Principal Investigator(s) and
Institution(s)
Charles Seim, T Y Lin International
Objective
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.
Approach
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
9/23/99 |