Bridge Earthquake Response: Engineering Bridges to Survive Seismic Events

The Loma Prieta earthquake struck the San Francisco Bay Area at 5:04 PM on October 17, 1989, as rush hour traffic filled the region’s bridges and freeways. On the Cypress Street Viaduct in Oakland, a double-deck freeway structure that had been built on unstable fill soil, the upper deck collapsed onto the lower deck, crushing vehicles between them. Forty-two people died in that single structure. Across the Bay Bridge, a section of the upper deck collapsed onto the lower deck when an eyebar in the suspension span failed. The earthquake, which measured 6.9 on the Richter scale, exposed the vulnerability of bridges designed before modern seismic engineering principles were understood.

Bridges are essential infrastructure that must remain functional after earthquakes to allow emergency response and community recovery. Yet bridges are inherently vulnerable to seismic forces because they are long, narrow structures that cross variable ground conditions. Understanding how bridges respond to earthquakes — and how to design them to survive — is one of the most important challenges in structural engineering.

How Earthquakes Damage Bridges

Ground Shaking

The primary cause of earthquake damage to bridges is ground shaking. The seismic waves that travel through the earth cause the ground to move in multiple directions simultaneously. The bridge, which is anchored to the ground at its foundations but free to move in its superstructure, experiences inertial forces that can exceed the strength of its components.

The structural collapse investigation methods applied to building failures are equally relevant to bridge earthquake damage analysis.

Soil Liquefaction

Soil liquefaction occurs when saturated sandy soil loses its strength during earthquake shaking and behaves like a liquid. Liquefied soil cannot support foundations, causing bridges to settle, tilt, or collapse. The 1995 Kobe earthquake caused extensive liquefaction that destroyed port facilities and damaged bridge foundations throughout the city.

Fault Rupture

Bridges that cross active faults may be subjected to direct fault rupture — the ground on one side of the fault moves relative to the ground on the other side. Fault rupture can shift bridge foundations apart, buckle the superstructure, and cause collapse.

Pounding

Adjacent bridge segments can pound against each other during earthquakes if expansion joints are not designed to accommodate seismic movements. Pounding can damage bearing supports, cause spalling of concrete, and lead to unseating of bridge decks.

Seismic Design Principles

Ductility

Ductility is the ability of a structure to deform without losing strength. Ductile structures can absorb seismic energy through controlled deformation rather than brittle failure. Steel reinforcement in concrete bridges provides ductility, allowing the structure to crack and deform while continuing to support loads.

Capacity Design

Capacity design ensures that the structure fails in a predictable and controlled manner. Certain components, called ductile fuses, are designed to yield and deform during an earthquake, protecting more critical components from overload. The dam failure analysis concept of progressive failure is inverted in capacity design — the goal is to control the failure sequence.

Redundancy

Redundant structures have multiple load paths so that if one component fails, others can carry the load. Bridges with multiple columns, continuous spans, and robust diaphragms can redistribute loads after damage.

Seismic Protection Technologies

Base Isolation

Base isolation decouples the bridge superstructure from ground motion by placing flexible bearings between the bridge and its foundations. The isolators allow the bridge to move relative to the ground during an earthquake, reducing the seismic forces transmitted to the structure. Base-isolated bridges have performed exceptionally well in major earthquakes.

Dampers

Seismic dampers absorb energy and reduce bridge response. Viscous dampers, similar to automobile shock absorbers, dissipate energy through fluid friction. Metallic dampers use the plastic deformation of steel to absorb energy. Friction dampers dissipate energy through sliding friction at specially designed interfaces.

Restrainers

Cable restrainers and shear keys prevent bridge spans from becoming unseated during earthquakes. These devices limit relative movement between the superstructure and substructure, keeping the bridge deck supported on its bearings even during extreme seismic events.

Major Earthquake Bridge Failures

San Francisco Bay Area, 1989

The Loma Prieta earthquake caused the collapse of the Cypress Street Viaduct and damage to the San Francisco-Oakland Bay Bridge. The failures led to a comprehensive retrofit program for California’s bridges and the replacement of vulnerable structures.

Kobe, Japan, 1995

The Great Hanshin Earthquake destroyed the Hanshin Expressway, collapsing multiple spans of elevated highway. The failures were caused by brittle column failures in structures designed to outdated seismic codes. The disaster led to a major revision of Japanese seismic design standards.

FAQ

Can bridges be designed to survive any earthquake?

Bridges can be designed to survive the maximum credible earthquake at their location, but the cost of designing for the most extreme events must be balanced against the probability of occurrence. Modern seismic codes aim to prevent collapse in the largest earthquakes while allowing repairable damage in more frequent events.

How are existing bridges made earthquake-resistant?

Existing bridges can be seismically retrofitted by adding base isolators, dampers, column jackets, and restrainers. Retrofit programs in California, Japan, and other seismically active regions have upgraded thousands of bridges to modern standards.

What is a seismic gap?

A seismic gap is the space provided between adjacent bridge segments to prevent pounding during earthquakes. The gap must be large enough to accommodate the expected relative displacement but small enough to prevent the deck from becoming unseated.

Why do some bridges fail while nearby bridges survive?

Bridge performance depends on design, construction quality, foundation conditions, and the specific characteristics of the ground motion at each location. Differences in any of these factors can cause dramatically different performance in the same earthquake.