The Earthquake Engineering Online Archive

Behavior and failure analysis of a multiple-frame highway bridge in the 1994 Northridge earthquake

Fenves, Gregory L.; Ellery, Michael

PEER-1998/08, Pacific Earthquake Engineering Research Center, University of California, Berkeley, 1998-12, 169 pages (400/P33/98-08)

The Route 14/Interstate 5 Separation and Overhead bridge, a curved ten-span concrete structure, partially collapsed in the 1994 Northridge earthquake. This study examined the cause of failure by comparing estimates of the capacities and demands of components in the bridge. Nonlinear static analysis (pushover analysis) is used to determine the capacity of a frame. Nonlinear dynamic analysis results are used to evaluate the earthquake response. 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 was 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 piers 2 and 3 in a brittle-ductile mode was the likely cause of the collapse. The 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 failure. The three-dimensional model of the bridge was used to investigate the behavior of the bridge if it had been seismically retrofit prior to the earthquake. If the bridge had been retrofit, the analysis shows it would have experienced minor to moderate damage in plastic hinge zones of several piers. The analyses indicate the bridge would have been functional (able to carry emergency traffic) after the earthquake. The displacements from the nonlinear model are compared with linear models typically used in seismic design. The linear compression provides a good estimate of the displacement demands.

Available online: http://peer.berkeley.edu/publications/peer_reports/reports_1998/9808.pdf