Last reviewed 30 Sept 2026 · 5 min read
Three problems with one root
Earthquakes, wind and traffic all produce dynamic or repeated load. The static design for dead and live load is not enough; the bridge must have strength, ductility and durability under these effects.
Seismic effects
Design philosophy
Bridges are designed so that:
- Under minor earthquakes they stay elastic with no damage.
- Under the design earthquake they may be damaged, but the damage is repairable and limited to designated locations — usually plastic hinges in the piers, not in the superstructure or foundations.
- Under a maximum considered earthquake they do not collapse, so that lives are safe and emergency traffic is possible.
The superstructure is kept elastic and the foundations are protected; the piers, which are cheap to inspect and repair, are the designated energy-dissipating parts. This is the idea of capacity design.
The design horizontal seismic force is obtained from the response spectrum of the site:
= zone factor; = importance factor (higher for major and lifeline bridges); = response reduction factor (larger for more ductile systems); = spectral acceleration coefficient at the fundamental period of the structure; = weight (dead load plus a fraction of live load). The fundamental period of a pier–superstructure system is ( = lateral stiffness) — a stiffer pier gives a shorter period and larger force.
Ductility and detailing of piers
- Plastic hinge region at the base of a pier, confined by closely spaced hoops or spirals so the concrete core keeps its strength and the longitudinal bars do not buckle.
- Longitudinal steel limited between minimum and maximum; lap splices placed outside the hinge region.
- Capacity design of shear: the shear strength is designed for the shear that can develop when the hinge reaches its over-strength moment so that a brittle shear failure cannot occur before flexural yielding.
- Foundation designed for the over-strength capacity of the pier.
Preventing unseating and other damage
- Seismic restrainers and stoppers — shear keys or cable restrainers stop the girder from moving off the bearings.
- Adequate seat length at the support, greater than the calculated relative displacement.
- Link slabs and shock-transmission units connect adjacent spans.
- Liquefaction check for foundations in saturated loose sand.
Base isolation and dampers
Seismic isolation bearings (lead-rubber, high-damping rubber, friction-pendulum) lengthen the period of the bridge and add damping, so the seismic force on piers falls sharply. Viscous dampers and tuned mass dampers absorb energy. Isolation is favoured for important, stiff bridges on short piers.