In recognition of the serious potential destructive effects of earthquakes, AASHTO specifications contain comprehensive provisions for seismic design. Although earlier specifications contained some provisions, the more comprehensive provisions were not adopted until the 1980s. They were based on a detailed study by consultants who were specialists in that field, with review and participation by bridge engineers and design firms. The standards developed apply to conventional steel and concrete girder and box girder construction with spans up to 500 ft, but do not cover suspension, cable — stayed, arch-type, and movable bridges.
Bridges and components designed to the AASHTO seismic provisions may suffer damage under severe seismic events, but should have a low probability of collapse due to ground shaking. The general philosophy adopted in the development was
• Small to moderate events should be resisted elastically without significant damage.
• Realistic seismic ground motion intensities should be used in design.
• Large events should not cause bridge collapse, and damage that occurs should be
readily detectable and repairable.
Seismic performance categories are assigned on the basis of a ground acceleration coefficient for the site determined from a contour map of the United States, and an importance classification of “critical,” “essential,” or “other.” Different degrees of design complexity are specified, depending on the seismic performance category. Each bridge is assigned to one of four seismic zones, and one of four different site coefficients is applied to approximate the effects of the site conditions (soil profile) on the response. Lateral forces and displacements may be determined from a single-mode spectral analysis, a multimode spectral analysis, or more rigorous procedures. Elastic response is assumed in the analysis, but forces are adjusted with response modification factors. The lateral forces are applied in orthogonal directions in combination to account for the directional uncertainty of earthquake motions. An important requirement specifies the minimum length of the bearing seat supporting the expansion ends of girders, determined as a function of the span length and the height of the supporting columns. Foundation design is also treated.
Seismic retrofit is a major consideration for older structures, particularly in the western United States. Serious distress and collapse of some bridges in California during the Loma Prieta (1989) and Northridge (1994) earthquakes received wide publicity. However, the problem structures were generally those designed and constructed to earlier standards. Where bridges were built according to modern methods, problems were minimal. Problems included failure of reinforced-concrete rigid-frame supports, failure of reinforced-concrete columns, columns punching through decks, and collapse of a structural steel span where the longitudinal displacement was excessive.
Active programs are in place to retrofit older structures to current criteria, but it is a massive undertaking that requires several years to accomplish. Some of the techniques being applied include (1) increasing the length of the seats for the bearings to provide a greater tolerance for longitudinal displacements, (2) adding cable restraints and hold-down devices at supports and hinges to restrict excessive movement and keep members in place, (3) adding spiral reinforcing steel and steel jackets or composite overwraps to strengthen concrete column piers, (4) replacing obsolete bearings with energy-dissipating types having lead cores or shock absorbers, and (5) adding foundation tie-down rods inserted into holes drilled into the soil. In the Northridge earthquake, several structures that had recently been retrofitted survived intact, giving confidence to the retrofit program. Retrofitting is not limited to the west but is under way in other parts of the United States as well.
