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RE: Load Factor Question

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Mr. Allen,
Wouldn't sudden settlement from densification of the liquefiable lens be your biggest problem following an event?  Then it would be a matter of the slab being stiff enough to distribute the live and dead loads during the event as it settles.  Also, is your stress envelope of the mat actually into the liquefaction layer? 
By the way, I like the stiff mat for a lightly loaded wood frame structure.  No argument on that from me.
As for using the reduced K value, your approach looks good to me.
-----Original Message-----
From: Bill Allen, S.E. [mailto:T.W.Allen(--nospam--at)]
Sent: Friday, February 04, 2005 1:36 PM
To: seaint(--nospam--at)
Subject: Load Factor Question

I am working on a project that has a liquefaction potential. There is a lens of good soil then a lens of liquefiable soil then a lens of good soil. Based on the strata, the geotechnical engineer has said that a mat foundation is acceptable. For the design of the mat, he has given me a reduced modulus of subgrade reaction (k) to represent what the soil might act like during the event. For the purposes of this question, there is no alternative to the foundation type because that issue has been beat to death and it would be a waste of cyberspace to discuss alternatives. The question here is just related to load factors.


The structure is a one story wood framed building. The only loads on the building are D, Lr, W and E.


Here is what I intend to do.


  1. I use the unreduced k value for the D+Lr and the D+Lr+W cases.
  2. I use the reduced k value for D+Lr+E load cases.
  3. After the event, the results from question 1 are still valid.


Does anyone disagree with the above approach?




T. William (Bill) Allen, S.E. (CA #2607)


Consulting Structural Engineers

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