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

This paper describes an exploratory study carried out to examine the effect of foundation rocking on the shake table response of a 1/4-scale, 2-column bridge bent. 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 3 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 designed to represent stiffness of spread footings supported on medium soil. 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. Data shows that for the same input earthquake intensity in the maximum steel bar strain near the base of the columns in an unretrofitted rocking footing was only 7% of the strain in the fixed frame. When the footing was retrofitted but was still allowed to rock, the maximum steel strain increased, but was still only 16% of the strain in the fixed frame. The retrofit design implications of the results are discussed.