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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 -----
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!"
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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