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bridgesmall.gif (4301 bytes)MCEER/NCEER Bulletin Articles: Workshop/Conference Reviews...

National Representation of Seismic Ground Motion for Highway Facilities

by Ian Friedland and Maurice Power

This article summarizes discussions from a recent FHWA/NCEER workshop conducted through NCEER's Highway Project, task 106-F-5.4.1. More detailed information is available in Proceedings of the FHWA/NCEER Workshop on the National Representation of Seismic Ground Motion for New and Existing Highways, NCEER-97-0010. Comments and questions should be directed to Ian Friedland, NCEER, at (716) 645-3391; email: imf@acsu.buffalo.edu.

A significant amount of research has been conducted within the FHWA/NCEER Highway Project between 1993 and 1997 on how to adequately portray the national seismic hazard in highway design specifications and guidelines. This work included a review of existing national, state, and regional seismic hazard maps, an evaluation of alternative strategies for the future portrayal of the national seismic hazard, and the development of alternative recommendations for presentation to AASHTO and other highway design and specification authorities. During this time, the USGS published new national seismic ground motion maps in 1996 which appear significantly different than those currently in AASHTO specifications.

In order to ensure that the key issues related to the development of national seismic hazard portrayals were adequately addressed, NCEER organized and conducted the FHWA/NCEER Workshop on the National Representation of Seismic Ground Motion for New and Existing Highway Facilities on May 29 and 30, 1997, in San Francisco, California. The workshop provided a forum under which more than 50 earth scientists, geotechnical engineers, and structural engineers were brought together to discuss a number of these key issues and develop consensus recommendations with respect to their implementation in new highway facility design specifications (see review of the workshop in the July 1997 issue of the NCEER Bulletin, Vol. 11, No. 3).

The ground motion issues that have emerged in recent years as potentially important to highway facilities design and that were considered at the workshop were:

These issues were considered sequentially at the workshop. For each issue, selected workshop participants prepared papers and made presentations illuminating the issues and proposing a course of action in terms of design criteria and procedures and/or further development. Papers covering the presentations by each speaker are contained in the workshop proceedings (report number NCEER-97-0010).

Following the presentations on each issue, the workshop participants as a whole discussed the issues and developed conclusions and consensus recommendations. A final set of recommendations will be presented to AASHTO in the spring of 1998. The following summarizes the key elements in the discussion of each issue and the conclusions and consensus recommendations resulting from the workshop.

Using the New USGS Maps
In 1996, the U.S. Geological Survey (USGS) developed new seismic ground shaking maps for the contiguous United States (maps for Alaska and Hawaii are under development). These maps depict contours of peak ground acceleration (PGA) and spectral accelerations (SA) at 0.2, 0.3, and 1.0 second (for 5% damping) of ground motions on rock for probabilities of exceedance (PE) of 10%, 5%, and 2% in 50 years, corresponding to return periods of approximately 500, 1000, and 2500 years, respectively. These maps for the contiguous United States and separately for California and Nevada are available from and can be viewed or downloaded from the USGS World Wide Web site at http://geohazards.cr.usgs.gov.

97OctFig1pg5.gif (11169 bytes)The workshop considered whether the new USGS maps should replace or update the maps currently incorporated in AASHTO specifications, which were developed by the USGS in 1990. The key issue regarding whether the new USGS maps should provide a basis for the national seismic hazard portrayal for highway facilities is the degree to which they provide a scientifically improved representation of seismic ground motion in the United States. Based on an analysis of the process of developing the maps, the inputs to the mapping, and the resulting map values, the workshop concluded that these new maps represent a major step forward in the characterization of national seismic ground motion. The maps are in substantially better agreement with current scientific understanding of seismic sources and ground motion attenuation throughout the United States than are the current AASHTO maps. The workshop therefore concluded that the new USGS maps should provide the basis for a new national seismic hazard portrayal for highway facilities.

The workshop also examined the issue of an appropriate probability level or return period for design ground motions based on the new USGS maps. Analyses were presented showing the effect of probability level or return period on ground motions and comparisons of ground motions from the new USGS maps and the current AASHTO maps. The workshop recommended that for design of highway facilities to prevent collapse, consideration should be given to adopting probability levels for design ground motions that are lower than the 10% probability of exceedance in 50 years that is currently in AASHTO (i.e., ground motion return periods longer than 500 years should be considered). This recommended direction is consistent with proposed revisions to the 1997 NEHRP Provisions for buildings, in which the new USGS maps for a probability of exceedance of 2% in 50 years (an approximate 2500 year return period) have been adopted as a collapse-prevention design basis. (The NEHRP proposal for buildings was described at the workshop and is summarized in the Proceedings.) Figure 1 provides an example of how Maximum Considered Earthquake (MCE) ground motion maps were developed for use in design (see paper by Hamburger and Hunt in Friedland et al., editors, 1997).

Using Energy or Duration in Design Procedures
At the present time, the energy or duration of ground motions is not explicitly recognized in the design process for bridges or buildings, yet many engineers are of the opinion that the performance of a structure may be importantly affected by these parameters, in addition to the response spectral characteristics of the ground motion. Based on the presentations and discussions at the workshop, the participants concluded that some measure of the energy of ground motions is important to the response of a bridge, but, at present, we do not have an accepted design procedure to account for energy. Research in this area should be continued to develop energy-based design methods that can supplement current elastic response-spectrum-based design methods. The workshop also concluded that energy, rather than duration, is the fundamental parameter affecting structural behavior.

Characterizing Site Effects
At the Workshop on Site Response During Earthquakes and Seismic Code Provisions, held in 1992 at the University of Southern California (USC), a revised quantification of site effects on response spectra and revised definitions of site categories was proposed. Subsequently, these revised site factors and site categories were adopted into the 1994 NEHRP Provisions and the 1997 Uniform Building Code (UBC). Since the development of these revised site factors, two significant earthquakes occurred, Northridge in 1994 and Kobe in 1995, which provided substantial additional data for evaluating site effects on ground motions, and research using these data has been conducted.

The site factors and site categories in the current AASHTO specifications are those that were superseded by the USC workshop recommendations for the NEHRP Provisions and the UBC (see Table 1, taken from Dobry et al. in Friedland et al., editors, 1997). The questions for consideration at this workshop were whether the USC workshop recommendations should be utilized in characterizing ground motions for highway facilities design and whether they should be modified to reflect new data and new knowledge since the 1992 workshop. The most significant differences in the USC workshop recommendations and the previous site factors (those currently in AASHTO) are: (1) the revised site factors include separate sets of factors for the short-period and long-period parts of the response spectrum, whereas the previous site factors were only for the long-period part; (2) the revised site factors are dependent on, rather than independent of, intensity of ground shaking, reflecting soil nonlinear response; and (3) the revised site factors are larger (i.e., show a greater soil response amplification) than the previous factors at low levels of shaking, which is important for the lower-seismicity regions in the United States.

The workshop found that the post-Northridge and post-Kobe earthquake research conducted to date generally was supportive of the site factors derived during the 1992 USC workshop, although revisions to these factors might be considered as further research on site effects is completed. The wor