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Re: Strap hold down detailing

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Bill's post implied a "lightly" loaded shearwall, say 1/2" CDX w/ 10d@6" loaded at about 100 to maybe 140 plf. On an 8' post that's 800 to 1100 #. These days I'm using more & more engineered joists (to eliminate the shrinkage problem) & on an upper floor CS16 x 4' seem about right with nails at every other hole. Since the shear is in the plywood, the strap nails suck it out before it gets into the post. The nailing above the strap are the only ones lifting the post (which is why Simpson's Strongwall tiedowns are set kind of high on their corner posts IIRC).

That does bring up another question. Where are you guys setting your cut-off value for tiedowns? After talking to Ed Diekmann & some other S.E.'s with a lot of wood-frame design experience, I decided on 800#. I just had a big dispute with a young, outside plan checker who thought the number should be 0. (My calculated net uplift was 300# & this particular location was very difficult to access, so I said no.)

I don't see the straps as any more susceptible to shrinkage than PHD's. Around here we're framing with green D.F. which dries very slowly (until the the building is closed-in & the occupants turn on the heat). With green, sawn 2x12 joists the only way I can see to address shrinkage is with a tensioning coupler like Simpson's TUD. At $50 each that seems a little pricey but YMMV

Chuck Utzman,P.E.

Ed Tornberg wrote:

This is a subjective response – sorry, no numbers.

“How much is enough?” is fairly straightforward with respect to gravity loads.

“How much is enough?” is complex, sometimes vague, esoteric, and sometimes mind-numbing with respect to lateral design.

For the majority of wood-frame buildings, because of the many unaccounted and redundant, lateral force resisting mechanisms, as well as the multiple factors of safety, I’m more and more inclined to not subject the inner workings of holdown detailing to thesis-level scrutiny. Especially for wind-governed elements.

But for a structure that doesn’t have the redundancies, say a large commerical shell building, or a “high liability risk” structure, like multilevel multifamily, I’d “do more”.

I agree with the other posts approving of straps over sheathing, especially if it’s a lightly loaded shearwall.

Ed Tornberg

Tornberg Consulting, LLC

503-551-4165

-----Original Message-----
*From:* Bill Allen, S.E. [mailto:T.W.Allen(--nospam--at)cox.net]
*Sent:* Thursday, December 23, 2004 10:34 AM
*To:* seaint(--nospam--at)seaint.org
*Subject:* Strap hold down detailing

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_*

Consulting Structural Engineers

http://www.AllenDesigns.com <http://www.allendesigns.com/>

V (949) 248-8588

	

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