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Bridges
Abstract:
Seismic geotechnical investigations for a bridge involve several types of analyses, including: establishment of design level rock motions; site characterization and response analyses; ground and foundation motion computations; assessment of liquefaction and its impact on bridge foundations; soil-foundation interaction and impedance calculations; assessment of foundation performance under the design seismic loads; and in the case of existing bridges, if deemed necessary, design of seismic retrofit measures. The outcomes of each of these various investigations have important implications on the assessment of the overall vulnerability of a bridge. Frequently in engineering practice, “conservative” assumptions are employed at various stages of the investigations with the intent of ensuring that uncertainties are accounted for, and to allow the use of simplifying assumptions and analyses. Whereas a certain level of conservatism should be adopted in geotechnical engineering analyses under static loading conditions, a different approach is warranted for seismic loading conditions. In seismic geotechnical analyses, it is often difficult to discern what the impact of a particular assumption will be on the overall assessment of the vulnerability of a bridge. It is likely that in certain situations perceived conservatisms adopted in the seismic investigations may actually lead to an unconservative outcome. Thus, there are inherent pitfalls of perceived conservatism in seismic geotechnical investigations. Through examples of seismic geotechnical analyses of bridges, a case is made in favor of the application of a rational approach. The rational approach is to obtain accurate and site-specific geotechnical information, apply the analysis procedures that most accurately model the specific bridge site and foundations, and employ good professional judgment that is based on a thorough understanding of the fundamentals of soil, foundation and structural dynamics Seismic Assessment and Retrofitting of the Brooklyn Bridge, New York, New York Seismic Geotechnical Investigations and Evaluation of Vulnerability of the Foundations of the Brooklyn Bridge Report |
