Need a book? Engineering books recommendations...
Is it just me?[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "Seaint" <seaint(--nospam--at)seaint.org>
- Subject: Is it just me?
- From: "Bill Allen" <t.w.allen(--nospam--at)cox.net>
- 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.
T. William (Bill) Allen, S.E.
Consulting Structural Engineers
- Prev by Subject: If so many agree, why remain apathetic - Code Discussion
- Next by Subject: RE: Is it just me?
- Previous by thread: Code Discussion
- Next by thread: RE: Is it just me?