials overall. Send email to hall at scieds dot com

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20 Years After Northridge Quake, Buildings Remain Vulnerable

PASADENA, Calif. — Twenty years ago this week an earthquake struck Northridge, Calif., killing 57 people and revealing a serious defect in a common type of mid-rise building. A new study by U.S. Geological Survey and Caltech engineers, shows that these mid-rise buildings with fracture-prone welds in their steel frames are much more dangerous than they would be if they met current standards. The study also shows that buildings without the defect, for example those built after 1994, have varying levels of safety.

In the study released today, the authors used computer models of buildings to simulate how the structures would perform in moderate, strong, and very strong seismic ground motions. The very strong shaking would occur within 10 miles of the epicenter of an earthquake with magnitude greater than 7.2. The study used nearly 65,000 simulated ground motions produced by the Southern California Earthquake Center and USGS.

The simulations showed that during very strong ground motions, buildings with fracture-prone welds are substantially more likely to collapse than buildings with sound welds, and are much more likely to sustain irreparable damage, making them more likely to be a total loss in a major earthquake.

"We're not saying that every building constructed before 1994 is going to collapse in an earthquake," said Heaton, professor of geophysics and civil engineering at Caltech.  "We're saying that buildings continue to be in use that pose a greater risk of physical injury and financial harm than is necessary."

Most of the welds that failed in 1994 were located where vertical steel columns and horizontal steel beams connect, an important location in the structural system. These welds did not perform as expected in the earthquake. Instead of distributing a building's movement throughout the structural system as intended, the welds fractured and concentrated damage at the connections between vertical columns and horizontal beams.

"Our study can inform future benefit-cost assessments of buildings of this type relative to other buildings," said Anna Olsen, lead author and research civil engineer at the USGS at the time of the study. "The relative seismic safety of different buildings can be identified before the next earthquake so that an owner or occupant can take it into consideration when buying or leasing a building."

"Fracture-prone welds increase the probability that a building will sustain major damage or collapse in large or very strong seismic ground motions," said John Hall, professor of civil engineering at Caltech. "Even with sound welds, buildings of this type have different levels of safety. Our simulations show that, for example, a more flexible, lower strength design is more dangerous than a design with higher stiffness and strength."

 "The decisions that most significantly affect people during and after an earthquake are actually made before the ground starts shaking," Heaton said. "Individuals who choose to occupy or own a safer building before an earthquake avoid major injuries, financial loss, and business interruption after the earthquake."

The research paper, " Characterizing Ground Motions that Collapse Steel, Special Moment-Resisting Frames or Make Them Unrepairable ," by Anna H. Olsen, Thomas H. Heaton, and John F. Hall, was published in the journal "Earthquake Spectra," and was funded in part by a grant from the National Science Foundation to SCEC.

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Public Roads - Summer 1994

Date: Summer 1994 Issue No: Vol. 58 No. 1 Table of Contents

The Northridge Earthquake: Progress Made, Lessons Learned in Seismic-Resistant Bridge Design

Introduction: FHWA and the Challenge of Natural Hazard Mitigation

Our society--our way of life--depends on a complex network of infrastructure systems. These systems are lifelines that provide transportation and communication services, a supply of energy and fresh water, and the disposal of wastewater and waste products. Among the oldest of these lifelines are our transportation systems--highways, railroads, mass transit, ports, waterways, and airports.

The Federal Highway Administration (FHWA) has a vested interest in ensuring that the critical resource represented by the nation's roads and bridges is not undermined, threatened, or destroyed by natural hazards. To this end, it conducts, sponsors, or otherwise participates in extensive research to identify new technologies or new applications of existing technologies that will mitigate the effects of such natural hazards as flood, fire, windstorm, and earthquake. Specifically, this effort tries to determine how highway structures should be built or how they should be strengthened (retrofitted) to minimize the effects of natural hazards.

This research has paid off! Many valuable lessons have been--and continue to be--learned about how to build and retrofit better, stronger, more hazard-resistant roads and highways. Slowly and steadily, these lessons have been translated into practical technological applications. New highway structures replace the old; existing structures are strengthened through retrofitting. These new and strengthened structures are helping to avoid much of the worst damage and are precluding additional damage when new disasters strike. But it takes a long time to do research and apply technologies. Also--and unfortunately--this research is, of necessity, grounded in tragedy and destruction, since we learn from yesterday's failures.

Thus, when a disaster such as the Jan. 17, 1994, Northridge, Calif., earthquake occurs, the results are simultaneously: unfortunate--the lives lost, the destruction of property and infrastructure; positive--the enhanced performance of new and retrofitted infrastructure; and hopeful--the improvements the Northridge lessons will allow us to make as our knowledge base grows.

Mitigating Against Earthquakes

The hazard to bridges

Highway systems contain many elements--pavements, tunnels, slopes, embankments, retaining walls, etc.; however, the most vulnerable element in the highway system appears to be bridges.

There are about 575,000 bridges in the United States. About 60 percent of these were constructed before 1970 with little or no consideration given to seismic resistance. Historically, bridges have been vulnerable to earthquakes, sustaining damage to substructures and foundations and, in some cases, being completely destroyed. In 1964, nearly every bridge along the partially completed Cooper River Highway in Alaska was seriously damaged or destroyed. Seven years later, the San Fernando earthquake damaged more than 60 bridges on the Golden State Freeway in California. This earthquake cost the state approximately $100 million in bridge repairs. In 1989, the Loma Prieta earthquake in California damaged more than 80 bridges and caused more than 40 deaths in bridge-related collapses alone. The cost of the earthquake to transportation was $l.8 billion, of which the damage to state-owned bridges was about $300 million.

Approaches to improved seismic response

Much has been learned from these failures. Currently, two approaches are being taken to improve the seismic resistance of highway bridges. The first approach requires considerable time, but is economically reasonable. Design guidelines are upgraded as more knowledge is gained about the response of specialized transportation structures to seismic activity. These new design guidelines can be applied to new construction as older bridges that are either structurally unsound or functionally obsolete are removed from service.

The second approach involves identifying those existing bridges that are important to the network and are susceptible to significant damage or collapse in the event of an earthquake. These structures can then be strengthened or retrofitted to enhance their response to seismic activity. Seismic retrofitting is a relatively new concept in bridge engineering and was motivated by the damage sustained by highway bridges during the 1971 San Fernando earthquake. The earthquake clearly pointed out the existence of a number of deficiencies in the then-current bridge design specifications. It also focused on the fact that numerous existing bridges may be expected to fail in some major way during their remaining life if subjected to strong seismic loads. However, because of the difficulty and cost involved in strengthening an existing bridge to new design standards, it is usually not economically justifiable to do so. This second approach thus requires significant capital expenditure; it consequently can prove economically infeasible in many cases.

A balance between these two approaches is needed to strengthen the highway system against seismic attack. This balance can be accomplished by upgrading those structures that form vital links in the network and are vulnerable to damage, while at the same time imposing new applicable, geographically appropriate, seismic design standards on replacement bridges and new construction.

Case Study: The Northridge Earthquake

Damage was extensive to residential and commercial buildings and lifelines in the epicentral region. The main shock and aftershocks affected the built environment in an area of about 900 square kilometers (350 square miles). ( 1 ) In addition to Northridge, residents of Sylmar, Newhall, San Fernando, Burbank, Van Nuys, Glendale, and Santa Monica were affected. By earthquake standards, this magnitude 6.7 event was a moderate quake. By comparison, the 1964 Alaska earthquake had a magnitude of 8.1; the 1971 San Fernando had a 6.4; the 1987 Whittier Narrows had a 5.9; the 1989 Loma Prieta had a 7.1; and the 1991 Sierra Madre had a 5.8. Technicalities related to type of faulting, fault mechanisms, geology, and structural design considerations make it impossible to relate or compare damage between these events and thus indicate that the Richter magnitude, by itself, is a poor guide for quantifying the level of expected damage.

It is pointed out that all of the above earthquakes are considered to be moderate-to-large events, yet all fall far short of the expected "Big One." In fact, many seismologists believe that the "Big One" may not occur in California at all, but rather in the Midwest, the East, or on some other yet-to-be-defined fault system. The fault that ruptured under Northridge had not been identified by seismologists prior to the earthquake!

Bridge performance in the Northridge earthquake (1)

There were about 2,000 bridges in the epicentral region of the Northridge quake. Of these, only six bridges failed and four others were so badly damaged that they will have to be replaced. These failures did, however, create severe hardships for the traveling public, involving as it did some of the busiest freeways in the world, including the Santa Monica Freeway (Interstate Highway 10) and the Antelope Valley Freeway (state Route 14)-Golden State Freeway (I-5) interchange. The failure of those bridges was primarily due to the failure of the supporting columns that had been designed and constructed before 1971. The timing here is critical, for following the 1971 San Fernando earthquake, the standards for earthquake design began to be toughened considerably. However, two bridges--both constructed shortly after the 1971 earthquake--on the Simi Valley-San Fernando Valley Freeway had severe column distress that resulted in bridge failure.

Other damage to bridges included spalling and cracking of concrete abutments, spalling of column-cover concrete, settlement of bridge approaches, and tipping or displacement of both steel- and neoprene-type bearings. Slight shear cracking of column bents also occurred at the Marina-San Diego Freeway interchange.

On the other hand, several other bridges experienced relatively minor damage, yet those designed to current criteria performed as expected and met the intent and philosophy of the bridge specification of the American Association of State Highway and Transportation Officials (AASHTO). On those bridges, damage was visible and will be relatively easy to repair. There were, however, unforeseen circumstances. Even though the Balboa Street bridge over Route 118 was designed to current criteria, it incurred significant damage when a colocated water main burst and washed out the embankment, exposing most of the concrete piles that supported the abutment. Although this bridge performed well, it indicates that secondary effects from an earthquake can create major damage.

Retrofitting technologies--including the use of hinge or joint restrainers and column jacketing--performed very well. Although some restrainers failed, primarily by pulling through concrete bolsters, it is believed that none of these failures were the primary cause of span collapse, with perhaps the exception of the Gavin Canyon bridge on I-5. (See title page photo) That structure, however, was very highly skewed, which greatly increased hinge seat movement. Restrainers are not designed to carry the loads imposed by multiple spans once the structural integrity of intermediate columns are lost. This earthquake provided the first test of columns confined by steel jackets and none experienced failure.

As in the 1989 Loma Prieta earthquake, the implementation of hinge and joint restrainers is credited with preventing the collapse of many of the bridges in the epicentral region. This technology clearly represents one of the most cost-effective retrofit measures that can be implemented nationally, although use is not a guarantee that span collapse or damage can be avoided. However, restrainers will significantly reduce bridge damage in small-to-moderate quakes.

Observations

• Older bridges with unusual geometries and large skews respond to earthquakes in complex ways that were not accounted for when designed.  Most of the heavily damaged bridges had skewed decks or skewed column supports that tended to combine forces and amplify the structural response. This has been observed in numerous previous earthquakes.
• Newer structures designed to current specifications performed well.  The poorest performance overall was observed in bridges designed prior to 1971. The performance of those bridges designed between 1971 and 1981--a decade of evolution in bridge seismic design standards--was more mixed: some did well, some did not. Bridges designed after 1981, when newer seismic design standards were fairly well in place in California, generally performed quite well.
• Retrofitting improves earthquake resistance.  The retrofit techniques of joint restrainers, column jacketing, and foundation strengthening generally improved the performance of the affected bridges. Although retrofitting is not foolproof, it once again reduced structural failure and damage.
• The significance of high vertical accelerations needs further investigation.  The earthquake caused high vertical accelerations. Although these accelerations are not identified as the primary cause of damage, the phenomenon and the implications on structural performance need further study.
• Preparedness facilitates recovery.  A major element in mitigating the effects of this earthquake was the efficiency and alacrity with which California's personnel and contractors responded to the emergency. Many arrangements had been made by the California Department of Transportation (Caltrans) in advance; these included contracts with construction crews, instructions provided to inspecting engineers, and rapid identification of detour routes.

Recommendations

The Northridge quake resulted in the following recommendations:

• Conduct a thorough forensic study.  Much remains to be learned from the damage of the Northridge earthquake. Although major collapses are spectacular and attract the most attention, many other structures that are damaged to a much lesser extent can provide the lessons that will improve seismic design and retrofit. Opportunity exists to study the behavior of many other bridges in the epicentral region, most of which went unnoticed by the media.
• Examine flared column designs . A few bridges, some of those designed in the transitional period between 1971 and 1981, did not perform as well as those that were designed post-1981. Details must behave as assumed by the designer.

• Consider the need for combined horizontal and vertical loadings.  Bridges are designed well for static, vertical loads. However, if moderate-to-high vertical accelerations are coupled with moderate horizontal accelerations, structural damage and degradation could occur in smaller events.
• Investigate the use of isolation and/or energy absorption technology to reduce damage.  Newer technologies exist that can isolate or reduce forces on vulnerable bridge components such as columns. These devices detune or decouple the earth's motion from the structure. This technology can be implemented by replacing bearings that are known to be vulnerable to earthquakes with these protective devices.
• Eliminate bridge joints.  In addition to providing increased seismic resistance, the elimination of bridge joints in new or existing structures will reduce maintenance costs.
• Consider all components when evaluating a bridge for possible retrofit.  This includes its joints and/or bearings, columns, and foundation. When one element is strengthened or retrofitted--such as adding hinge restrainers--seismic loads (or displacements) are forced to the next weakest link, e.g., columns. If columns are strengthened, the next weakest link could be the foundation or the superstructure.
• Re-evaluate the retrofit prioritization scheme based on lessons learned from this event.  Some of the collapsed bridges on state Route 118 were not scheduled to be retrofitted under the current state program. Structure vulnerability ratings need to be revised for certain bridge types. ( 2 )
• Develop and train teams with damage evaluation experience prior to the disaster.  Experienced, technically balanced, post-disaster response teams make a difference in facilitating recovery. For example, Caltrans uses a two-person team comprised of a bridge designer and a maintenance engineer to quickly evaluate the severity of damage and make a rapid preliminary assessment of needs in order to reopen a bridge. Typically, hundreds of bridges will have to be evaluated following a moderate earthquake. Preparedness training for these teams is essential.

(1) All photographs used with this article illustrate the bridge performance and damage discussed in this section.

Lessons Learned from Earthquake Engineering Research

Designing a highway bridge to withstand large earthquake forces is a technically challenging and, until relatively recent times, daunting problem. However, recent earthquakes, coupled with FHWA and state-sponsored research efforts, have taught us much about bridge performance under these conditions.

Although it is virtually impossible to design or retrofit a bridge to be "earthquake-proof," a number of basic principles have been identified that, if followed, will improve the seismic performance of bridges and minimize the likelihood of structural collapse.

Bridge damage observed in recent earthquakes is generally attributed to one or more of the following:

• Approach slab failures and abutment damage due to abutment fill slumping from soil failure behind the abutment or permanent abutment movements.
• Collapsed or unseated girders due to bearing failures or inadequate seat widths.
• Column failures due to excessive shear or flexural demands from earthquake motions. (Inadequate capacities in reinforced-concrete columns are often due to insufficient confinement and poor anchorage and splice details.)
• Footing failures due to excessive flexural or shear demands. (Inadequate capacities in reinforced-concrete footings are often due to a lack of top reinforcing steel, poor footing-pile connection details, or inadequate bearing capacities.)
• Ground failure due to liquefaction or excessive soil deformations.

Recommendations for new design

The following are recommendations, based on past experience and research, for the seismic-resistant design of new or replacement highway bridges:

• Use approach slabs with positive ties to the abutment. This can provide continuity and minimize the effect of soil slumping behind the abutment.
• Use continuous spans rather than simply supported spans; this will reduce the need for expansion joints and thus minimize the potential for span separation. A side benefit of this practice will be a reduction in joint maintenance costs.
• Provide adequate seat widths for simply supported spans at piers, in-span joints, and abutments to prevent girders from becoming unseated.
• Design all bearings for simply supported spans for lateral seismic loads--that is, provide adequate strength in restrained directions. Check the stability of the bearing in the unrestrained direction at its maximum anticipated displacement.
• Provide adequate confinement for bridge columns by using either spiral reinforcement or transverse ties.
• Do not lap or anchor column longitudinal steel in the plastic hinge zones at the column-to-cap connection and/or the column-to-footing connection.
• Design footings to resist the full moment and shear demands transmitted from the column. Do not allow plastic hinging in the footings.
• Use earthquake-protective systems (e.g., seismic isolation), where appropriate, to minimize the seismic demand on bridge members.
• Use soil improvement technologies to reduce the potential for soil failure or liquefaction.

Recommendations for existing construction

The following are retrofitting recommendations. Note that some of the recommendations for new designs also can be applied to existing structures (e.g., using soil improvement technologies).

• Identify and rank those bridges in need of retrofit, based on structural vulnerabilities and socioeconomic considerations, using one of the many screening and prioritization schemes available.
• Either extend the seat width or add cable restrainers across the joint if seat widths are inadequate in order to prevent spans from becoming unseated at piers and in-span hinges. Check for adverse effects in columns and foundation.
• Consider bearing replacements if bearing failures could result in collapse or loss of function of the superstructure. Older steel rocker bearings with inadequate anchor bolts are known to be particularly vulnerable and should be replaced or strengthened.
• Eliminate expansion joints. This not only improves seismic performance, but also reduces maintenance costs. As an alternative to extending seat widths or adding cable restrainers, a number of simple spans could be made continuous by structural modifications (e.g., by casting a continuous deck slab with or without web connectors on the girders).
• Provide column jacketing for existing bridges in high seismic zones if there is inadequate confinement in the column (i.e., insufficient transverse hoops or ties) and/or there are splices or laps in hinge zones. In low-to-moderate seismic zones, consider column jacketing if these deficiencies exist and the bridge is judged to be important or essential. Check for adverse effects on other components.
• Provide footing overlays or extensions in high seismic zones if the column-footing connection is intended to be moment-resistant and there is a lack of top reinforcing steel, inadequate shear steel, or insufficient bearing capacity. In low-to-moderate seismic zones, consider footing retrofits if deficiencies exist and the bridge is judged to be important or essential.
• Strengthen cap beams to provide increased resistance to transverse flexure and shear through external concrete jacketing and/or prestressing. Joints between columns and cap beams can also be strengthened by external jacketing.
• Use seismic isolation technologies to retrofit bridges with short, stiff columns. Various isolation technologies exist and have demonstrated good seismic performance.

Conclusion: Past Research Has Paid Off, Continued Effort Is Needed

The Northridge earthquake showed us that we are indeed on the right track with regard to the development of effective seismic-resistant highway bridge design and retrofit procedures and technology.  New designs hold up; retrofitting works.  The evidence suggests, in fact, that had Caltrans had the time to complete its current retrofit program before the earthquake took place, many of the structural failures would not have occurred at all and much of the damage would have been minimized.

Some people believe that structures can or should be made earthquake-proof. Unfortunately, earthquake design and retrofit are still more of an art than a science. At this time, research and engineering have provided the tools to improve the seismic performance of bridges and minimize the liklihood of structural collapse. Until such time as we better understand the science, damage and failure will continue to occur--albeit at a reduced level.

Research in earthquake engineering is still needed. Far too many buildings and lifelines, including the transportation system, were damaged by the Northridge earthquake. Research programs--notably the FHWA Seismic Research Program being conducted by the National Center for Earthquake Engineering Research, the Caltrans research program, and the research programs of the other states--are expected to advance the state of the practice in bridge and highway engineering. These programs will provide improved tools to assess the vulnerability of highway systems and corresponding technologies to retrofit deficient systems in a cost-effective, timely, and efficient manner.

FHWA Earthquake Engineering Research

Yesterday....

The 1906 San Francisco earthquake, which caused millions of dollars in damage, was considered ill fortune--the city was rebuilt in an almost identical fashion. After the devastating Santa Barbara earthquake in 1925, however, engineers began to include earthquake design provisions in building codes. It took almost another 20 years for similar provisions to be included in highway bridge design. And it took another 30 years--in the aftermath of the 1971 San Fernando earthquake--for earthquake design criteria to be toughened and a seismic retrofit program to be instituted.

In 1971, FHWA began a modest $3-million, basic research program to develop national bridge seismic design guidelines. The study evaluated then-current criteria used for seismic design, reviewed recent seismic research findings for their potential use in a new specification, developed new and improved seismic design guidelines, and evaluated the impact of these guidelines on construction and cost. The guidelines were completed in 1979 and adopted by AASHTO as its  Guide Specification for Seismic Design of Highway Bridges  in 1983. This specification became the national standard in 1992, following the Loma Prieta earthquake.( 3 )

... and Today

FHWA's prominent role in earthquake research did not end with the adoption of this standard. The agency's commitment to mitigation of the highway-related effects of earthquakes was renewed with the establishment of a Seismic Research Program, mandated by the Intermodal Surface Transportation Efficiency Act of 1991 and conducted for FHWA by the National Center for Earthquake Engineering Research. The Seismic Research Program covers all major highway system components (bridges, tunnels, embankments, retaining structures, pavements, etc.). Its first product, however, deals with bridges.  Seismic Retrofitting Manual for Highway Bridges , which summarizes lessons learned from more than 20 years of earthquake engineering research and implementation and which provides procedures for evaluating and upgrading the seismic resistance of existing bridges, will be published this fall.

The FHWA Seismic Research Program is focusing research in four priority areas to improve the seismic performance of bridges:

• Improving methods for the seismic-resistant design of new bridges.
• Developing and improving techniques for vulnerability assessment and retrofit of existing structures.
• Developing assessment and analysis techniques for the seismic design and retrofit of bridge foundations.
• Developing preliminary guidelines for the seismic design and retrofit of long-span bridges.

The program's approach involves: (1) assimilating the large body of research work that has been, and is being, conducted in response to recent earthquakes, including the Northridge and 1989 Loma Prieta earthquakes, (2) undertaking physical testing where data are needed, and (3) supplementing data using analytical computer techniques to extrapolate information. The results will be used to update and clarify the AASHTO specifications for new bridge design, while parallel research is focusing on the development of nationally applicable seismic retrofit measures and guidelines. Thus, FHWA research will continue to lead the development of the next generation of national seismic design and retrofit technology.

( 1 ) Jack P. Moehle (editor).  Preliminary Report on the Seismological and Engineering Aspects of the January 17, 1994, Northridge Earthquake , Report No. UCB/EERC 94.01, University of California-Berkeley, January 1994.

( 2 )  Seismic Retrofitting Manual for Highway Bridges , Publication No. FHWA-RD-94-052, Federal Highway Administration, Washington, D.C., not yet published.

( 3 )  Standard Specifications for Highway Bridges , Fifteenth Edition, American Association of State Highways and Transportation Officials, Washington, D.C., 1992.

( 4 )  Seismic Design and Retrofitting Manual for Highway Bridges , Publication No. FHWA-IP-87-6, Federal Highway Administration, Washington, D.C., 1987.

( 5 )  Seismic Retrofitting Guidelines for Highway Bridges , Publication No. FHWA-RD-83-007, Federal Highway Administration, Washington, D.C., 1983.

( 6 ) M.J.N. Priestley, F. Seible, and C.M. Wang.  The Northridge Earthquake of January 17, 1994--Damage Analysis of Selected Freeway Bridges , Report No. SSRP-94/06, University of California-San Diego, February 1994.

James D. Cooper  is chief of the Structures Division, Office of Engineering and Highway Operations Research and Development at the FHWA's Turner-Fairbank Highway Research Center in McLean, Va. He received his bachelor's degree and master's degree in civil engineering from Syracuse University.

Ian M. Friedland  is assistant director for bridges and highways at the National Center for Earthquake Engineering Research. He received his bachelor's degree in civil engineering from Cornell University and his master's degree in structural engineering and structural mechanics from the University of Maryland.

Ian G. Buckle  is deputy director of the National Center for Earthquake Engineering Research and professor of civil engineering at the State University of New York-Buffalo. He received his undergraduate degree and doctorate in civil engineering from the University of Auckland, New Zealand.

Roland B. Nimis  is the regional structural engineer for the FHWA's Region 9 Office in San Francisco and is also currently serving as acting director of engineering. He received his bachelor's degree in civil engineering from California State University.

Nancy McMullin Bobb  is the division bridge engineer in the California Division of FHWA in Sacramento. She received her bachelor's degree in civil engineering from the University of Nevada-Reno and her master's degree in civil engineering from the University of California-Davis.

What's Changed Since the 1994 Northridge Earthquake

On jan. 17, 1994, a magnitude-6.7 earthquake revealed some of the safety issues that southern california needed to address, by john antczak, jonathan lloyd and conan nolan • published january 17, 2023 • updated on january 18, 2023 at 9:52 am, what to know.

• More than a quarter-century has passed since the Jan. 17, 1994 Northridge earthquake.
• The early morning earthquake was centered in SoCal's San Fernando Valley, but shaking was felt throughout the region.
• In the years since, the threat of earthquakes remains part of living in Southern California, but there have been significant changes in safety, research and technology.

Editor's Note: This article was originally published as part of NBC4's 25th anniversary coverage of the 1994 Northridge earthquake. It has been updated.

In the nearly three decades since the Jan. 17, 1994 Northridge earthquake , there has been a push toward progress -- sometimes frustratingly slow -- on everything from making buildings safer to increasing society's overall ability to deal with seismic threats.

Below, a look at some of the changes, lessons learned and safety efforts in the wake of one of the United States' worst natural disasters.

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The Great ShakeOut Dril l

In 2008, an annual earthquake drill known as the Great ShakeOut began in Southern California to teach the basic safety technique of "drop, cover and hold on." Initially based on a scenario of a magnitude 7.8 quake on the southern end of the mighty San Andreas fault, the drill has since spread across the United States and around the world.

Mandatory Retrofit Ordinance

In 2015, Los Angeles enacted a mandatory retrofit ordinance aimed at preventing loss of life in major earthquakes at the city's most vulnerable buildings. It covered about 13,500 "soft-story" buildings like Northridge Meadows and some 1,500 buildings with "non-ductile reinforced concrete" construction. The ordinance, however, allowed a process spanning seven years for retrofitting of soft-story buildings and 25 years for non-ductile reinforced concrete buildings.

Fault Model Evolution

Researchers are able to better document faults in the Los Angeles area -- what seismologist Dr. Lucy Jones called a "CAT scan" of the LA basin. The models allowed researchers to study the Oakridge fault, a larger fault that shows up in Ventura County.

"We could start seeing that Northridge wasn't just a localized little fault," Jones told NBC4. "Up until Northridge, we would have said you couldn't have that big an earthquake on a blind fault -- that if it's a big enough fault to give you a big earthquake, it has to come all the way through to the service. We had to revise that idea."

Mapping the 1994 Northridge Earthquake: Origin, Shaking and Damage

Northridge Earthquake: Daylight Reveals Damage From Collapse at 5 and 14 Freeways

Better freeway overpasses.

Major collapses Investment in overpass retrofits means those structures should hold up better in an earthquake. But the same isn't true for all bridges.

"We won't see a freeway collapse, probably," said Jones. "Caltrans has invested $10 million in retrofitting freeway bridges because of Loma Prieta and Northridge. We might very well see a collapse of county and city bridges because they haven't got any of that money."

Early Warning System

Last year, the U.S. Geological Survey announced its fledgling West Coast earthquake early warning system was ready for broad use by businesses, utilities, transportation systems and schools after years of development and testing of prototypes. The system detects the start of an earthquake and sends alerts that can give warnings ranging from several seconds to a minute before shaking arrives, depending on distance from the epicenter. That can be enough time to slow trains, stop industrial processes and allow students to scramble under desks.

"You can draw a line straight from Northridge to what we're doing with early warning," Jones said. "Because the system failed in Northridge, we got more money. Because of those problems, we got a chunk of money to improve those computers. We've sped it up to the point that sometimes the information will get to you before the shaking itself does, compared to months to get that information in 1995."

Northridge Earthquake: The First Day in Photos

Early warning app s.

Los Angeles unveiled a mobile app 25 years after Northridge that uses the early warning system to alert Los Angeles County residents when there is an earthquake of magnitude 5.0 or greater. In 2021, that original app was replaced by the MyShake alert app .

Other Digital Technology

In 2014, the Los Angeles Economic Development Corp. released a guide aimed at helping businesses minimize disruptions from major earthquakes, taking advantage of information technologies such as the digital cloud to keep a company working even if its physical systems are destroyed or inaccessible.

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Plan quality and mitigating damage from natural disasters: A case study of the northridge earthquake with planning policy considerations

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Research output : Contribution to journal › Article › peer-review

Planners have long believed as an article of faith that land use planning can reduce damage from natural hazards. After evaluating the relationship between the seismic safety elements of comprehensive plans prepared in the Los Angeles region of California and damage caused by the 1994 North-ridge earthquake, we provide evidence that this faith is not misplaced. The State of California requires every local government to include a seismic safety element in its comprehensive land use plan. The 1994 Northridge earthquake provided an opportunity to evaluate the extent to which the quality of state-mandated, locally prepared seismic safety elements reduce earthquake damage. We found that fewer homes were damaged when local governments had developed high-quality factual bases, formulated goals for improving seismic safety, crafted regulatory policies to manage development in hazardous areas, and advanced policies that made the public aware of seismic risks. We conclude that including a high-quality seismic safety element in land use plans can reduce property damage associated with seismic events. Our work has broad implications for land use planning.

ASJC Scopus subject areas

• Geography, Planning and Development
• Development
• Urban Studies

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• Policy Planning Business & Economics 100%
• Natural Disasters Business & Economics 94%
• Earthquake Business & Economics 85%
• natural disaster Earth & Environmental Sciences 79%
• Safety Business & Economics 71%
• Damage Business & Economics 69%
• damages Social Sciences 66%
• safety Earth & Environmental Sciences 55%

T1 - Plan quality and mitigating damage from natural disasters

T2 - A case study of the northridge earthquake with planning policy considerations

AU - Nelson, Arthur C.

AU - French, Steven P.

N1 - Funding Information: This article is based on research supported by National Science Foundation grant CMS-9416458 to the Georgia Institute of Technology. We would like to acknowledge the assistance of S. Muthukumar and Maureen M. Holland of the Georgia Institute of Technology who helped us assemble and manipulate the data. We are also grateful to three anonymous referees and to the editors and staff of the Journal of the American Planning Association. Views expressed do not necessarily reflect those of the National Science Foundation, the Georgia Institute of Technology, or the Georgia Tech Research Corporation.

N2 - Planners have long believed as an article of faith that land use planning can reduce damage from natural hazards. After evaluating the relationship between the seismic safety elements of comprehensive plans prepared in the Los Angeles region of California and damage caused by the 1994 North-ridge earthquake, we provide evidence that this faith is not misplaced. The State of California requires every local government to include a seismic safety element in its comprehensive land use plan. The 1994 Northridge earthquake provided an opportunity to evaluate the extent to which the quality of state-mandated, locally prepared seismic safety elements reduce earthquake damage. We found that fewer homes were damaged when local governments had developed high-quality factual bases, formulated goals for improving seismic safety, crafted regulatory policies to manage development in hazardous areas, and advanced policies that made the public aware of seismic risks. We conclude that including a high-quality seismic safety element in land use plans can reduce property damage associated with seismic events. Our work has broad implications for land use planning.

AB - Planners have long believed as an article of faith that land use planning can reduce damage from natural hazards. After evaluating the relationship between the seismic safety elements of comprehensive plans prepared in the Los Angeles region of California and damage caused by the 1994 North-ridge earthquake, we provide evidence that this faith is not misplaced. The State of California requires every local government to include a seismic safety element in its comprehensive land use plan. The 1994 Northridge earthquake provided an opportunity to evaluate the extent to which the quality of state-mandated, locally prepared seismic safety elements reduce earthquake damage. We found that fewer homes were damaged when local governments had developed high-quality factual bases, formulated goals for improving seismic safety, crafted regulatory policies to manage development in hazardous areas, and advanced policies that made the public aware of seismic risks. We conclude that including a high-quality seismic safety element in land use plans can reduce property damage associated with seismic events. Our work has broad implications for land use planning.

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UR - http://www.scopus.com/inward/citedby.url?scp=0036282749&partnerID=8YFLogxK

U2 - 10.1080/01944360208976265

DO - 10.1080/01944360208976265

M3 - Article

AN - SCOPUS:0036282749

SN - 0194-4363

JO - Journal of the American Planning Association

JF - Journal of the American Planning Association

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Improving the Performance of Steel Moment Frame Connections

WASHINGTON, DC - One of the critical lessons from the 1994 Northridge earthquake was the unacceptable performance of steel moment-resisting frame construction. In response to that performance, FEMA established the FEMA/SAC Steel Moment Resisting Frames Project. When the extent of the problem became known, the earthquake engineering community faced a crisis. The building code for this type of construction had effectively been invalidated, and there was little idea of how safe existing buildings were or how to repair damaged buildings. Since FEMA funds the repair of publicly owned buildings, this was a crisis for FEMA as well as for building owners. It also quickly became clear that this was not just a California problem but also a national problem. FEMA determined that the first need was for guidance on how to repair damaged buildings. With funds from the Congressionally-authorized NEHRP Northridge Research Fund, the work was completed in less than a year and its primary product, the Interim Guidelines for Steel Moment Resisting Frame Construction (FEMA-267), quickly became the de facto standard. To date, FEMA has distributed over 20,000 copies of the Guidelines.

FEMA then began the second phase of the project, an effort to study and develop final design criteria for the design and inspection of new construction and upgrading of existing buildings for use by the nation's model building codes and standards. The final products include technical guidance for new construction (FEMA 350), upgrade guidance for existing buildings (FEMA 351), evaluation and repair guidance for damaged buildings (FEMA 352), and a technical specifications and quality control guidance document (FEMA 353). FEMA also published non-technical guidance for building owners and local officials (FEMA 354) and a CD-ROM with all of the publications and a series of background reports (FEMA 355).

This groundbreaking initiative was the first FEMA, if not federal, effort to effectively combine the academic research world and the earthquake engineering design community on a scale never before attempted. As a result of this effort, the building codes and standards for the entire country have been revised to take into account project findings. The quality of steel moment frame construction has been significantly improved because of the project. Both the model code organizations and the industry standards group are now using the final design guidelines as the basis for the next update of their products. In fact, the American Institute for Steel Construction is now sponsoring training courses across the country using the FEMA publications and has distributed several thousand copies to date.

FEMA has been widely recognized for its role in organizing and leading the solution to a serious problem for the nation's building codes and standards. The steel industry, through the American Institute for Steel Construction, presented an award to the Director of FEMA for its role in resolving this complex problem.

Construction of the "Northridge" Earthquake in Los Angeles' English and Spanish Print Media:

Damage, attention, and skewed recovery, geop home | chico home, this document is maintained by: chr webmaster ([email protected]) last updated: 02/21/98.

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How Safe Are Pre-Northridge WSMFs? A Case Study of the SAC Los Angeles Nine-Story Building

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Bruce F. Maison , David Bonowitz; How Safe Are Pre-Northridge WSMFs? A Case Study of the SAC Los Angeles Nine-Story Building. Earthquake Spectra 1999;; 15 (4): 765–789. doi: https://doi.org/10.1193/1.1586071

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This paper demonstrates a procedure for modeling, analysis, and evaluation of existing steel frame buildings of the type damaged in the 1994 Northridge earthquake. The procedure accounts for Northridge data and incorporates post-Northridge research. It is distinguished from more conventional procedures by the use of fracturing connection elements with randomly assigned rotation capacities. The study confirms and quantifies a number of observations from Northridge. Damage patterns are highly variable, but their global effects are predictable. Many steel frame buildings can sustain substantial damage and still satisfy criteria for “safe” response. Expected performance, however, is measurably less reliable than intended performance, and this has important implications for public policy and performance-based engineering.

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Fracture Case Studies, Part 3

The previous two STRUCTURE magazine articles ( General Principles of Fatigue and Fracture, Part 1, August 2016 and AISC and Damage Tolerance Approaches, Part 2, November 2016 ), reviewed the fundamental principles of cracking and how to design for fatigue and fracture. This article presents three case studies that illustrate how an engineer can use this guidance to address project challenges. The intent of this article is to move from the theoretical to the practical, and demonstrate that there is a realistic place for the more developed methodologies of fatigue and fracture mitigation.

Northridge Earthquake

The 1994 Northridge earthquake had a tremendous impact on the American Institute of Steel Construction’s (AISC) steel code over the past 20 years. After the magnitude 6.7 (Mw) earthquake, inspectors discovered 1,300 fractured moment frame connections in 72 buildings. Naturally, this made many people uncomfortable.

To address the fracture issues, the SAC Steel Project studied material behavior, connection geometry, and construction practices to figure out what happened and why it happened. Results of the project are widely published and infused throughout current AISC Seismic provisions.

One of the questions that came up during the studies was the effect of the welding backup bar. Field erectors preferred leaving them in place because they take time and money to remove. However, they create an inherent notch in the joint. This section uses fracture mechanics to study the impact that leaving the backing bar in place has on joint behavior, and what happens when it is fully fillet welded to the beam flange.

Figure 1. Beam to column flange weld in Pre-Northridge moment connection.

In the first condition, the backing bar is tack welded to the column flange and fused to the beam as the weld is deposited, illustrated in Figure 1. Note how the backing bar and any lack of fusion at the weld root creates a crack. Using fracture mechanics, one can plot the stress intensity K I as a function of crack depth and far-field stress, shown in Figure 2. Using a fracture toughness of 50 MPa (m) 1/2 – a middle ground value – most of the stress intensities are greater for stresses in the yield range (250 MPa to 350 MPa). Even with twice the toughness, it still seems like a poor choice to leave the backing bar in place.

Figure 2. Tack welded stress intensity solution as a function of crack depth.

What happens when a continuous fillet weld is placed along the bar? Won’t that take care of the problem? There now exists an eccentric crack condition. Looking at Figure 3, notice that about half of the stress intensity values are higher than the assumed toughness. There may be an argument to allow this condition. However, considering the possibility of lower toughness, the certainty of constraint near the web intersection, and the potential for the crack to grow due to low-cycle fatigue, it also seems imprudent to leave the backing bar in place.

Figure 3. Fully welded stress intensity solution as a function of crack depth.

In the end, a joint where the backing bar is removed, with the weld root gouged out and rewelded, can perform orders of magnitude better than one that has a crack-like lack of fusion in it from the backing bar. This conclusion is born out not only by the analysis but also by a rational view of the problem.

Ammonia Tank

The question to answer, on a sizeable ammonia tank, is what stress corrosion cracks need to be repaired and which ones can be left alone. When steel is in contact with ammonia with very low oxygen content, cracks do not grow. However, cracks do grow in tanks when the ammonia is contaminated with air. The tank in question had been out of service for some time and had a number of stress corrosion cracks. The owner wanted to recommission the tank, and hence the project.

Utilizing API RP 579 Fitness for Service , the engineers on the project created crack ratio charts that let field crews know which cracks needed repair. Cracks under a certain size for a given aspect ratio, though detected, could remain in place.

The effort began by mining Charpy toughness data from material test reports. Using the master curve approach, the engineer correlated Charpy values to fracture toughness K 1 values. The correlations are a function of thickness and Charpy energy values. This provided one side of the equation – the other being the stress intensity factor.

Figure 4. Assumed crack geometry in the tank wall.

Utilizing this data, the engineer developed stress intensity solutions based on the basic crack geometry shown in Figure 4. These are from solutions in API 579. Selecting a crack length 2c , a crack depth a is calculated. Doing this for numerous crack lengths, the curves in Figure 5 and Figure 6 are generated. Where the crack depth is greater than the tank wall thickness (Figure 5), a leak-before-break condition exists. This approach is good because the tank will leak before rupturing. However, for lower toughness material, like in the weld or heat affected zone, a break-before-leak condition existed (Figure 6). This is of more concern, given the lack of warning before catastrophic failure.

Figure 5. Critical crack size, a leak-before-break condition in the tank wall.

Figure 6. Critical crack size, a break-before-leak condition in the weld.

This analysis tells two things. In the base metal, long, shallow cracks need to be repaired, as a break-before-leak condition exists for aspect ratios ( a/2c ) less than 0.5. In the weld base metal, all cracks of a given size need to be repaired. The engineer can decide what crack size, for a given aspect ratio, needs to be repaired by choosing an acceptable safety factor.

Finally, perhaps a third lesson: Not every crack is a problem and needs to be repaired.

Bridge Crane

The bridge crane in Figure 7 was one of the dozens in the area that were decommissioned over the years. It was about 100 years old and had experienced somewhere between 5 to 10 million fatigue cycles. The owner wanted to know if the structure was safe before investing in a major electrical upgrade.

Figure 7. Bridge crane with eyebar bottom chord and diagonal members.

The study looked at the member forces, AISC fatigue requirements, and non-destructive testing of the eyebars.

The force analysis did not identify any problems. The model results matched the Maxwell diagram in the original drawings. The fatigue analysis indicated stresses in most members below the threshold values in AISC of 4.5 ksi. A few members towards the middle of the truss had stresses near 10 ksi. They had failed at one point, causing the truss to lose over a foot of camber. Up to this point, nothing was of major concern. However, enter non-destructive testing (NDT).

Figure 8. Eyebar cracking.

Before any NDT testing occurred, ironworkers stripped the paint of some key joints and discovered cracks, visible to the naked eye, shown in Figure 8. The phased array ultrasonic and magnetic particle testing found cracks inside and at the surface of a substantial number of joints. The cracks ranged in size from 1/8 to 2½ inches long and 1/64 to 1/32 inches wide.

After lengthy discussions and a second engineering opinion, the owner elected to retire the truss – creating a serious operational challenge to the site. Given the size and extent of the cracks and difficulty in repairing eyebars, it was truly the only rational decision.

A key lesson to learn from the bridge crane is the importance of thorough inspection. The stress and fatigue analyses showed the bridge crane was in good shape. However, reality showed a very different picture, one that eventually saved lives.

In the end, the principles of damage tolerance can be applied to traditional civil engineering structures in a way that provides clarity to the cracks they may contain. These are rooted in fracture toughness testing, stress intensity factor solutions, fatigue testing, life correlations, and non-destructive testing. These case studies show the approach in utilizing some of these tools and the insight gained through their application. Greater application of these tools to civil engineering structures would lead to increased safety of the structures for which engineers are responsible.▪

About the author  ⁄  Paul W. McMullin, Ph.D., S.E.

Paul McMullin, Ph.D., S.E. is a Founding Partner at Ingenium Design in Salt Lake City. He is an Adjunct Professor and the lead editor of the Architect’s Guidebooks to Structures series. Paul can be reached at [email protected].

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