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RE: moment due to differential settlement - signature

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i don't know where it got picked up but someone's email program is listing
"DA" as david adie and changed it in the reply header - please note that
this is a different engineer and not MY question, thanks.

da (lowercase)



-----Original Message-----
From: Michael L. Hemstad [mailto:hemstad.ml(--nospam--at)tkda.com] 
Sent: Monday, March 28, 2005 10:56 AM
To: seaint(--nospam--at)seaint.org
Subject: moment due to differential settlement


Chris Wright responded to Dave Adie, P.E.:


"On Mar 27, 2005, at 10:09 AM, DA wrote:

> there an equation to calculate the moment due to differential
> settlement for footing 15"x18"  . 20'  long  settled 1" can someone 
> have it  handy remind me
This one is just as easy to figure out as the cantilever with the end
moment. Especially for a P.E. Get yourself a copy of Blodgett's 'Design of
Welded Structures' and check the beam bending load cases in the back. Faster
that repeated e-mail postings.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)"


Yeah, it would be a lot faster than the 6 or so emails over this question,
especially when you multiply it by the 15,000 or so engineers scratching
their heads trying to figure the question out.

Dave, I know you said "simple span," but what does the footing really look
like?  It sounds like a strip footing, but is it really 20 foot long or is
that the distance between points of no settlement?  If it is 20 feet, is the
settlement at the end or in the middle?

What is the wall supporting?  If it's a block wall, the wall may help it
span, albeit while cracking badly.

You said the footing is 15"x18".  Not knowing otherwise, I assume it's 18"
wide.  15" is an unusual depth for a strip footing assumed to be uniformly
supported; and a really unusual depth for a member trying to span 20 feet.
If you use the gross moment of inertia in Steve Gordin's equation (against
his prudent recommendations), I would guess you'll need some awfully big
bars in that little footing.

Bear in mind that the reinforcing design methodology you'll use to design
the reinforcing assumes the member cracked, while Steve's equation (the
portion of it asking for I) assumes whatever you assumed when you calculated
a value for I.  What in your judgement should you use for I?  If you divide
1" deflection by 10 feet of beam each side of the deflected point, times 15"
of depth, neglecting bending in the rest of the beam, the crack is 1/4 inch
wide.  Doesn't sound promising.

I apologize for always being the jerk on your case, Dave, but if we can't
understand your questions, we can't give you good answers.  It doesn't seem
like that much to ask.

Mike Hemstad, P.E.
St. Paul, Minnesota



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