Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: "Lateral Torsional Support from Gypsum Board"

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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.)?
> 


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********