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It's hard to go wrong if you design out of the AASHTO LRFD Bridge Design Specifications. Appendix A11 has a pretty concise summary, and it incorporates Anderson's new thinking about general limit equilibrium in NCHRP Report 611. The horizontal seismic component is either the site-adjusted PGA or half the site-adjusted PGA, depending on whether you can tolerate 1-2 inches of permanent deflection. AASHTO designs bridges for the 1,000-year event (7% chance of exceedance in 75 years). It may be that you're working on a building and your building official will require you to us the 2% in 50 yr (2,500-year) event. The "Sds/2.5" approach assigns building ductility reductions to structures that typically are not ductile like a building. I've seen it used in Puget Sound by engineers trying to report low PGA's and hoping that the building official doesn't call them on it. Sds/2.5 is explicitly incorrect, plus you can get a lower PGA by following Caltrans (who use the BDS) and claim the shorter exceedance interval. |