I am a structural engineer working on a small commercial building on Long Island, NY. We had assumed Site Class D for the seismic coefficients since we did not have a geotech report. The owner provided some soil borings (no geotech report, just borings) that were done about 8 years prior for a building that is directly adjacent to our site. The borings indicate there is a 15 ft thick layer of organic peat about 10 ft below grade. Looking at the IBC, this would fall under Site Class F. We have told the owner they need to get a geotech engineer to evaluate this and give recommendations. They are refusing because they said they just built the adjacent building without it (they apparently assumed Site Class D). My question is, how much of an amplification effect can we expect this to have on the seismic loads? Are we being sticklers for the Code given that we are in a low seismic area (Ss~0.3 and S1~0.1) and wind loads are probably going to govern anyway? Thoughts...
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