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Figure 3 summarizes the static push-over curves that result from the constitutive models adopted in this
study. Each push-over curve extends to the range of deformation that the corresponding element
experiences during the strong (Case 1) and moderately strong (Case 2) earthquake loading. All
substructure elements except the isolation bearings exhibit a nearly elastic behavior. This observation is in
agreement with observations from the studies on highway overcrossings, such as the Meloland Road and
the Painter Street Overcrossings that have been shaken by strong earthquakes (Werner et al. 1987,
McCallen and Romstad 1994, Goel and Chopra 1997). Even the center bent that shares a large fraction of
the horizontal inertial loading behaves nearly elastic. This finding is in agreement with recent design
practice adopted by Caltrans.
514
Below East Abutment (.r or y)
ilar for West Abutment)
0.06 0.08 0.1
Displacement (m)
Figure 3. Summary of force-displacement (push-over) curves of various substructure elements of
interest m this study. Each curve extends to the range of deformation that the corresponding element
experiences. Case 1: strong earthquake shaking; Case 2: moderately strong earthquake.
SEISMIC RESPONSE ANALYSIS
Figure 4 presents a stick model idealization of the 91/5 Overcrossing. The bridge superstructure is
modeled by beam elements with a massless rigid link at each end that preserves the skewed geometry of
the bridge deck and serves as the connecting elements between bridge deck and the abutment. The
elastomeric bearings are replaced by the bilinear models that will produce the same hysteretic loop as a
sliding bearing. The fluid dampers are replaced by nonlinear dashpot elements and are arranged in a way
exact as the layout during the construction. The elastomeric bearings serve as the connecting element
between deck and abutment while the abutment is supported by "springs" and "dashpots" that will replace
the presence of embankment and pile foundation under the abutment footing. The detailed information on
the values of these spring and dashpot constants can be found in the thesis by Zhang (2002).
Because of the proximity of the bridge to active faults, eleven strong ground motions that have been
recorded in California relatively close to the fault of major earthquakes are selected for the simulation.
The bridge is subjected to the recorded free-field accelerations at the center bent and the amplified
accelerations at end abutments along the transverse and longitudinal directions simultaneously. In general
the fault-normal component is applied to the transverse direction while the fault-parallel component is
applied to the longitudinal direction.