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.
- 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.
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
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.