MCEER/NCEER Bulletin Articles: Workshop/Conference Reviews...
Research Needs for Long-Span Bridges
Current recommendations and guidelines for the seismic evaluation and retrofitting of highway bridges are limited to structures of conventional steel and concrete girder and box girder construction, with spans not exceeding 500 feet (150 meters). However, longer bridges, which are typically considered to be "important" or "critical" structures under most definitions of bridge importance, are not presently covered under any current codes or guidelines, and are usually evaluated or retrofitted on a case-by-case basis. Such bridges include, but are not limited to, suspension and cable-stayed bridges, arches, and long-span box girder and truss bridges. Furthermore, bridges in the 200 to 500 foot (60 to 150 meter) span range may not be adequately covered by the current FHWA recommendations for seismic retrofitting.
Many structural components incorporated into long-span bridges, like floor beams and stringers, can be evaluated and retrofitted using available criteria appropriate for those components. However, due to their very nature, the seismic evaluation and retrofitting of long-span bridges must consider structural members and details, and additional factors, that are specific to such structures. At this time, there is no clear-cut consensus as to what the most important factors and issues that must be evaluated are, and for which additional guidance may be necessary or must be developed, for these bridges.
In order to address these concerns, NCEER conducted the Long-Span Bridge Seismic Research Workshop on December 12 and 13, 1994, in San Francisco, California. The workshop was organized by Ian M. Friedland and chaired by Ian G. Buckle, both from NCEER, and was sponsored by the Federal Highway Administration as a task in the NCEER Highway Project. Thirty-six people attended the workshop, of which more than 15 were NCEER affiliates. Attendees included a mix of researchers and practitioners with experience in long-span bridge technical issues, including representatives from academia, State and Federal governments, and the consulting engineering community. The focus of this workshop was on issues unique to long "monumental" structures. Long, multi-span bridges were not of primary concern in the workshop, but were included where overlapping interests occurred (e.g., spatial variation).
Prior to the workshop, all attendees were asked to submit a list of what they considered to be the critical seismic concerns and research needs for long-span bridges. More than 160 individual concerns were identified by participants, and these were classified into six technical categories:
- Performance criteria and risk assessment
- Ground motion and spatial variation
- Geotechnical engineering
- Analysis and modeling
- Structural details
- Materials and retrofit measures
To kick off the workshop, overview presentations were made in each of these technical areas. Presenters included Ian Buckle, performance criteria; Klaus Jacob (Lamont-Doherty Earth Observatory), ground motion; Geoffrey Martin (University of Southern California), geotechnical; Frieder Seible (University of California at San Diego), analysis; Roy Imbsen (Imbsen & Associates, Inc.), structural details; and Charles Seim, (TY Lin International), materials/retrofit measures. Participants broke into technical working groups and further discussed, identified, and prioritized critical issues and research needs in each of these areas.
Participants then reconvened in a general session, during which the most important issues and research needs that were identified during the technical break out sessions were presented and further discussed. The top three-to-five issues in each area were then agreed upon.
A general consensus was reached on a number of critical issues and research needs. Among those considered to be the most important were the following:
- Modeling of soil-structure interaction for large caisson foundations
- Modeling of large pile-groups with closely spaced piles
- Characterizing the duration of ground motion
- Characterizing wave passage effects for spatial variation of ground motions
- Developing a database documenting the historical performance of long-span bridges
- Developing long-span bridge-specific performance criteria and methodologies for achieving targeted criteria
- Developing an improved understanding of the cyclic characteristics of materials, members, and connections typically used in long-span bridges
- Developing improved models of hinges, joints, and bearings
- Developing criteria and retrofit measures for steel members and connections
Subsequent to the workshop, the top issues in each area are being further developed and drafted into research task statements by the technical area presenters. These will then be sent to all workshop participants for review and a final balloting and ranking by mail.
It is expected that the results of the workshop will be incorporated into the Year 3 and Year 4 research programs of the NCEER Highway Project, through the initiation of several tasks identified as the most critical for long-span highway bridges. In addition, the proceedings of the workshop and the results of the mail-ballot ranking of critical issues will be published by NCEER in the spring of 1995.
NCEER Bulletin, January 1995, Vol. 9, No. 1