Return to index: [Subject] [Thread] [Date] [Author]

Re: drywall type shearwalls- l

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Dennis:

The intent of the UBC shear wall provision for applying wood structural panels
over gypsum sheathing was to cover the application of plywood siding (now
expanded to include wood structural panels such as L-P OSB siding and other
types of approved wood structural panel siding) on the exterior face of one-hour
fire-rated exterior walls (fire-rated from both sides). This would be important
for some applications in multi-family housing, and for residences located in
regions with a wildfire hazard. The construction permits a wall with
"two-layers" (one layer of gypsum sheathing and an exterior facing of wood
structural panels, in this case).  The fire-rated construction is permitted by a
generic listing of an exterior fire-rated wall, fire-rated for one-hour from
both sides, in the Gypsum Assn. Fire Resistance Design Manual (GA File WP8105)

For shear walls, the application was tested by APA in 1965 and reported in APA
Report 105 (now summarized in APA Report 154). It has not been subjected to
cyclic shear wall tests, although I lobbied COLA to do this as part of their
cyclic shear wall test program at UC-Irvine.

You are correct that a fire from the exterior side would first consume the
plywood siding (in a short time). However, the required fire resistance would be
provided by the gypsum sheathing.  It is presumed that the wood structural panel
sidng and gypsum sheathing would be replaced after the fire, along with any
necessary structural framing repairs. It is also presumed that an earthquake or
wind storm would not be coincident with the fire.

A better structural solution (seismic and wind loads) is to apply the wood
structural panel sheathing first, then apply the gypsum sheathing over the wood
structural panel sheathing. Then, exterior facing is applied over the gypsum
sheathing. This assembly protects the seismic and wind resistance of the
structure, even after a fire exposure. Such an assembly has been fire-tested
from both sides of the wall by UL, in tests sponsored by APA (UL Design U344).
The regular shear wall values for wood structural panels apply in this case. The
construction was also included in cyclic shear wall tests conducted by UC-Irvine
as part of the COLA shear wall test series (it performed very well).

For shear walls where fire exposure is from the interior side only, as now
permitted for some applications in the IBC (following a long-time provision in
the NBC and SBC, and National Building Code of Canada), the applicable UL
fire-rated design is U356. It provides a number of options for exterior facings
over wood structural panel sheathing, for combined shear wall/fire-rated wall
applications. This design would not be appropriate for wildfire zones, however,
since the exterior side of the wall also would need a one-hour fire rating. In
that case, U344 would be the construction recommended (although more expensive
because of the three-layers of material on the exterior face).

Hope this addresses your question.

John Rose/Tacoma, WA (APA retired)

Structuralist wrote:

> While the end of Scott's message provides a reasonable solution, what fails
> me is the rationale that would allow plywood to be used over Gypsum. Not
> only is the shear greatly reduced, I would assume that the requirement is to
> protect the structural integrity of the wall from the flame or heat applied
> from outside the wall rather than within the wall cavity. What purpose would
> there be to providing a flammable material (plywood) over a fire barrier
> (Gypsum) only to protect the stud wall within, when the strength of the
> system is maintained by applying the plywood directly to the studs and then
> protecting the wall assembly from fire by installation of the Gypsum over
> the plywood and studs.
> I'm confused because the wording of the code seems convoluted at best and I
> would question if this was the original intention and what purpose it
> serves.
>
> Maybe I have this backward, but I am making this argument from the section
> of Scott's message that states:
> "The way the UBC is now, for a rated exterior wall you could put plywood
> over the
> exterior sheetrock as a finish but sheetrock would be required both sides.
> IBC 704.5 allows exterior rated wall assemblies, farther than 5 feet from
> property lines, to be rated from the inside, if you can find such an
> assembly.  This means if you can find an exterior assembly that has plywood
> on the outside, that is rated from inside, it is OK."
>
> I don't understand where this would ever be used?
>
> Dennis S. Wish, PE
>
> -----Original Message-----
> From: Haan, Scott M. [mailto:HaanSM(--nospam--at)ci.anchorage.ak.us]
> Sent: Friday, March 09, 2001 8:40 AM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: RE: drywall type shearwalls- l
>
> When the "type of construction" indicated in UBC Table 6-A or IBC Table 601
> requires wall assemblies to be fire rated then the walls are required to be
> tested fire-rated assemblies unless a "fire resistive sprinkler
> substitution" is used per UBC 508 or IBC Table 601 footnote d.  Some types
> of separation walls are not allowed to use this "fire resistive
> substitution".
>
> IBC 704.5 indicates if an exterior wall is less than 5 feet from a property
> line then the wall needs to be rated from both sides and if it is farther
> than 5' from a property line then the wall can be rated from the inside.
> The UBC requires walls to be rated from both sides regardless.  The way the
> UBC is now, for a rated exterior wall you could put plywood over the
> exterior sheetrock as a finish but sheetrock would be required both sides.
> IBC 704.5 allows exterior rated wall assemblies, farther than 5 feet from
> property lines, to be rated from the inside, if you can find such an
> assembly.  This means if you can find an exterior assembly that has plywood
> on the outside, that is rated from inside, it is OK.
>
> UBC Table 7-B footnote 17 and IBC Table 719.1 footnote L indicates: "Wood
> structural panels may be installed between the fire protection and the wood
> studs on either the interior or the exterior of wood frame assemblies in
> this table, provided the length of the fasteners used to attach the fire
> protection are increased by an amount at least equal to the thickness of the
> wood structural panel."  This means for rated wood stud walls you can put
> plywood under the sheetrock.  This exception is for wood stud walls and
> unless you find a metal stud wall that meets this exception you should not
> place plywood under sheetrock on metal stud walls!!!!!!
>
> Scott M Haan  P.E.
> Plan Review Engineer
> Building Safety Division http://muni.org/building,
> Development Services Department,
> Municipality of Anchorage
> phone: 907-343-8183   fax: 907-249-7399
>
> > -----Original Message-----
> > From: Mark Gilligan [SMTP:MarkKGilligan(--nospam--at)compuserve.com]
> > Sent: Friday, March 09, 2001 6:41 AM
> > To:   INTERNET:seaint(--nospam--at)seaint.org
> > Subject:      Re: drywall  type shearwalls-  l
> >
> > What is not clear is why does the fire rating prevent the use of plywood
> > sheathing.  In California it is not common practice to use gypsum for
> > shear
> > walls yet we still build buildings with plywood sheathing.  What are we
> > doing wrong?
> >
> > What fire rating are you dealing with?  Is it that plywood could be used
> > but since that may require another layer of gypsum that it is cheaper to
> > use gypsum board for the shear wall?
> >
> > Mark Gilligan
> >
> >
> > +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >
> > >
> > We use gypsum for lateral force resisting where fire rating prevents the
> > use of plywood sheathing.  For design help the Light Gauge Steel Engineers
> > Association publishes data for bacing with gypsum, it references table 3
> > 1997 UBC and allow for two sided application values up to 850 plf with
> > screws at 4/4.   this 850 must be divided by the recommended safety factor
> > of 2.5 yeilding 340 plf.  LGSEA has a TECHNICAL NOTE booklet call Vertical
> > LAtera Force Resisiting System with this data and desing examples.
> >
> >
> > Jeff Fertich, PE
> > Gettysburg, PA
> > <
> >
> >
>
>