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Re: "Lateral Torsional Support from Gypsum Board"

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Bill,
 
The allowable lateral values for gypsum drywall shear diaphragms can be obtained from UBC Table 25-I (pretty substantial, 100-to-250 PLF).
 
However, do you really need this lateral bracing?  Ok, for our - 1997 UBC - area:
 
The rafters support, say, 8 psf dead load (very light roof) and, say, 20 psf live load.  Using 91 NDS, a 2x10@16" rafter appears good for such load without intermediate supports at a 14-ft long span.  OK, to cross all NDS T's, one edge of 2x10 should be "held in line" - say, formally, it will be the top edge. 
 
Now, we have a 28/1.25=22 psf downward load; the equivalent net upward transient load will be
qw=22 psf*1.33+0.9*qdl = 36.5 psf.
 
Then, use Cq=0.7 (for the roof): 36.5/0.7= 52 psf wind load.  For a 20-ft house (Cq=1.3, UBC Method 2), that corresponds to 118 MPH for Exposure C (open area) or 152 MPH wind for Exposure B (suburban area). 
 
The need for lateral support at the bottom of conventional rafters appears a quite rare occasion.
 
HTH,
 
V. Steve Gordin, PhD
Registered Structural Engineer
Irvine CA
 
----- Original Message -----
Sent: Tuesday, August 17, 2004 7:31 AM
Subject: DQoD: "Lateral Torsional Support from Gypsum Board"

Okay, so here?s the Dumb Question of the Day <tm>.

 

I?ve noticed that quite often design of roof rafters are governed by the wind uplift because they are supported laterally only at the supports, or perhaps with intermediate blocking.

 

Is it common to lend lateral torsional support via the use of gypsum board, such as ceiling drywall applied directly to the underside of the rafters? If so, what are the limitations (thickness, type, etc.)?