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 current seismic design philosophy allows nonlinear behavior of the structure to reduce strength requirements. This implies that the structure must be able to retain its integrity under cycles of displacement into the nonlinear range. The structure has to be tough. Evaluation of experimental results from 15 series of tests reported by various investigators has shown that the limiting drift capacity of a reinforced concrete column proportioned to resist earthquake effects through energy dissipation in the nonlinear range of response may be determined simply by considering two parameters: 1) the aspect ratio, and 2) the transverse reinforcement index.

This paper presents an analytical study on the seismic vulnerability of flared reinforced concrete bridge columns with structural flares. Four representative bridges, located in Northern Nevada, are included in the study. The flared columns in these bridges have similar architectural features and cross sectional dimensions but are reinforced with different steel arrangements. The "push-over" method of analysis is applied to predict the locations of plastic hinges in the columns and the corresponding shear demands under lateral earthquake loading material over strength on the ductility and the flexural capacity of different cross sections along the column height. It is shown that in parabolic structural flares the location of plastic hinges is strongly dependent on the longitudinal steel arrangement and that the shear capacity of flared columns without core reinforcement may be inadequate under lateral earthquake loading.

The damage to highway bridges during the recent California earthquakes has highlighted the need for improved seismic analysis and design procedures. This paper is a study f one such approach that aims to enhance the seismic performance of bridges. Chosen as an application of this approach is a 12-span viaduct that is part of the San Joaquin Hills Toll Roads project in Orange County, CA. Current California Department of Transportation (Caltrans) and AASHTO design criteria require bridges to be designed for a maximum credible earthquake to safeguard against collapse. Damage prevention under moderate earthquakes is not ensured. Thus, to provide a higher and more uniform level of safety and reliability, the design criteria for the toll bridges involved a two level design approach: one corresponding to a 72-year return period event and the other corresponding to a maximum credible level design event. Design forces and displacements resulting from the lower level event were used to ensure that no damage occurs as a result of this event. The forces and displacements resulting from the maximum credible analysis were used to ensure that the structure will not collapse. In the paper this two level approach is used to design the column steel based on strength criteria. Displacement ductility checks are then performed on the columns to ensure that they can undergo the inelastic deformations produced by the upper level spectrum loads. In 1992 when the bridges on the toll roads were designed, the two level approach was a relatively new design method. However, since the recommendations put forward by ATC-32(1) in 1996, the two level spectrum design approach is finding increased use in the design of bridges.

A series of five bridge columns with concrete hinges at their bases were tested under reversed cyclic loading to determine their behavior. A method for predicting the responses, which accounts for the effects of confinement on the concrete hinge region and yield penetration is presented. A simple retrofit technique for two-way hinges, using dry-pack grout, was investigated. A method for retrofitting one-way hinges by using an added reinforced concrete footing block, together with concrete in-fitted steel jacketing was tested. The reversed cyclic loading response, before and after retrofit are compared and means of both designing the retrofit and predicting the responses are presented.