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Re: Steel straps on wood shear walls

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The test are for the strap holdowns that embed directly into the foundation (LSTHD & STHD). The test results are a little goofy looking when I compare 2500 psi conc. with 3000psi load/deflection results. Their geometry always made me uneasy & the installations I'd seen made me queasy. I don't think I'd be willing to assume that this test data is directly applicable to a floor-to-floor strap tiedown like a CS16. I'd want to see a real test report before trying to extrapolate to other situations. I think I'm still comfortable with a 4' piece of CS16 as an upper floor tiedown loaded to about 1500#. That has enough nails (even at every other hole) to directly transfer a reasonable shearwall's "post" load. The goal being to hold the edge of the plywood relative to the sill. YMMV

Thor Matteson wrote:
I may have misunderstood the original question--my earlier answer was directed at more or less *continuous*, *horizontal* straps. For strap-type tiedowns, optimal installation is directly to the studs/post. Simpson catalogs state this, with advice that deflection is increased if the strap was placed over the plywood, and refer to their form "T-PLYWOOD" ( http://www.strongtie.com/ftp/bulletins/T-PLYWOOD08.pdf ) which tabulates the deflections for several products and conditions, and states that it's the designer's responsibility to determine whether the added deflection is acceptable. The additional deflection occurs because the plywood under the tiedown acts like a shim between the post and the tiedown, and the nails from tiedown to post deform within the thickness of the plywood. All of the uplift cannot be taken out by gripping the bottom corner of a shear panel--the post above the tiedown collects the majority of the uplift, and this force needs to get to the tiedown somehow. Delivering the force through a plywood or OSB spacer is questionable. The increased deformation of the tiedown nails in the plywood 'shim' will undoubtedly reduce overall performance, especially in cyclic loading. My standard detail shows strap tiedowns attached directly to a studs/post and a 3-foot block attached to the studs/post for the shear panels to connect to, with the panels being cut out around the tiedown. I almost never use this, because I tend to specify the HDU or HTT tie-downs. Thor Matteson, SE
www.shearwalls.com <http://www.shearwalls.com>


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