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RE: Is it just me?

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Bill-
Simpson has a pdf of an article on their website by Stephen E. Pryor (2002) reporting on some Finite Element Analyses including the effects of nail and sheathing stiffness in reducing the moment occurring in a post as compared to the simplified bare post model due to eccentric hold downs. The reduction is substantial. Note, however, that tension is not reduced.

The link is:
http://www.strongtie.com/literature/TechTopics/eccentric_restraint.pdf
It is a 33 page paper and discusses both solid posts and dbl 2x4 posts.
HTH
Bill Cain, SE
Berkeley, CA


From: t.w.allen(--nospam--at)cox.net
To: seaint(--nospam--at)seaint.org
Subject: Is it just me?
Date: Fri, 9 Jan 2009 16:01:38 -0800

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
 
V (949) 248-8588 F(949) 209-2509

 



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