Under earthquake loading, bridge superstructures will in general experience both translational and rotational motions. The rotational motions arise from a number of effects, including shaking response of asymmetrical structures, wave passage effects, and ground failure including fault rupture at the bridge site. As a consequence, supporting columns may be subjected to torsion, which is often ignored in typical design practice. The degree to which torsion reduces bridge safety because of reduced lateral strength and, more importantly, because of reduced deformation capacity, is unclear.
A research program was designed to investigate whether torsional response of reinforced concrete (RC) bridge construction has an important effect on the strength and deformability of the supporting bridge columns. Five one-third scale RC bridge columns were tested in the laboratory under simulated seismic loading. Three circular columns and one oblong column were subjected to a compressive axial load and varying degrees of lateral and torsional loading, characterized by the imposed twist-to-drift ratio. Results are compared with results from tests of another column, reported previously, that was subjected to lateral displacement cycles without twist. It is observed that increasing the twist-to-drift ratio results in appreciable reductions in the lateral deformation capacity.
Analytical models are developed to understand the effect of twist on column response. In addition, the degree of anticipated twist occurring in actual bridge structures is investigated using analytical models of simplified bridge systems under a variety of loadings, including near-fault ground motions and ground motions across fault-rupture zones.
Design implications are addressed and a design procedure is proposed for enabling columns to achieve their intended deformation capacity under simultaneous lateral displacement and twisting deformations.
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