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Re: Plywood shear walls with 3x sills...

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Dave,
The following is my interpretation as a plan checker. (I'm not an engineer)


1.  The requirement is that in seismic zones 3 & 4 where "allowable shear
values" exceed 350plf, 3x sill plates and 3x framing members where panel
edges meet are required. Suppose I had a shear wall with 19/32" C-D plywood
on both sides of the wall with 10d nails at 6"o.c. The individual "tabulated
allowable shear value" is 340plf, however with two layers, the "total
allowable shear value" would be twice as high. Since the "tabulated" value
of the individual panel used is less than 350plf, would the 3x member
requirements still apply?  [By the way, I think this is a silly question
since, practically speaking, It seems that the intent is for the allowable
shear value of the "total" wall pier (i.e. regardless of whether one or two
layers of plywood is used), not just the tabulated value. I believe this
came in response to Northridge.]

The footnote clearly indicates "Allowable shear" which would be the table
values. Using the two sided would double the allowable shear value and the
3x plates would be required, unless the anchor bolts were designed at 50%
value. I just did a plan check that the engineer called for 3x sills with
AB's at 2'5" OC or 2x sills with AB's at 9" OC. Just think of all the steel
in the foundation with a 3 foot shear wall with SSTB28's at each end and
AB's at 9" OC. It almost seems at a certain point, the requirement
would/could create other problems with shear cones or most of the plate
missing with all the holes bored in the sill. The design shear (tabulated)
is good when the wall has 600PLF or more, the 3x sill would be required with
no options.

2.  How does this requirement relate to "upper-level" shear walls? They do
not have "foundation plates" nor "anchor bolts".

I talked to a local Civil who has consulted an attorney. The attorney said
the courts would/could interpretate the "abutting panels" would be any
abutting panels, hoirizontal or vertical, even though the intent was
vertically or horizonal  abutting, but not from floor to floor abutting. The
UBC should be clearer.

3.  I've had some contractors whine (no...really?) about this, saying that
they either cannot get 3x pressure-treated members or they are very
expensive. Some building officials also are not enforcing this
(no...really?). Anyone run across this?

This complaint is very common regarding the availibility of 3x sills. The
UBC does say "minimum" so 4x sills could be used. Remember, some of the
problems are the anchor bolts must be longer to maintain the proper
embedment in the concrete. The bolts must be above finished floor enough for
the sill plate, washer and nut. The standard studs need to be trimmed to
maintain consistant plate heights with the balance of the wall. The stud
nailing, toe nailed or end nailed, has to meet Code. UBC Table 23-II-B-1,
Item 8 calls for "4-8d, toenail or 2-16d end nail". Well the end nail won't
work very well with 16d's end nailed, so another problem surfaces.

Just a little food for thought. The UBC should be clarified better.
Again, just what I've seen so far.

Good luck,
Tom