International Concrete Abstracts Portal

The bridge investigated consists of two very different structure types joined by a voided pier at Bent 4 that serves as a central abutment for both. The final retrofit design strategy used some "standard" Caltrans retrofit techniques and incorporated some innovative and unique design elements. It is the unique elements of the " South" structure's retrofit that is the focus of this paper. The bearings at Abutment 1 and Bents 4 were replaced with Dynamic Isolation Bearings. The fixed bearing at Bents 2 and 3 were replaced with low friction Spherical Expansion Bearings. This system "Highly" isolate the superstructure from the Bents 2 and 3 eliminating a costly foundation retrofit. Additional structural modifications included four buttress walls attached to a pile supported grade beam at Bent 4 and modification to the bearing pedestals at Abutment 1. These served to resist the Forces transferred from the Dynamic Isolation Bearings. Also, steel frame to act as a seat extension for the slab spans and concrete blocks to prevent crushing of the slabs was installed. Construction was completed in 1995. The innovative techniques used in the seismic retrofit of this bridge, resulted in savings compared to a more costly conventional retrofit. It significantly reduces impact during construction to the high volume of traffic on both Route 242 and Route680 and will reduce the susceptibility to damage during an earthquake as compared to a conventional column strengthening retrofit.

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.

The partial collapse of the bridge superstructure due to the inadequate seat width of expansion joints at the Gavin Canyon Undercrossing during the 1994 Northridge earthquake was investigated through numerical simulation of the bridge motion under three-dimensional ground excitations. The bridge consists of five spans with two highly skewed expansion joints, at which restrainer cables were installed for the seismic retrofit purpose after the 1971 San Fernando earthquake. In order to compensate for the ineffectiveness of the cables in absorbing seismic energy, energy dissipation systems were introduced at the expansion joints in this study. The effectiveness of different energy dissipation systems including friction dampers and yielding steel plates in reducing the relative displacements at the expansion joints without significantly increasing the seismic forces on the bridge substructures is demonstrated.

Linear and non-linear analytical studies were conducted for evaluating the performance of the southbound separation and overhead bridge at the SR14/I5 interchange during the Northridge earthquake of January 17,1994. The analyses are focused on potential implications of the spatial variability of ground motion on the collapse of the structure. The influences of vertical ground motion, soil-structure interaction, non-linear contact effects at the expansion joints and abutments, are also examined. The parameter studies help to determine some of the causes of collapse and offer insight in the complex seismic behavior of long multi-span concrete bridges.

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.