MCEER HIGHWAY PROJECT
Task C1-1: State-of-Practice Survey and Strawman Seismic Design and Retrofit Manual for Long Span Bridges
Subject Area: Special
Bridges - Design and Retrofit Manual
|Principal Investigator(s) and Institution(s)
Charles Seim, T Y Lin International
Many long-span bridges or unusual bridges of long length or tall piers or movable superstructures have been constructed in the United States in earthquake-prone areas over the years. These bridges are generally characterized by being designed to codes that did not require the high level of seismic inputs or ductile design that current bridge design specifications now require.
These bridges are also characterized as being important structures on long established roadways that have become vital links in the commerce of the region they serve. The closure of the roadway due to seismic damage to the bridge would have a major impact on emergency services following an earthquake as well as impacting the flow of traffic for commercial goods during the repair period.
The 1989 Loma Prieta, 1994 Northridge, and 1995 Kobe earthquakes have given the bridge engineering profession an understanding of the large accelerations and displacements that bridges undergo during earthquakes. This improved understanding also extends to structural analysis and seismic retrofit methods. A number of long-span bridges have been seismically retrofitted in high seismic zones such as the Pacific Coast region and the New Madrid Fault area, as well as lower seismic areas such as New York. In addition, new bridges are being designed and constructed in these areas.
This work has resulting in the creation of a database of long span bridge and unusual bridge seismic retrofit measures of existing bridges and seismic design methods of new bridges that can be drawn on and developed into a Long Span and Unusual Bridge Retrofit Manual. Some of these seismic retrofit and design measures may be untested. The details in the manual must be developed in conjunction with a laboratory testing program to gain a better understating of their behavior and performance under dynamic loading.
There are two main objectives for Year 2:
For the purpose of this project, long span bridges shall be defined as those structures with the main span exceeding 150 meters. Unusual bridges are defined as bridges of any structural type with a total length longer than about 500 meters, or with piers taller than about 30 meters, or movable superstructure types, or a bridge defined as an essential or life-line bridge, or any bridge that is not explicitly within the scope of the AASHTO seismic provisions.
In contrast with short-to-medium span bridges, which are generally comprised of simply supported or continuous girders, these bridges are generally made up of an assemblage of elements in the form of trusses, arches, cables, or a combination of these.
Most long span bridges can be classified into the following superstructure types:
Unusual bridge types can be generally classified into the following categories:
The most prolific of all these bridges in the United States is the truss-type structure. Therefore, it is planned to primarily, but not exclusively, focus on the truss-type of long-span bridges in the manual.
Most substructures for these types of bridges can be classified into the following types:
To the extent possible, the seismic retrofit of these foundation types will be included in the manual.
During Year One, this task produced and distributed a state-of-practice survey. Some surveys have been returned and a few are still being received. There needs to be a follow up in Year Two so that important states such as California and Washington in high seismic areas and New York, have responded.
State-of-practice survey: The survey needs to be coordinated with the NCHRP Project 12-49 Comprehensive Specification for the Seismic Design of Bridges. To increase the amount of surveys returned, responses will be solicited from state DOTs, turnpike authorities, bridge authorities, and private consulting firms. The data will be collected and cataloged by bridge types, specific seismic issues, and by new bridge designs or retrofit measures of existing bridges. The data will be reviewed and a brief summary report produced.
Seismic Retrofit and Design Manual: Based on the issues raised by the above survey and other resources, a list of topics to be included in the manual will be developed. Additionally, a second list will be developed, specifying issues that require further analytical work or laboratory testing. Subsequently, an outline will be developed, including drafts of some selected sections, which will be reviewed. Based on the results of the review, a strawman manual will be developed.
The major product of this task is a strawman design and retrofit manual. A summary report will also be produced that will document the findings of the state-of-the-practice survey.
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