The Route 14/Interstate 5 Separation and Overhead bridge, a curved ten-span structural concrete structure, partially collapsed in the 1994 Northridge earthquake. The primary objective of this study is to ascertain the cause of failure by comparing estimates of the capacities and demands of various components in the bridge. A secondary objective is to examine earthquake modeling and analysis recommendations for highway bridges. As part of the examination, nonlinear static analysis (pushover analysis) is used to determine the capacity of a frame. Linearized analyses are compared with nonlinear dynamic analysis results to evaluate the capability of simpler models to predict maximum earthquake displacement demands.
To simulate the earthquake response of the bridge, a three-dimensional nonlinear model was developed using the DRAIN-3DX computer program. A suite of four recorded and two simulated ground motion records were used for the time history analysis, assuming uniform free-field ground motion. The earthquake analysis provided estimates of the force and deformation demands of components. The demands were compared to the capacity of the piers, superstructure, and intermediate hinges to determine which component initiated the partial collapse of the bridge.
The demand-capacity comparison shows that shear failure of pier 2 in a brittle-ductile mode was the most likely cause of the collapse. Based on the analysis, pier 3 reached its shear capacity shortly after the time at which pier 2 reached capacity. The analysis indicated that there may have been minor yielding in the pier shafts below ground. The negative bending moment in the box girder over pier 3 nearly reached the flexural capacity or had started to yield at the time piers 2 and 3 reached their shear capacity. The displacement at intermediate hinge 4 was much less than the hinge seat width; it is unlikely that hinge unseating precipitated the collapse. The conclusions about the cause of the partial collapse of the bridge are consistent with the observed damage after the earthquake.
The three-dimensional model of the bridge was used to investigate the expected behavior of the bridge assuming seismic retrofit. For the model of the hypothetically retrofitted bridge, the maximum drift angle demands were 4% for piers 2 and 3 and approximately 2% for the other piers. The maximum curvature ductility demand, occurring at pier 2, is approximately 10. Had the bridge been retrofit, it would have experienced minor to moderate damage in plastic hinge zones of several piers. The analyses indicate the bridge would have been functional after the earthquake. These analyses also show that the vertical component of certain near-source ground motions can have a large effect on some of the structural response quantities, particularly column axial load and superstructure bending moments. The displacement response of the nonlinear model is compared with three-dimensional linear “compression” and “tension” models typically used in seismic design of bridges. The comparison indicates that the compression model adequately represents the displacement demands on the bridge.
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