To: "seaint(--nospam--at)seaint.org" <seaint(--nospam--at)seaint.org>
Subject: CMU Wall Supported by Plywood Sheathed Roof Diaphragm and Shearwalls
From: Monty Hart <montyh(--nospam--at)gci.net>
Date: Wed, 21 Feb 2001 21:48:22 -0900
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
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,
Associated Design Consultants, Inc.