International Concrete Abstracts Portal

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 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 17 papers presented cover a wide array of subjects related to seismic behavior of concrete bridge structures. Topics include the impact of recent earthquakes on the seismic design process, results of recent experimental research on behavior of bridge components during earthquakes, case studies of bridge seismic upgrades, and the use of composite materials and seismic isolation systems to enhance seismic performance. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP187

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.