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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