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RE: Strap hold down detailing

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Chuck -

FWIW, my question was general in nature and was relative to all coil straps
up to and including CMST12s.

Regarding the 2nd question, I try to provide a hold down for all calculated
uplift forces >0. Sometimes I go to wall frames to eliminate uplift forces.

Regards,

T. William (Bill) Allen, S.E. (CA #2607)	
ALLEN DESIGNS	
Consulting Structural Engineers	
http://www.AllenDesigns.com	
V (949) 248-8588	 .	 F (949) 209-2509	

-----Original Message-----
From: chuck utzman [mailto:chuckuc(--nospam--at)pacbell.net] 
Sent: Tuesday, December 28, 2004 11:41 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Strap hold down detailing

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
>
> 	
>
> .
>
> 	
>
> F (949) 209-2509
>


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