EFFECT OF CAP BEAM TO COLUMN INERTIA RATIO ON TRANSVERSE SEISMIC RESPONSE OF MULTI COLUMN BRIDGE BENTS

摘要

In seismic design of bridge bents typically plastic deformations are allowed to occur at bridge column ends while the rest of bridge components remain essentially elastic due to maintenance and retrofit concerns. Based on observations of failures at past major earthquakes, CALTRANS bridge seismic code is structured in such a way that a ductile seismic response is preferred over a more rigid and brittle seismic response that targets to eliminate or minimize shear failures at bridge columns. Cap beam, expected to remain in essentially elastic range, can indirectly affect displacement ductility capacity of these bents. However, there exists considerable amount of examples of bridge bents having stumpy and rigid columns and flexible cap beam. In such a case, development of flexural plastic hinges takes place at cap beam and columns can experience shear failure before occurrence of flexural plastic hinges, showing a brittle mode of bridge failure. To investigate the consequences of such conditions, two different multi-column bridge bent examples are studied in transverse direction of bridge through pushover analysis. Bridge bent with a flexible cap beam, which is designed according to current design philosophy, has lower displacement ductility capacity due to the greater yield displacement capacity of bent. Pushover analysis showed that bridges having bents with stronger columns and weaker cap beam can display poor seismic behavior due to shear failure of columns or localizing inelastic region only at cap beam through formation of plastic hinges. Damaged cap beams can also risk unseating of superstructure.