Title: Shake Table Studies of Effects of Foundation Flexibility on Seismic Demand in Substandard Bridge Piers

Author(s): M. Saiidi, B. Gopalakrishnan, and R. Siddharthan

Publication: Symposium Paper

Volume: 209

Issue:

Appears on pages(s): 553-570

Keywords: bearing pad; bridge; earthquake; footing flexibility; pier; rocking; shake table; steel strain

DOI: 10.14359/12520

Date: 9/26/2002

Abstract:
An exploratory study of the effect of foundation rocking on the shake table response of a quarter-scale two-column bridge bent was conducted. Several innovative details and designs were used in the columns and the cap beam but the footing represented typical substandard and retrofitted spread footings. The structure was tested in three modes: rocking with as-built footings, rocking with retrofitted footings, and fixed base. The foundation flexibility was modeled in the tests using elastomeric bearing pads that were designed to represent the stiffness of spread footings supported on medium soil. The rocking tests were conducted in the elastic range, but the fixed-base model was tested to failure. The Sylmar record of the 1994 Northridge earthquake was simulated on the shake table. The data showed that for the same input earthquake intensity the maximum steel bar strain near the base of the columns in an unretrofitted rocking footing was only 7 percent of the strain in the fixed kame. When the footing was retrofitted but was still allowed to rock, the maximum steel strain increased but was still only 16 percent of the strain in the fixed kame. The retrofit design implications of the results are discussed.