Title: Lessons Learned from Bridge Performance during Northridge Earthquake
Author(s): F. Seible and M. J. N. Priestley
Publication: Symposium Paper
Appears on pages(s): 29-56
Keywords: bridges; design; earthquake; flexural capacity; retrofit; seismic
The January 17,1994, Northbridge earthquake taught the bridge engineering community invaluable lessons concerning vintage-based bridge damage, performance of retrofitted bridge structures, and design considerations for a new bridges. As a result of the Northridge earthquake, seven bridges collapsed or were damaged beyond repair; of the seven, three were designed and built prior to the San Fernando (1971) earthquake, two were designed before 1971 but construction completed after the 1971 event, and two bridges were designed and built a few years after the San Fernando earthquake. Many other bridges in the strongly shaken region sustained repairable damage. In all cases the damage could be explained based on simple rational assessment models. All but one of the seven bridges collapsed due to column failure, based on inadequate shear design, lack of flexural capacity and/or buckling of the compression reinforcement. One bridge failed by unseating of highly skew superstructure movement joints. All bridges retrofitted since 1989 (when the Caltrans seismic retro fit program included substructures) performed without signs of damage. The analytical assessment of six of the seven collapsed structures showed that available column retrofit technology could have prevented the observed failures. However, the key lessons learned form the Northridge earthquake, over and above the proper member detailing, are in the systems response understanding. By taking movement joints out of the collapse path of the bridge by avoiding highly skew geometries, and by assigning equal demands to substructure assemblages to take advantage of ductility and redundancy, reliable seismic performance can be achieved. The encountered problems, lessons learned, and new bridge systems design approaches are discussed.