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RE: Is it just me?

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Bill,

 

Don’t bother. Check this first:

 

http://www.strongtie.com/literature/TechTopics/eccentric_holdowns.htm

 

Oshin Tosounian, S.E.


From: Bill Allen [mailto:t.w.allen(--nospam--at)cox.net]
Sent: Friday, January 09, 2009 4:02 PM
To: Seaint
Subject: Is it just me?

 

When specifying a Simpson PHD hold down, one of the footnotes reads "Post design by Specifier." In looking at the HDQ8 in 2-2x4s, the capacity is listed as 5,715 lbs. Based on an eccentricity of 3"(CL=1.5" + 1.5" for one 2x), the weak axis bending moment due to the eccentricity is 1,428 ft.-lbs. Assuming the 2-2Xs are face nailed adequately to transfer VQ/I stresses, this moment results in a bending stress of 3,266 psi on the gross section. The allowable stress on a 2x4 DF-L section is of course quite a bit lower than this, not even considering combined stresses.

 

Have I forgotten how to properly draw a free body diagram or is there something else going on here?

 

Otherwise, is it misleading to list 2-2Xs with a hold down of this capacity?

 

Regarding the VQ/I stresses, if the height of the studs are 8 feet, then the shear on the post is Pe/h = (1428)/(8)= 179 lbs. Then VQ/I = (179)((3.938)/(7.875)=90 lbs/in. Using 10d FN (capacity = 115 x 1.60 = 184 lbs each), the spacing would be 184/90 = 2" o.c.

 

This doesn't seem right to me.

 

If the two studs aren't nailed adequately to transfer VQ/I stresses, then the bending stress due to the eccentricity is even higher (6,528 psi) since S reduces to 2x1.313= 2.625 in3 from 5.25 in3.

 

Maybe I should put away the calculator on Friday afternoons.

 

If anyone would care to shed some light on the calculations, I would be most appreciative.

 

Thanks,

 

T. William (Bill) Allen, S.E.

ALLEN DESIGNS

Consulting Structural Engineers
 
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