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Re: Strap hold down detailing[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Strap hold down detailing
- From: Mlcse(--nospam--at)aol.com
- Date: Thu, 23 Dec 2004 22:12:19 EST
I would double check with Simpson, but I have a feeling the strap capacity is based upon allowable nail values as oppose to tested values. I don't know if the a nail group reduction factor is included in the allowable strap design values. For a CMST12 that extends 48" or more on the shear wall, with nails at 3"o.c., I can't believe all the nails have equal loading (you would have to use a group reduction factor if it was bolts).
I would probably favor the 3rd stud being added for the straps, if the shear wall is highly loaded (2"o.c. nailing for wall sheathing) with the edge nailing on its own 2x or shared with the strap if on a 4x or larger post. If the design shear for the wall is low (6"o.c. nailing) then I might go with omitting the sheathing E.N. and just use the straps nailing (less than 1000 pounds of uplift) also for the E.N.) as long as the nail spacing in the strap wasn't more than the sheathing nail spacing. But, I would want to feel comfortable enough that the remaining plywood nailing above the strap was adequate for the shear wall design force (example: shear wall design force (100 plf ) is significantly less than the selected shear wall rated capacity (200 plf ) based upon nail size and spacing pattern.
With the strap over the plywood sheathing, the sheathing is going to help redistribute some of the strap nail shear to the other sheathing nails in the local area not nailed through the strap. The nail has a given shear capacity (transferring the shearing forces from both the strap and plywood in to the stud). The shear in the nail just above the stud surface will be the sum of the sheathing and straps shears. The strap nail shears will be vertical, the sheathing nail shear will be in a diagonal direction at the corners of the plywood sheathing (diagonal tension field in corners of plywood sheathing)
As a global check, I would think you could add up the (uplift force of the strap)/(wall height) + shear wall horizontal shear /ft and use this design value to determine the vertical nailing along the end of the shear wall. You might end up with the vertical nailing being 3"o.c. at the boundary post above the strap , and strap nailing from there to the bottom of the wall, and 4"o.c. for the rest of the shear wall edge nailing (top plate, sill plate, non-boundary edges).
I would play it conservative and provide the nailing separately, or at least show how I distributed the forces,
In a message dated 12/23/2004 10:34:46 AM Pacific Standard Time, T.W.Allen(--nospam--at)cox.net writes:
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