by Ian M. Friedland
On May 29 and 30, 1997, NCEER conducted the Workshop on National Representation of Seismic Ground Motion for New and Existing Highway Facilities in San Francisco, California. The workshop, which was sponsored by the FHWA/NCEER Highway Project, was organized by Maury Power of Geomatrix Consultants and Ron Mayes of Dynamic Isolation Systems, and was attended by 55 professionals representing the technical areas of seismology, geotechnical engineering, and structural engineering, and composed of academia, the practicing engineering community, and State and Federal transportation officials.
The objective of the workshop was to provide a forum to discuss and ultimately recommend future directions for a national representation of the seismic ground shaking hazard for the design of new highway facilities and the evaluation and retrofit of existing facilities. Emphasis during the workshop was on the selection of appropriate representations of seismic ground motions for use in nationally-applicable guidelines and specifications, such as the AASHTO seismic design specifications for highway bridges. A secondary objective of the workshop was the identification of areas where further research and/or development is still needed in order to determine the optimum characterization of ground motions for guidelines and specifications. Among the topics addressed were the following:
- National hazard portrayal.
Are the new 1996 USGS maps a sufficiently accurate and improved portrayal of the national seismic hazard and should they become the basis for national seismic hazard portrayal in future highway design codes?
- Ground shaking parameters.
Which mapping parameters are considered the most important in the national seismic hazard portrayal; e.g., peak ground acceleration versus spectral acceleration and duration.
- Ground motion levels for design.
What are appropriate probability levels or return periods for highway facility design? Should deterministic considerations be integrated with probabilistically-based ground motions in determining design ground motions? Should probability levels be region-dependant?
- Site effects.
Are current characterizations of site effects appropriate for use in design procedures for highway facilities? Are there special situations requiring variation in site factors; e.g., high impedance ratios or shallow soil profiles?
- Vertical and near source ground motions.
Should special provisions apply for the design of vertical ground motions and, if so, what should they be? Should the unique characteristics of near source ground motions be considered in design and, if so, can they be quantified in a simplified manner for the design of ordinary structures without special site-specific studies?
- Spatial variations of ground motions.
When should spatial variations of ground motions (e.g., wave passage effects and incoherence) be explicitly quantified for design, and are there classes of structures for which these can be neglected?
Each topic was addressed by a keynote paper and presentation, and then alternate opinions were solicited from the workshop participants. In all, 16 papers were prepared and distributed to participants in a set of workshop preprints. An executive summary describing the discussions, conclusions, and recommendations reached during the workshop will be prepared and, in addition to the papers given during the workshop, published as a set of workshop proceedings by NCEER in the fall.
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