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Most geotechs would consider 1/2 inch between any two points (say
footings or walls) as acceptable, and will qualify their foundation
recommendations as such.  As expected for the GE profession, this is
conservative.   From personal experience with residential
investigations, some owners do not notice (or object to) 3 inches over
the longest slab dimension.   The 1/4 inch in 20 feet is similar to the
ACI construction tolerance of 1/8 inch in 10 feet, measured with a
straightedge.  For another perspective, I and other "experts" have
testified in litigation proceedings that 1 inch over the full slab
dimension is tolerable, but that 2 inches or more could be considered
"defective".  If this isn't confusing enough, let me add that, as a rule
of thumb, brittle finishes such as stucco, plaster,  or gypsum board will
almost certainly show distortion cracking when the distortion along a
building line exceeds 1 inch in 20 to 30 feet.  At 1/2 inch, the cracks
may or may not be present, but will be small enough to be "acceptable". 
 It is distortion - not settlement per se - that will cause visible
structural or architectural problems.  So, the 1/2 inch used by the
GE's, per my limited experience, is an achievable and reasonably
conservative guideline for new construction.

Russ Nester, SE, GE
rnester(--nospam--at)juno.com
___________________________________________________________________
On Wed, 26 Mar 1997 15:42:18 -0800 Mark Baker <shake4bake(--nospam--at)earthlink.net>
writes:
>I am interested in the opinions of others regarding the following:
>
>1. What would you (as an engineer, not homeowner) consider as an
>acceptable settlement for wood framed residential slab on grade
>construction?
>
>I am well aware of the many factors which one needs to account for in
>response to this question, here is what prompted my post...
>
>I have had a rule of thumb bounced off me by numerous design
>professionals over the years of 1/4" in 20', but... no one seems to 
>know
>where this came from.
>
>2. Does ACI or any other source give placement tolerances for this 
>type
>of construction?
>
>Mark
>
>


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Subject: Re: Construction Tolerances
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