I am preparing to perform an ASCE31 Tier 1 Evaluation of an existing building. The building is a small single story structure housing bathrooms and lockers for workers at a warehouse in Oakland, CA. The walls are CMU block and the roof is wood trusses with a plywood diaphragm. The geologic hazards checklist requires that liquefaction be evaluated so I looked at the USGS maps and found that the site falls within the liquefaction hazard zones. My question is, what effect can liquefaction have on the structural performance? The site is flat as can be, so it seems to me that the worst that could happen is that we’ll end up with signifant differential settlement and damage to the building that may be beyond repair, not really a Life Safety issue. Any thoughts on this would be appreciated.
All the suggestions provided are valid. You need to do a site specific geotechnical investigation with at least one boring to a minimum depth of 50 feet below the ground surface. The most significant thing to notice is depth to groundwater. A liquifaction analysis should be done to determine potential total and differential settlement. There are various softwares available to perform liquefaction analysis. The simplest method is to use either the STP or the CPT method. The CPT method gives you soil parameters directly from the field investigation. In the SPT method, the soil engineer may have to perform laboratory testing depending on the blow counts and summarize his/her findings in a report. The report will have diagrams showing how much settlement to expect. The structural engineer will use the report to determine the best method to use for the evaluation. I will treat this as a big safety issue. Good luck.
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