Seismic Design of Columns of Short Bridges Accounting for Embankment Flexibility

Empirical observations and analysis of the recorded response of two California bridges indicate that the flexibility of the bridge embankments has a significant effect on the displacement demands sustained by the columns of short bridges under seismic excitation. Because the deck and abutments provide a capable load path, the columns only need to maintain gravity load support while accommodating the lateral displacement demands associated with the response of the embankments. This is in contrast to conventional R (or Z) factor approaches, which require the columns to provide substantial lateral resistance and, hence, lead to relatively large diameter columns that have limited displacement capacity. A displacement-based approach to the design of the columns is proposed. Displacement demand is assessed for one or multiple performance objectives using a simple extension of the “N2” model to address the nonlinear response of the embankments. Relative to conventional designs, the smaller diameter columns obtained with this approach have better seismic performance. The columns have a larger displacement capacity and sustain less damage because they have larger yield displacements. The columns also are much less vulnerable to shear failures because of their more slender aspect ratios. Normalized embankment capacity curves are provided, and their use is illustrated with an example.