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

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I have ALWAYS done it that way almost. I take moments about the level I am
currently analyzing. Then divide by the moment arm and subtract the
appropriate portion of dead load (60%) to obtain my uplift load. Gets pretty
high on three story buildings on the West coast. I have seen other engineers
ignore the laws of statics and nature and often wonder what will happen to
their designs.

Mark E. Deardorff, SE 
R & S Tavares Associates, Inc
9815 Carroll Canyon Road
Suite 206
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Phone: 858-444-3344
Phone: 209-863-8928
mark(--nospam--at)rstavares.com
www.rstavares.com

 

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> -----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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