Effects of Liquefaction on Existing Building 1

[EBF]

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.

 

[muuddfun]

The worst case is loss of bearing and total structural failure and complete collapse killing everyone in side. Sure it must have stood up to loma-prieta, so that maybe wont happen, but I definietly would call it a life safety issue that must be investigated.

So you need to look more carefully at what the liquefaction at the exact location will be. The CGS published the liquefaction hazard maps for a reason, but the exact effects from liquefaction at a site can't be determined from the CGS maps. You need a site specific subsurface investigation to determine that. Do you have any old geotechnical reports for the building on file with the city or county? Start there.

Also CGS put out a new version of SP117 just very recently I haven't even had a chance to read it yet. You need to look at this as well.

 

[EBF]

Thanks for the input. Can you be a little more specific about how liquefaction can cause "total structural failure and complete collapse" for this building? I'm trying to understand the mechanism by which this could happen for a 10' tall CMU box. I can see that loss of bearing capacity can lead to large differential setllements causing the building to lean, which can cause structural instability. I also understand that for a frame type building differential setllements in columns can induce additional forces in the frame that can lead to structural failure. But neither of these scenarios seem to be a likely hazard for this particular building. Are there any other failure mechanisms I should be considering?

Note that we still plan to recommend to the owner that they hire a geotechnical engineer to assess the liquefaction hazard at the site, but I'm just trying to anticipate what this is something that could become a huge issue given the building type as I have described it.

 

[muuddfun]

What it would take to get total collapse is not likely in most cases especially for one story buildings that are wider than taller. I could see it happen if you had very shallow groundwater and no overlying sediments that would prevent the surface effects from manifesting. So if you had loose sands near the surface with very shallow groundwater and a longer duration event, you may get the loss of bearing under the footings so they would settle but with a cmu wall on them they may also lean over. When they lean out then they may pull away from your roof supports and cause the wall to tip over completely and the roof to collapse. Again unlikely, but not out of the question.

 

[STVU]

It is good that you are recommending a site specific geotechnical evaluation. Let the soils engineer determine, what layers are susceptible to liquefaction, and seismic settlement above and below GWT depth. Usually one 50 ft boring will do for small job sites.

After receiving the soils report, as the structural engineer, you may then compute rotation, support settlement for column footings.

Take liquefaction very serious and do not assume structural performance is similar to past behavior. Oakland, CA is in zone 4 and is susceptible to strong ground motions. If you were 60 miles east, like Merced county, I would not be as concerned, unless your GWT is shallow. The difference between the two sites being PGA of 0.6g(Oakland) vs. 0.2g(Merced county).

By the way are the CMU walls reinforced?

 

[dgillette]

For an extreme case, see

http://earthquake.usgs.gov/regional/world/events/1964_06_16.php

 

[Friar6]

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.