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Re: CMU Wall Supported by Plywood Sheathed Roof Diaphragm and Shearwalls

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I cannot comment on being able to apply sheathing with only 4" of
clearance, but can offer some comments about firewalls.  The only real
comment that I can offer about the drywall w/stud option is to look at
shaft wall construction.  Shaft wall "stud" are designed to install
sheetrock/drywall from one side (at least that is what I understand).  It
is typically used at elevators and other shaft.  It is designed so that
the contractor doesn't have to setup scaffolding in the shaft.  This is
done by allowing then contractor to install from the floor level outside
the shaft.  I don't know, however, if this type of system could be adapted
to an exterior type condition like yours.

Now about firewalls...

This was actually debated/discussed about 4 to 6 months ago at my
"instigating", so you might want to check the archives as well.

The first thing is to realize that there is a difference between a
firewall and a fire separation wall.  A fire separation wall is a wall
between two spaces that is meant to stop or slow the spread of fire.  A
fire wall is essentially a fire separation wall that MUST remain standings
AFTER one side of the building has collapsed (or is one of the two
buildings has collapsed).

If you must have a firewall, then that wall between the two buildings must
be designed in a fashion that it would remain standing if either building
collapses.  Thus, if the new building falls, then the fire wall MUST
remain standing or if the existing building falls, then the fire wall MUST
remain standing.

This can typically be done one of several ways.  You can:

1) Do the fire wall as two independent walls that are tied to each
2) Do a single wall that is tied to each structure that has "break away"
connections so that one side can fall without causing the other to
collapse or bringing down the wall.  With a masonry wall with wood joists,
this has traditionally been done with a "fire cut".
3) Do a single wall that is tied to NEITHER structure that is completely
structurally independant.  This means that the wall must be able to stand
with one of the two buildings collapsed.  Depending on ones
interpretation, this could mean designing a cantilever wall with full wind
loads.  There is, however, some CMU industry documents that suggest that
the wall should be designed for just an internal pressure (i.e. 5 or 10

Therefore, in your case, if the existing structures exterior wall is not
rated for the full fire separation, then the new wall would need to remain
standing if either the old or new structure collapses.

Now, I will add my disclaimer.  The BOCA code is used around here.  I am
not sure if the UBC or SBC have the same requirements or not, but I would
bet that they do.  Regardless, you will want to take a close look at the
firewall portion of the code.

Hope that helps,


On Wed, 21 Feb 2001, Monty Hart wrote:

> I'm working on a proposed one story 50' x 80'  warehouse in seismic zone
> 4.  Exterior walls are 8" metal studs sheathed with plywood and are 23'
> high to the roof deck.  Roof is open web wood joists with a plywood roof
> diaphragm.  One of the 80' long walls is adjacent to (with a 4" gap) the
> CMU wall of an existing building.   The existing building and the new
> building are on different lots, so the buildings can not be connected.
> The new wall needs to be a firewall, and must be finished on the
> interior and exterior sides, with only a 4" clearance for installing the
> exterior sheathing.  I would prefer to try sheathing a stud wall flat on
> the slab and then tilting it up, or installing temporary bracing, sheath
> the studs in a vertical position about 4 ft from the existing wall and
> then slide the new wall into place.   Neither I nor the contractor has
> seen a metal stud wall erected in this way, so the contractor is
> insisting on a single 80' long, 23' high CMU wall.  I am extremely
> reluctant to support such a wall with a plywood sheathed roof diaphragm
> supported by plywood sheathed shear walls because of the potential for
> excessive deflection at the top of the CMU wall, which may result in
> cracking of the concrete block.    I have not yet calced the deflection,
> but I assume it would be in the 1" to 2" range at the top of the CMU
> wall.
> The contractor says he has seen other buildings constructed with a
> single CMU fire wall.  Unfortunately, I can not find any section of the
> '97 ICBO Code that would prevent it or even address the allowable
> out-of-plane deflection for a CMU wall.   I would appreciate any
> comments regarding applicable code requirements or experience with
> failures of similarly constructed buildings.  I would also like any
> recommendations about installing plywood sheathing on a metal stud wall
> with only 4" of clearance.
> Thanks in advance for any comments,
> Monty Hart
> Associated Design Consultants, Inc.
> Anchorage, Alaska