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Re: Control Joints in a Mat Foundation

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I'm a little confused.  Presumably you're using a mat fdn. because you need the *whole* foundation to act together and distribute loads throughout the building footprint.  (I'm assuming a "normal" size building, not a Wal-Mart-size building.) 

I also assume you have "decent" rebar in 2 mats, top and bottom, maybe #5 @ 12" oc EW or so.  I don't know why you would want to have any sort of weakened joints in such a foundation.  It seems to me that that would be like putting shrinkage joints at midspan of a beam.   Don't you need the full strength of the mat at all locations? 

Or maybe I misunderstand the situation.

Ralph Hueston Kratz, S.E
Richmond CA USA

In a message dated 10/27/05 12:29:41 PM, T.W.Allen(--nospam--at)cox.net writes:

I am working on a project which consists of a single story wood framed building sitting on a mat foundation. The mat is 16” thick.
 
A field question has come up regarding control joints. In a typical slab-on-grade, I specify cuts one third the slab thickness spaced no more that 3T feet where “T” is the slab thickness in inches. These recommendations don’t make a lot of sense for a 16” “slab”.
 
What would be a more appropriate guideline for this situation?
 
TIA,
 
T. William (Bill) Allen, S.E.
ALLEN DESIGNS
Consulting Structural Engineers