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Publication Number:      Date:  Summer 1994
Issue No: Vol. 58 No. 1
Date: Summer 1994


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

James D. Cooper, Ian M. Fiedland, Ian G. Buckle, Ronald B. Nimis and Nancy McMullin Bobb

The Northridge Earthquake

At 4:31 a.m. PST on Monday, Jan. 17, 1994, the ground shook for approximately 20 seconds in the Northridge section of the San Fernando Valley in Los Angeles, Calif. The earthquake had a Richter magnitude of 6.7. Its epicentral region was the same area that had been rocked during the 1971 San Fernando earthquake. Fifty-seven people lost their lives as a result of the Northridge quake.

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.

Crushed column supporting Santa Monica freeway.Crushed column supporting Santa Monica freeway.

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.

Collapsed connector structure at I-5 and SR14 interchange.Collapsed connector structure at I-5 and SR14 interchange.

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.

Collapsed section of westbound Santa Monica freeway.Collapsed section of westbound Santa Monica freeway.

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)

One of two collapsed connectors at the I-5 and SR-14 interchange.

One of two collapsed connectors at the I-5 and SR-14 interchange.

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.

Portion of demolished Santa Monica Freeway reveals typical box girder construction.

Portion of demolished Santa Monica Freeway reveals typical box girder construction.

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.

Failed columns allowed superstructure to drop although it did not collapse.Failed columns allowed superstructure to drop although it did not collapse.

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.

Use of flared column tops exhibited unexpectedly poor performance.

Use of flared column tops exhibited unexpectedly poor performance.

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.


Column's concrete core failed due to failure of circular confinement steel.Column's concrete core failed due to failure of circular confinement steel.


The Northridge quake resulted in the following recommendations:

Columns retrofitted with steel jackets exhibited no structural damage.
Columns retrofitted with steel jackets exhibited no structural damage.

(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:

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:

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).

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


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:

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.




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