Proportioning Substructure Columns of Short Bridges for Improved Seismic Performance

摘要

Empirical observations and analysis of the recorded response of two California bridges show that the flexibility of the bridge embankments has a significant effect on the displacement demands sustained by the substructure columns of short bridges responding to earthquake excitations. Computation indicates that the stiffness of the columns has little effect on peak displacement responses of short bridges, and bridges with flexible columns will transfer most of the lateral inertial force to the embankments via the deck and abutments. 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. This understanding leads one to select columns with smaller diameters than typically result using the conventional R (or Z) factor design approach. Several benefits result from the use of smaller diameter columns: (1) damage to the columns is reduced or avoided, (2) the columns are less vulnerable to shear failures, and (3) the reduced size of the columns and foundations reduces construction costs. This paper suggests a new displacement based approach for design of the substructure columns for improved seismic performance to meet the specified seismic performance objectives. Strategies to determine the number of columns are discussed for cases in which seismic loading is the governing load combination. A case study compares the dynamic response of a short bridge designed according to the conventional and proposed approaches. The comparison illustrates that although the bridges with small diameter columns may have slightly larger displacement demands compared to bridges with conventional large diameter columns, the curvature and displacement ductility demands on the small diameter columns are either similar to or smaller than those in conventional columns. Designs using conventional columns were found to violate the Caltrans requirement for target displacement ductility under even moderate ground shakings, while the smaller diameter columns satisfied this requirement for all the ground motions considered.