Comment les Ponts Survivent aux Tremblements de Terre

Bridge Seismic Challenges

Bridges face seismic demands that differ significantly from those on buildings. While buildings typically experience earthquake forces as inertia of the structure itself, bridges must also accommodate differential ground motion — the fact that the ground at one end of a long bridge may move quite differently from the ground at the other end during a seismic event. Bridges have relatively simple structural configurations compared to buildings, but they serve critical functions in disaster response: collapsed bridges cut off access for emergency vehicles and severely impede community recovery.

The 1994 Northridge earthquake collapsed portions of several major freeways in the Los Angeles area, including the I-10 Santa Monica Freeway at La Cienega and the I-5/SR-14 interchange. These collapses disrupted traffic patterns for months and cost hundreds of millions of dollars to repair, demonstrating dramatically that infrastructure continuity is a fundamental component of earthquake resilience. The earlier 1989 Loma Prieta earthquake collapsed a 50-foot section of the Bay Bridge's upper deck and the Cypress Street Viaduct, shutting down the Bay Area's primary cross-bay link for weeks.

Bridge Types and Their Vulnerabilities

The most common bridge type in North America is the reinforced concrete column-supported bridge, with prestressed concrete or steel I-beam spans resting on concrete bents (column and cap beam assemblies) and abutments. This type's seismic vulnerability depends primarily on the ductility of the concrete columns and the adequacy of seat width at supports — the horizontal distance between the beam end and the edge of the support surface.

Non-ductile concrete columns, built before 1971 design improvements, can fail in brittle shear or flexure-shear modes without providing the ductile deformation capacity needed to absorb seismic energy. These columns — characterized by widely spaced spiral reinforcement that provides inadequate confinement — failed in the 1971 San Fernando, 1989 Loma Prieta, and 1994 Northridge earthquakes, dropping spans onto vehicles below. The Seismic RetrofitStrengthening an existing building to improve its earthquake resistance. Common methods include adding steel bracing, reinforcing foundations, and bolting structures to foundations. of California's non-ductile bridge columns became a massive state program following Loma Prieta, encasing columns in steel or concrete jackets that provide the missing confinement.

Bearing seat width is critical because earthquake forces push bridge superstructures longitudinally and transversely, potentially sliding spans off their supports. This "unseating" failure mode dropped numerous spans in the 1971 San Fernando earthquake when seats were as little as 6 inches wide. Modern designs require much larger seat widths — 24 inches or more — and include restrainer cables or devices that limit relative movement between spans and bents.

Seismic Design Approaches for Bridges

Modern bridge Seismic DesignThe practice of designing structures to withstand earthquake forces. Modern seismic design aims to prevent collapse and protect life, while accepting some structural damage in major earthquakes. in the United States follows AASHTO LRFD Bridge Design Specifications, which incorporate seismic provisions based on hazard level, bridge importance, and structural type. The fundamental design philosophy — similar to building design — accepts that bridges designed for a large earthquake may sustain repairable damage but must not collapse. For critical bridges (emergency routes, major economic links), higher performance objectives requiring immediate post-earthquake usability are specified.

Ductile concrete columns are achieved through closely spaced spiral reinforcement that confines the concrete core, preventing explosive failure under combined compression and bending. Properly designed ductile columns can sustain drift ratios of 4-6% while maintaining vertical load capacity — far exceeding the deformation demands of most earthquakes. The column design is deliberately intended to yield and dissipate energy while protecting the superstructure and foundations.

Isolation Seismic DesignThe practice of designing structures to withstand earthquake forces. Modern seismic design aims to prevent collapse and protect life, while accepting some structural damage in major earthquakes. is widely applied to bridges, particularly for retrofitting existing non-ductile structures. Friction pendulum bearings, lead-rubber bearings, and elastomeric isolation bearings replace conventional expansion bearings, shifting the bridge's fundamental period to the isolation period and dramatically reducing force demands on columns and foundations. Bridge Base IsolationAn earthquake engineering technique that decouples a building from ground motion using flexible bearings at the foundation. Reduces forces transmitted to the structure by 75-90%. has been applied to hundreds of structures in California, Japan, and New Zealand, with demonstrated success in multiple subsequent earthquakes.

Cable-Stayed and Suspension Bridges

Major cable-supported bridges — suspension bridges, cable-stayed bridges — present unique seismic challenges due to their enormous spans, complex dynamics, and multiple supported modes of vibration. The towers of a large suspension bridge may oscillate in multiple modes simultaneously, with the cable system coupling vertical, lateral, and torsional responses in complex ways. Wind and seismic demands interact, requiring combined analysis that bridge designers call "aeroelastic" modeling.

The Seismic DamperA device installed in buildings to absorb and dissipate earthquake energy, reducing structural movement. Types include viscous dampers, friction dampers, and tuned mass dampers. concept is widely applied to cable-supported bridges. Viscous dampers connect the bridge deck to towers or anchorages, limiting longitudinal deck motion during earthquakes. Cable dampers control stay cable vibration induced by both wind and seismic loading. The Tsurumi Tsubasa Bridge in Japan, completed in 1994, incorporated viscous fluid dampers in its cable-stay anchorages specifically for seismic response control.

The New Bay Bridge (San Francisco-Oakland Bay Bridge eastern span), completed in 2013, incorporates state-of-the-art seismic design for its self-anchored suspension span. Shear link fuses — yielding steel elements designed to fail at specified force levels — protect the main structure from excessive force by sacrificially absorbing energy. The design anticipates and accommodates the bridge experiencing a major earthquake during its service life, with the shear links replaceable after a damaging event.

Seismic Isolation for Bridges

Base IsolationAn earthquake engineering technique that decouples a building from ground motion using flexible bearings at the foundation. Reduces forces transmitted to the structure by 75-90%. has proven particularly effective for bridges because bridge weight and geometry typically allow straightforward installation of isolation bearings at the span-to-bent support interface. Replacing conventional expansion bearings with isolation devices can be accomplished during bridge rehabilitation without altering the bridge's appearance or function. The cost premium for isolation bearings over conventional bearings is modest, particularly when the alternative is column jacketing to add ductility.

The Bolu Viaduct in Turkey, a major highway bridge across a valley, experienced the 1999 Duzce earthquake while under construction. Portions with installed isolation bearings survived with minimal damage; portions still using conventional fixed bearings sustained severe column damage. This natural experiment dramatically demonstrated isolation's effectiveness and accelerated adoption of isolation for Turkish highway bridges.

Seismic DamperA device installed in buildings to absorb and dissipate earthquake energy, reducing structural movement. Types include viscous dampers, friction dampers, and tuned mass dampers. systems are used on long-span bridges where isolation alone is insufficient and where controlling deck displacement is essential for maintaining traffic usability. Viscous dampers connecting the deck to towers limit peak displacement while allowing the gradual thermal movement that bridges must accommodate. The damper force-velocity relationship can be tuned to provide large damping forces for the rapid earthquake-induced displacements while offering minimal resistance to slow thermal movements.

Post-Earthquake Assessment and Rapid Retrofitting

After a major earthquake, rapidly assessing bridge safety determines whether transportation networks can support emergency response. California, Japan, and New Zealand have all developed rapid bridge assessment protocols that allow trained inspectors to make open/closed decisions based on visual inspection within hours of an earthquake. These protocols prioritize the visual indicators most predictive of imminent collapse risk — visible column damage, bearing failures, settlement, and deck misalignment.

The California Department of Transportation (Caltrans) completed a systematic bridge seismic retrofit program following the 1994 Northridge earthquake, adding column jackets, seat extenders, and isolation bearings to over 2,000 bridges over 20 years at a cost exceeding $10 billion. This program substantially reduced the state's bridge seismic risk and provided a model for systematic infrastructure retrofit that other states and countries have followed.