MCEER HIGHWAY PROJECT
FHWA CONTRACT DTFH61-98-C-00094
Seismic Vulnerability of the Highway System

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  
Research Year 2

Charles Seim, T Y Lin International

Objective

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:

1. State-of-Practice Survey: Complete the survey and develop a synthesis of current practice and issues related to these types of bridges
   
2. Seismic Retrofit and Design Manual: Review data from the above survey, develop a synthesis of additional analytical work or laboratory testing and draft a strawman manual.

Approach

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:

• Steel trusses: through trusses, deck trusses: with articulation of simple span, continuous girder, cantilever, or combination
• Steel and concrete arches: through arches, deck arches: with articulation of fixed arch, two- or three-hinged arch, tied arch of single or multi-spans
• Steel and concrete cable stayed: single and double towers, single and multi-planes of cables with varied articulation
• Steel suspension spans: earth anchored, self anchored with varied articulation

Unusual bridge types can be generally classified into the following categories:

• Long bridges: steel or concrete girders of any span length and any articulation with a total length exceeding 500 meters
• Tall piers: steel or concrete piers taller than 30 meters supporting any type of superstructure
• Movable superstructures: vertical lift, swing, and bascule

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:

• Spread footings
• Timber piles, concrete piles, steel pile, drilled shafts
• Caissons

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.

Products

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.

Technical Challenges

• To ensure that all practical seismic retrofit measures will be found in the survey and data review.
• To ensure that the seismic retrofit measures selected for inclusion will be fully presented and explained.
• To identify specific seismic issues that will require additional analysis and experimental work.

8/14/01