Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design (2020) / Chapter Skim
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58 AASHTO Seismic Map Update Background Information The current versions of both the AASHTO LRFD Bridge Design Specifications and the AASHTO Guide Specifications for LRFD Seismic Bridge Design use maps that are based on the 2002 U.S. Geological Survey seismic hazard modeling efforts, which are now considered out of date.
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AASHTO Seismic Map Update 59 Figure 27. Current 3-period spectrum construction.
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60 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design classes. The third option would be to utilize the 2018 data with the multi-period spectra and the soil effects built into the mapping data.
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AASHTO Seismic Map Update 61 Soil Classes A, B, BC, C, CD, D, DE, and E These data are contained in Guidelines for PerformanceBased Seismic Bridge Design (to be published by AASHTO)
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62 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design Alaska and Hawaii were not included in the 2018 hazard model; however, approximate data sufficient to develop the 22-period spectra are being developed by the U.S. Geological Survey based on the latest hazard models available for these states and generic spectral shapes developed using the 2018 hazard model for the conterminous (or contiguous)
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AASHTO Seismic Map Update 63 in AASHTO to account for soil effects is to modify the reference hazard at the B-C boundary using site factors based on the properties of the soil in the top 30 meters at the three spectral periods that define the standard curve. The factors used in AASHTO are the original site factors developed in the early 1990s as a result of efforts at NEHRP agencies and are given in tables depending on the site class as well as the values of the spectral acceleration at 0.0, 0.2, and 1.0 seconds.
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64 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design approximated by applying 1997 NEHRP site coefficients) , and basin effects for several highpopulation areas.
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AASHTO Seismic Map Update 65 the differences in design motion across the country based on this choice, and arising from the differences in hazard with return period, also become somewhat arbitrary. A risk-targeted approach takes into account the full hazard curve and is based on the idea of an equal probability of collapse or some other damage state, rather than simply the equal probability of a certain motion occurring during a specified time period (e.g., 75 years)
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