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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
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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