Seismic Evaluation and Retrofit Techniques for Concrete Bridges (ACI 341.3R-07)

Description

Performance of bridges in past earthquakes indicates that existing bridge structures can be susceptible to severe structural damage. This vulnerability is evident in regions of high seismic risk, as demonstrated by extensive damage in bridge structures in the 1971 San Fernando Earthquake(Fung et al. 1971), the 1989 Loma Prieta Earthquake (EERI1989) and the 1994 Northridge Earthquake (Moehle 1995).In those earthquakes, damage included pounding at expansionjoints, severe spalling and cracking in bridge columns andjoints, and structural collapse.

The 2001 Nisqually Earthquake in the state of Washington resulted in damage to columns,restrainers, and the superstructure due to pounding, indi-cating that some bridges in the United States may besusceptible to damage even in moderate earthquakes (Ranfand Eberhard 2002).The bridge damage resulting from the San Fernando earth-quake caused concern about the seismic vulnerability ofbridges and initiated research into and development ofseismic retrofit guidelines and measures (Applied TechnologyCouncil (ATC) 1983; Zelinski 1985; Buckle et al. 1986;Selna et al. 1989a,b).

These earlier guidelines and proceduresfor seismic retrofit of bridges used strength-based evaluationapproaches in which the forces were used as a basis for theevaluation. If the seismic force demand exceeds the elasticstrength of the structure, the structural system may besubjected to large inelastic displacements and subsequentstrength degradation, instability, or both, of the system thatcould lead to structural collapse. In this case, retrofitmeasures solely based on a strength-based approach may notprovide adequate deformation capacity to ensure structuralstability.

Damage to bridges in the Loma Prieta andNorthridge earthquakes emphasized the need to address bothstrength and deformation capacities in bridge seismic retrofitprograms, which has resulted in more comprehensiveseismic retrofit prioritization schemes as well as improvedevaluation procedures and retrofit measure.