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Re: "Lateral Torsional Support from Gypsum Board"

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Jon,
 
(DL+Lr+W) combination was not considered in my example.  
 
I just took the load the rafter was supposed to sustain (DL+LL, downward, 1.25 factor, NO WIND), and then calculated an equivalent (WL up - 0.9*DL, NO Lr) load, with 1.33 factor, considering that the beam is laterally supported only at the ends in both cases. 
 
It came out that the corresponding wind speed is way beyond anything rationally applicable - that's all.
 
Steve Gordin, SE
Irvine CA
 
----- Original Message -----
From: Jon Brody
Sent: Tuesday, August 17, 2004 10:02 AM
Subject: Re: "Lateral Torsional Support from Gypsum Board"

Hi,

Question (for V. Steve G.):  What if the live load isn't present when
the wind blows?  Does the code really permit including the live load
for this calc.?

Jon Brody, SE
San Francisco


>
> 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 psfdead 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,oneedge 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; theequivalent net
> upward transient load will be
> qw=22 psf*1.33+0.9*qdl = 36.5 psf.
>
> Then,useCq=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 MPHfor
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 -----
> From: Bill Polhemus
> To: seaint(--nospam--at)seaint.org
> 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.)?
>


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