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: Long Span Bridges — Design and Retrofit Manual

Principal Investigator(s) and Institution(s)
Charles Seim, T Y Lin International

Many long span bridges have been constructed in the U.S. over the years in earthquake-prone areas. These bridges are generally characterized as having been designed to codes that did not require the high level of seismic design that current bridge design specifications now require. They 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 of commercial goods during the repair period.

The 1989 Loma Prieta, 1994 Northridge, and 1995 Kobe earthquakes have provided the bridge engineering profession an understanding of the large accelerations and huge displacements that bridges undergo. This improved understanding also extends to structural analysis and seismic retrofit methods. A number of long span bridges have been evaluated for their seismic vulnerabilities in moderate-to-high seismic zones such as the Pacific Coast, the New Madrid area, and in New York, and a few of these have had retrofit designs completed and implemented.

This work has resulted in the creation of a data base of long span bridge seismic retrofit methods that can be drawn on and developed into a Seismic Design and Retrofit Manual for Long Span Bridges. As a starting point for the development of such a manual, the objective of this task is to conduct a survey and synthesis of current practice and research related to long span bridges, and the development of a strawman manual.

For the purpose of this project, long span bridges shall be defined as those structures where the main span exceeds 150 m. In contrast to short-to-medium span bridges that are generally comprised of simply supported or continuous girders, long span bridges are generally made up of an assemblage of elements in the form of trusses, arches, cables, or a combination thereof. Most long-span bridges can be classified into the following superstructure types:

1. steel trusses (through trusses, deck trusses) - articulation: simple span, cantilever, continuous, gerber, combination
2. steel and concrete arches (through arches, deck arches) - articulation: full arch, two or three-hinged arch, tied arch (single or multi-spans)
3. steel and concrete cable-stayed (single and double towers, single and multi-planes of cables) - articulation: varied
4. steel suspension spans (earth anchored, self-anchored) - articulation: three spans

The most prolific of all long-span 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 bridge in the manual. In addition, most substructures for long span bridges can be classified into either spread footings, timber piles, concrete piles, steel piles, or caissons. To the extent possible, seismic retrofit of these foundation types will be included in the manual.

A state-of-practice and current research survey is needed to locate and obtain useful information and details that can be developed for inclusion in the manual. However, many of the seismic retrofit measures developed for current long span bridges are untested and, therefore, unproven. It is therefore expected that some of the details in the manual will be developed in conjunction with a laboratory testing program to gain a better understanding of their behavior and performance under dynamic loading.

Among some of the areas of concern are the chords and diagonals of steel trusses, which are usually built up as lattice members with low ductility capacities. In addition, many piers are constructed of unreinforced masonry or lightly reinforced concrete, and it has been demonstrated that foundations, piles, and anchorages cannot undergo the large displacements and uplift that earthquakes are known to produce in bridge substructures.

The research to be conducted in Year 1 under this task will therefore begin with a state-of-practice and research survey followed by the development of the strawman manual. This will include a review of the data collected by the ATC/MCEER Joint Venture for NCHRP Project 12-49, the conduct of an independent literature search, and contacting and obtaining information and data from other resources as available. In addition, a survey targeted to Federal, state, and local government agencies, and design consultants will be developed and widely distributed.

Based on a review of the data from the state-of-practice and research surveys, and other resources that are available, an outline of manual contents will be developed. Selected sections and subsections will be developed based on the available reference materials. The strawman will be sent to a variety of design consultants in order to obtain feedback and comments on the draft.

Based on the strawman draft review, a list of issues that require resolution will be compiled. It is anticipated that some of these issues will require additional analytical or experimental work in order to adequately address them.

Anticipated Start Date and Duration
January 1, 1999 - 24 months