This study investigated the seismic response of the westbound lanes of I-90 crossing Mercer Slough. Mercer Slough is filled with a very soft, thick peat deposit. Both linear and nonlinear dynamic analyses were performed, including spectral analyses, linear time-history analyses and nonlinear time-history analyses. Variables considered in the analyses included different column and foundation stiffnesses, different seismic input, different simultaneous seismic input, and nonlinear joint behavior.
The response of the bridge was found to be extremely sensitive to seismic input and, to a lesser extent, foundation stiffness. Consideration of nonlinear effects tended to lessen the bridge response. The analyses also indicated that a long, loosely connected bridge, such as that crossing Mercer Slough, can be adequately analyzed using a fairly short section of the bridge.
All of the different analyses indicated that elements in the bridge would probably be close to or exceed their capacity during an earthquake. Problem areas which were identified included the inability of the expansion joints to sustain large relative displacements and the possible overloading of the columns in flexure.