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RE: Ceiling seismic restraint

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Ceiling bracing, T-Bar for instance, can sway like a pendulum under
seismic loading. The most common way I have seen it done is to use wire
hangers to the grid with a spacing of about 144 sq. ft. for the sway
resistance. At these locations, 45 degrees from vertical and 90 degrees
in plan, the wires are tied to the structure above. This prevents it
from moving side to side and from falling down. At these same locations,
a rod, unistrut, or CFS are attached were the diagonals wires meet the
grid and go vertically up to the structure above-This prevents upward
movement. The strut is designed based on buckling, so if your ceiling is
well below the structure above, you may need unistrut or something
stiffer than a rod.

If it is not a t-bar ceiling, the same principles apply. Use diagonals
(either tension only with compression struts or compression diagonals as
well) to take care of the potential movement.

HTH,
-gerard

-----Original Message-----
From: Bruce Holcomb [mailto:bholcomb(--nospam--at)brpae.com] 
Sent: Monday, April 07, 2003 2:07 PM
To: Seaint (E-mail)
Subject: Ceiling seismic restraint

I have a building designed to the '99 BOCA with Av=0.13 and Aa=0.12;
Seismic Hazard Exposure Group II; Seismic Performance Category C.  I
need to specify seismic restraint of the ceiling (some lay-in and some
gypsum board).  I found some info in a drawing from ATC on the web, but
it showed only using wire restraints.  A co-worker says I need to
specify struts... basically a small stud vertical and diagonal brace at
some spacing, but he can't remember the particulars and can't find a
reference.  The reason for the struts is to avoid having the panel fall
out due to vertical movement of the ceiling grid system under seismic
loading.  Can anyone give me some guidance?


Bruce D. Holcomb, PE
Butler, Rosenbury & Partners
300 S. Jefferson, Suite 505
Springfield, MO 65806-2217
ph (417) 865-6100
fax (417) 865-6102




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