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RE: October Structural Engineer " Resisting Lateral Forces Cumulative overturning design"

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The foundation under the prefab shearwalls are missed all the time.  We
have had to design fairly large foundations under prefab shearwalls.

-----Original Message-----
From: Jeff Smith [mailto:jeffsmith7(--nospam--at)comcast.net] 
Sent: Tuesday, November 06, 2007 1:28 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: October Structural Engineer " Resisting Lateral Forces
Cumulative overturning design"

And BTW, I could not find any Simpson commentary for a foundation
required
to resist 36,000# and 72K-ft overturning within a 2 foot wide strong
wall.


-----Original Message-----
From: Jeff Smith [mailto:jeffsmith7(--nospam--at)comcast.net] 
Sent: Tuesday, November 06, 2007 1:20 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: October Structural Engineer " Resisting Lateral Forces
Cumulative overturning design"

Funny...I was just watching this simpson strong tie video:
http://www.strongtie.com/overturn/index.asp   which is how I have always
done it.

My October Structural Engineer is in the same place, thanks for bringing
that to my attention.

Jeff



-----Original Message-----
From: Haan, Scott M POA [mailto:Scott.M.Haan(--nospam--at)poa02.usace.army.mil]
Sent: Tuesday, November 06, 2007 12:42 PM
To: seaint(--nospam--at)seaint.org
Subject: October Structural Engineer " Resisting Lateral Forces
Cumulative
overturning design"

Last night I finally pulled the October 2007 Structural Engineer
magazine
off the flush tank and read the  " Resisting Lateral Forces Cumulative
overturning design..."

The author says that wood shear wall overturning forces  should be based
on
the moment arm from the center of the compression chord to the center of
the
anchor rod, fine...  The article says that the only way to get accurate
overturning forces for a multistory wood shear wall is to calculate the
overturning moments and draw a free body diagram of the multi-story
wall.  

The article says that using the unit shear times height method
understates
the uplift for multistory walls and is only for preliminary
approximations,
uh. uh. if you do it right. 

The unit shear times height method is derived with a free body diagram
and
summing moments. It assumes forces are distributed to walls proportional
to
their lengths. If you add the tension from the wall above to the tension
to
the wall chord below and use the full floor to floor heights and
subtract
out the tributary resisting loads you get the exact same result. If the
problem is the moment arm is too long, you would be conservative by
either
designing with the wall lengths as the moment arm length to calculate
your
unit shear or dividing your tension by the ratio of the moment arm
length to
wall panel length. I say to anyone who disagrees with this to draw a
free
body diagram and sum the moments.

I think that this article is going to cause a bunch of needless
permitting
hassles for design engineers.

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