Assessment of the current US seismic displacement requirements for bridges in a low-moderate seismic zone

Current US seismic guidelines indicate that ground acceleration of 0.10 g or greater is likely to produce plastic structural response, yet they require that an elastic seismic analysis be performed for all multi-span bridges located in regions deemed likely to sustain such ground acceleration. The relative superstructure-substructure displacements obtained from an elastic analysis of typical bridges located in a low-moderate seismic zone are usually in the range of 2.5-10 cm. The design code presents an empirical formula which approximates the upper limit of displacements due to plastic response. This formula typically gives displacements in the range of 37.5-60 cm. Since the requirement of the use of the empirical formula is predicted on the assumption of plastic response, an assessment of its validity is warranted. The primary objectives of this paper are to determine if plastic response is a real possibility for earthquakes of low to moderate magnitude (i.e. acceleration coefficient, A=0.10-0.20g) and further to examine if the empirical displacement formula is suitable for bridges in those seismic zones.