The Northridge earthquake: progress made, lessons learned in seismic-resistant bridge design

Public Roads, Summer, 1994 by James D. Cooper, Ian M. Friedland, Ian G. Buckle, Roland B. Nimis, Nancy McMullin Bobb

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.

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 $1.8 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.