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Re: 2x2 Plate Washers for WSP Shear Walls (UBC)

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Amazing isn't it?  But seriously, the basic thought behind seismic zone 3 and 4 is the same as the zones themselves, the demand is higher with a far greater likelihood of dynamic cyclical loading into the inelastic range.  The same is true of masonry requirements, ductile steel and ductile concrete design.  The code has to have a cut off somewhere, and the magic arbitrary line is from zone 2 to 3.  The fact is that if I was designing for high wind conditions I would probably include the washers as well.  The local contractors would think I've lost it, but it wouldn't be the first time.  Wind loading is distinctly different from seismic, but good detailing is still good detailing.  A wood wall taken to the maximum is going to behave the same way and have the same problems without knowing if the forces are from wind or earthquake, and if pushed hard enough will develop the same failure mode.  The reason we have not seen the performance problem in wind design is probably because wind tends to be an instantaneous peak load relaxing to zero rather than a sustained (though brief) cycle of frequently intensifying alternating maximum load conditions.  That and in high wind the roof usually blows off before the wall can reach the failure point :-) (just kidding)
 
----- Original Message -----
From: Bill Allen
Sent: Tuesday, August 26, 2003 12:21 PM
Subject: RE: 2x2 Plate Washers for WSP Shear Walls (UBC)

Paul -
 
And this phenomena only occurs in Seismic Zones 3 & 4?
 
T. William (Bill) Allen, S.E. (CA #2607)
V/F (949) 248-8588
San Juan Capistrano, CA
-----Original Message-----
From: Paul Feather [mailto:pfeather(--nospam--at)SE-Solutions.net]
Sent: Tuesday, August 26, 2003 11:14 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: 2x2 Plate Washers for WSP Shear Walls (UBC)

Bill,
 
Examination of failure modes on shear wall sill plates indicated that as the holdown and endpost deflect, there is a general tension field developed along the wall which creates tension on the sill plate in cross grain bending relative to the anchor bolts.  The behavior is similar to the observed failure in masonry or concrete walls as well, principal tensile forces are on the diagonal with internal resolution of the principal shear stresses.  This tension field action was causing sill plates to split along the bolt line thereby losing shear capacity at the bolts.  The idealized analysis model we have always used, direct shear with all tension and compressive forces concentrated at the ends does not adequately account for the actual conditions.
 
A rather extensive testing program was conducted, I believe at UC Irvine, that demonstrated the 2" plate washer was an adequate measure to prevent premature splitting of the sill plate and allow our traditional analysis model to remain valid.  I am sure there is probably more information available on the subject then we would ever have time to actually read.
 
The shear wall table in chapter 23 is actually closer to the "real" requirement as a result of tests.  Chapter 18, which is horribly out-dated and the "conventional construction" catch all for foundation design, was also updated to require plate washers regardless of application.  The two committees probably didn't talk to each other.  It is one of those code things, like if a two pound hammer is required under this condition, we will make it required under all conditions so they can't screw it up.
 
Personally I am a bit conservative with wood shear wall design.  I prefer to maximize the wall lengths where possible and keep demand below the 3" E.N. threshold, preferably 4". 
 
----- Original Message -----
From: Bill Allen
Sent: Tuesday, August 26, 2003 10:22 AM
Subject: RE: 2x2 Plate Washers for WSP Shear Walls (UBC)

Seth-
 
Wouldn't cupping only occur when the anchor is in tension?
 
These anchors (at least the ones I am talking about) are (designed) as shear anchors only. I make other provisions for the tension loads (i.e., hold down anchors).
 
Regards,
 
T. William (Bill) Allen, S.E. (CA #2607)
 V/F (949) 248-8588
San Juan Capistrano, CA
"Lord, I can't go yet. I've still got one more detail to do!"
 
 
-----Original Message-----
From: Seth W. Cutler [mailto:seth(--nospam--at)rlmorrisonengr.com]
Sent: Tuesday, August 26, 2003 10:06 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: 2x2 Plate Washers for WSP Shear Walls (UBC)

The reason the code requires square washers is to prevent cupping of round washers.  The square corners provide resistance to this.

I recently had a framing detail like this and I just decided they could notch the sheathing and the sole plate to accommodate the anchor bolt.  I used a 2x plate since the hangers I called out only required shorts for nailing.  If they even cut out the sole plate at the anchor bolt I'm not too worried because the plate nailing will take care of the shear transfer.  The contractor could even extend the anchor bolt all the way thru the sheathing up thru the sole plate.  I figured I show what I want and let the contractor figure out the best way to accomodate it.  If there are questions, we can work through them.

At 09:23 AM 8/26/2003, you wrote:
Bill,

Under chapter 18, the plate washers are required at all bolts.  (1806.6.1)

I do not know of any approved alternate round washer.  Since you are nailing
directly to the plate, I have seen 4x plates with the entire top cross
section dapped out for the bolts and washers.  A 3" gap in the plate surface
isn't such a big deal.  Alternatively I have seen the 3x plate with bolts
and then an additional level of flat 2x blocking between the bolts prior to
setting the joist hangers.  Seems to be easier than dapping the plate with
less room for defect.

HTH

Paul Feather PE, SE
pfeather(--nospam--at)SE-Solutions.net
www.SE-Solutions.net
----- Original Message -----
From: "Bill Allen" <T.W.Allen(--nospam--at)cox.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, August 26, 2003 8:57 AM
Subject: 2x2 Plate Washers for WSP Shear Walls (UBC)


> Ref: UBC Table 23-II-I-1 (p. 2-288), footnote 3
>
> My situation:
> Visualize a concrete stem wall, 3x sill plate and raised floor framing.
> The floor joists are framed flush, hung off the 3x sill plate with
> hangers. Sheathing over sill plate. Stud wall framed above. Based on
> this condition, the anchor bolts have to be dapped into the 3x (or 4x)
> sill plate.
>
> Using a 4x sill plate, if the dap is such that I still have 2-1/2" of
> "meat" left (I know this is difficult for the nut on a 5/8" anchor bolt
> plus a few threads, but bear with me), then I (believe I) still have a
> "qualified" 3x sill plate.
>
> The problem comes in getting in those *damn* square washers. The
> diagonal on a 2x2 washer is almost 3", not leaving much edge or
> tolerance in a 4" (nom.) stud wall.
>
> Question 1: Is there a code provision for an alternate, equivalent round
> washer? If a 2-3/8" diameter washer is equivalent, then a 2-1/2"
> diameter hole saw would work.
>
> Since the Beginning of Time (i.e., publication of the 1997 UBC), I have
> been under the impression that these washers are required for all
> anchorbolts in shear walls (in Seismic Zones 3 and 4 which are the only
> ones I care about). In reading Footnote 3, it almost appears that the
> washers are only required if one is trying to use a 2x sill plate for
> shear walls with less than 600 PLF.
>
> Question 2: Is this just (more) cumbersome code wording or is it true
> that these plate washers are _only_ required when one is trying to use a
> 2x sill plate on shear walls designed for less than 600 PLF in Seismic
> Zones 3 and 4?
>
> That's all for now. I have to go back and continue reading the Statement
> of Qualifications for the gubernatorial candidates. Sheesh!
>
> Regards,
>
> T. William (Bill) Allen, S.E. (CA #2607)
> V/F (949) 248-8588
> San Juan Capistrano, CA
>
> "Lord, I can't go yet. I've still got one more detail to do!"
>
>
>
>
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