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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 -----
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
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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