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Re: Strap hold down detailing

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Bill,
 
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,
 
Mike Cochran
 
 
In a message dated 12/23/2004 10:34:46 AM Pacific Standard Time, T.W.Allen(--nospam--at)cox.net writes:

Now that I’m just relaxing (just waiting for USC to show OU exactly what an “average” offense is like), I thought I would throw out a question to find out how others do things.

Currently, I detail straps to be installed over the plywood (oops, Wood Structural Panel) sheathing. Otherwise, it would be difficult to install the SW edge nailing. Simpson has load tested this condition and they believe it is O.K. to install it like this. I know there may be some who don’t like this installation, but for the purposes of this topic, let’s assume it’s O.K.

For CS straps, which are 1-1/4” wide, a single 2X stud may be adequate for anchorage. Similarly, for CMST straps which are 3” wide, double 2X studs would be a mimumum and may be adequate for anchorage (both for tension and compression). Let’s assume that they are for the purposes of this topic. With this scenario, how do you address the stud which has the shear wall edge nailing as well as the strap nails? I’ve heard where some folks omit the edge nailing at the strap. For short straps, this doesn’t seem to be much of a problem. A CS16 has an end length of 11” with 10d nails and 14” with 8d nails. However, with a CMST12, the strap is more than half the height of the shear wall. I’m having trouble reconciling forces with a FBD (load transfers from the SW into the end post back through the SW into the strap).

Of course, one could make the end post wider to allow the edge nailing to run past the strap. For a CS, that would require a double stud. For a CMST, that would require 3-2Xs. I don’t have so much of a problem with the larger strap (of course, all of the multiple studs would have to be face nailed to transfer the load from the edge nailing to the hold down strap), but it seems to me that this *might* be “overkill” for a lightly loaded CS strap. After all, CS20, CS18 and CS16 straps have capacities of 1,030 lbs, 1,370 lbs and 1,705 lbs respectively.

Opinions and comments are welcome.

Regards,

T. William (Bill) Allen, S.E. (CA #2607)

ALLEN DESIGNS

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