Seismic Design and Retrofit Manual for Long-Span and Special Bridges
Charles Seim, T.Y. Lin International
Many long-span bridges or special bridges with long total length or with tall piers or with movable superstructures have been constructed in the United States over the years in earthquake-prone areas. These bridges are generally characterized as being designed to codes that did not require the high level of seismic inputs or ductile designs that current bridge design specifications now require.
These bridges are usually 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 Loma Prieta (1989), Northridge (1994) and Kobe (1995) earthquakes have given the bridge engineering profession a new perspective of the high accelerations and large displacements that bridges endure under seismic activity. Structural analysis and seismic retrofit methods have been improved in recent years with better understanding and new developments in seismic engineering.
Some long-span bridges have been seismically retrofitted in high seismic zones of the Pacific Coast region and a few in the New Madrid Fault area, and possibly a few others in lower seismic areas such as New York. or New Jersey. In addition, a few new bridges are being designed and constructed in these areas.
This work has resulted in the adding to the data base of long-span 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-Spans and Special Bridges Retrofit Manual. Seismic retrofit of the substructure is as equally important as the seismic retrofit of the superstructure for these bridges. This function will also be addressed in Task C2.
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 understanding of their behavior and performance under dynamic loading.
Workshops and Training courses will be developed and presented to evoke discussion, produce suggestions for further refinement of the manual, and develop ideals for a further laboratory testing program.
It may be helpful to quickly review the proposed five-step process to implement the final publication of the Seismic Design and Retrofit annual.
Task C1-1 - State of Practice Survey A state-of-practice survey can act as a starting point to locate and obtain useful information and details that can be developed for inclusion in the Manual.
Task C1-2 - "Strawman" Manual Developing a Strawman Manual will serve to identify basic knowledge gaps that need technical development and to identify laboratory research program to validate the new technology.
Task C1-3 - Review Workshop The Strawman Manual will reviewed and discussed in a by-invitation-workshop of state engineers and leading practitioners. Revisions will be made to the manual based on the comments from the workshop and then published as an interim document.
Task C1-4 - Prepare the Final Manual A draft of the manual as finally developed will be sent out to engineers and researchers for final review and comments. These will be used to make the last revision to the draft manual before final publication in the last year of the Project.
Task C1-5 - Training Course A pilot training course will be developed that can be used for training engineers in the use of the manual. Feedback from the pilot will be used to enhance the course well as the manual prior to its final publication.
Bridge Type Definitions
For the purpose of this task, long span bridges shall be defined as those structures with the main span exceeding 150 m. Special bridges are defined as bridges of any structural type with total length longer than about 500 m, or with piers taller than about 30 m, 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 that are generally comprised of simply supported or continuous girders, these long-span and special bridge types are generally made up of an assemblage of elements in the form of trusses, arches, cables, or a combination of these.
Long-span bridges can be generally classified into the following superstructure types:
- Steel Trusses: through trusses, deck trusses; with Articulation of simple span, continuous, gerber, cantilever, or combination. The most prolific of all these types of 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.
- 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
- Steel Suspension Spans: Earth anchored, self-anchored with various span articulations
To the extent possible, the seismic retrofit of these long-span bridge types will be included in the manual.
Special bridges can be generally classified into the following categories:
- Long Bridges: Steel or Concrete Girders: Any span length and any span or joint articulation with total length of bridge exceeding 500 m.
- Tall Piers: Steel or Concrete piers taller than 30 m supporting any type of superstructure
- Movable Superstructures: Vertical Lift, swing, and bascule
To the extent possible, the seismic retrofit of these special bridge types will be included in the manual.
Substructures for bridges can be generally classified into the following types:
- Spread footings
- Timber piles, concrete piles, steel piles, drilled shafts
- Caissons, and very large diameter (d > 2 m) piles
To the extent possible, the seismic retrofit of these foundation types will be included in the manual.
State Of Practice Survey
The responses to the seismic survey questionnaire that was mailed out to State DOTs and some consultants during the last quarter was analyzed for content. The questionnaire was 17 pages in length with some of the pages containing blank lines for the responders to write answers to the questions. The questionnaire also requested that drawing of the project, reports or paper written about the project, or other related information be returned with the responses. All but one of the16 returned responses were from states that do not perform any seismic designs or do not have long span or seismically special bridges.
Most of the responses were from States that were located in low seismic areas or did not have any bridges that met the definition of either long-span or special bridges. The survey form could be quickly filled out and returned. The analysis of the responses proved to be disappointing as the survey will not yield significant data to be useful for inclusion in the Strawman Manual. Nor will these responses provide any information for the development of any laboratory research needs.
Most likely the seismic survey questionnaire was too long or too formable for a detailed response from those states that are in active seismic areas and do have bridges that met the definitions and the survey was simply ignored by some of the recipients. Another possible reason for the disappointing information from the responses is that there is not too many seismic projects have been completed or are in progress.
Start of the Draft Strawman Manual
Work started on a draft of the strawman manual using other resources, as the Seismic Survey produced no useful data. The first effort was to outline the Table of Contents, which will act as a guide for the development of the detailed text for each Chapter. A good guide to start with is the Seismic Retrofitting Manual for Highway Bridges, Report No. FHWA-RD-94-052, 1995. Although this retrofitting manual is intended to cover only seismic retrofitting of ordinary highway bridges of spans less than about 150 m, it can act as guide for a start of the strawman manual. Many of the subjects presented are applicable to longer spans and seismically special bridges when upgraded with current information on retrofitting and extended to new bridge designs.
Draft Strawman Contents (incomplete):
Definitions of bridge types
Design vs. Retrofit Strategies
New bridge seismic designs strategies
Seismic retrofitting of existing bridges strategies
New bridge seismic designs measures
Seismic retrofitting of existing bridges measures
Performance at two levels
Functional level - 50 % chance of occurrence during life of the bridge
Safety level-Maximum credible event that can occur
Functional level performance
No collapse to immediate full service
Safety level performance
No collapse to immediate full service
Near source effects
Low seismicity areas
Methods of analysis
Essentially linear behavior
Linear analysis satisfactory
Nonlinear behavior - plastic hinging, isolation, dampers
Nonlinear analysis required
Evaluating the Seismic Retrofitting of Existing Bridges
Nearby fault location and Site Hazard Analysis
Evaluating long-span bridges
Evaluating seismically special bridges
Bridges with tall piers
Caissons and large diameter piles
Seismic Evaluation for New Bridges
(Similar to above)
Seismic Response Modification Measures
This listing is, of course, incomplete at this stage. It will be lengthened and modified under further work and more detailed development in year two.
Products for Research Year Two
The Research Year Two products will be several brief summary reports and a draft of the strawman Seismic Design and Retrofit and Manual based on existing information available in papers, reports, and oral communications. Review workshops will be scheduled as the development of the strawman nears completion.