International Concrete Abstracts Portal

Almost nine years have passed since the disastrous Loma Prieta earthquake of October 17, 1989 and eight years have passed since the Governor's Board of Inquiry into the cause of highway structure failures during that earthquake issued its final report with the warning title" Competing Against Time". The California Department of Transportation has developed improved Seismic Performance Criteria, Seismic Design Specifications, seismic design procedures, and construction detail based on lessons learned from the 1971 San Fernando earthquake and subsequent seismic events. The success of the Bridge Seismic Design and Retrofit program and the success of future seismic design for California bridges is based, to a large degree, on an unprecedented accelerated and "problem-focused" seismic research program. The Department has spent over $40 million on this research and physical testing of details. This research has provided the bridge design community the assurance that the new specifications and design details perform reliably and meet the performance criteria. Caltrans staff engineers, consulting firms, independent Peer Review Teams, and university researchers have cooperated in this program of Bridge Seismic Design and Retrofit Strengthening to meet the challenge presented in the June, 1990 Board of Inquiry report. The eight year old Seismic Advisory Board has been an invaluable asset in reviewing the performance criteria, design specifications, design procedures, and construction details for both new design and retrofit strengthening of older, non-ductile bridges.

The Northridge Earthquake, January 17, 1994, caused partial collapse of seven freeway bridges, and damage to 230 others. Vertical accelerations, failed hinge restrainers, and column flare behavior were cited as causes of collapse and major damage. It is true that these factors, plus a few others, contributed to the collapse and damage can be summarized as insufficient ductility in the bridge structural frames. Furthermore, the bridge elements which failed in a non-ductile fashion behaved predictably. The details which led to the failures are no longer used in new bridge designs. These brittle details are converted to ductile ones in retrofit designs. This paper will investigate the undesirable bridge behavior observed in the Northridge Earthquake, and discuss how most of these issues are already covered in new bridge design codes. Finally, procedures adopted or proposed to correct the remaining issues and improve bridge behavior in a more reliable manner, as compared to undesirable Northridge Earthquake bridge response, will be outlined.

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.

The January 17, 1995 Hyogoken-Nanbu earthquake severely damaged highway bridges throughout the Kobe area. Most of the damaged bridges were of older construction (20 to 30 years old), but some modern bridges were also damaged. This appear reports on the seismic response of three representative bridges with reinforced concrete (RC) substructures. Two of these bridges had RC superstructures, and one had a steel superstructure. Reasons for the damage are discussed, and lessons which may be applicable to RC bridge design in the United States are outlined.

The characteristics of 1994 Northridge motions with respect to permanent slope deformations are consistent with much larger world-wide database of excitations. The paper describes the application of a recently developed procedure to compute abutment fill settlements caused by earthquakes to predict fill settlements. The study used 58 excitations from 1994 Northbridge earthquake recorded in the free field with acceleration higher than .1g to investigate the influence of vertical acceleration on permanent abutment fill settlements. It was found that the influence of vertical accelerations is insignificant. The comparison between predicted and observed abutment fill settlements aft4r the 1994 Northbridge earthquake using data from 18 bridge sites near the epicentral area has been reported. The study indicates that the existing computational model yields conservative estimates and it may be used as a first approximation for evaluating abutment fill settlements in moderate to large earthquakes.