Gypsum Board/Drywall, Brittle Finish? 1

[LowLax]

Table 1604.3 in the IBC has a deflection limit for "brittle" finishes of L/240, and L/120 for "flexible" finishes. (This is with regard to an interior partition wall). "Brittle" and "flexible" don't seem to be defined anywhere. Is there somewhere I'm not looking that defines these terms?

I've always considered gypsum board and drywall a brittle finish for deflection purposes. Am I being too conservative?

 

[woodman88]

If it cracks or breaks at a l/121 or less than it is brittle? I do not considered wallboard to be brittle.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[shobroco]

I always thought L/360 was the limit for rigid finishes (it is in our code) and I have always included drywall in the rigid class. Drywall will certainly bend more than L/360 with no problems, but the taped joints will crack if you look at them wrong which is the same as the drywall cracking as far as the building owners/occupants are concerned.

 

[Ron]

Drywall is a flexible finish. A brittle finish would plaster on lath.

 

[LowLax]

Thanks for the replies. So 2/3 for flexible? Is there any literature to support that it performs well at l/120? Gypsum association or something?

 

[shobroco]

The Ontario Building Code disagrees with Ron, & it's based on the National Building Code of Canada as are all provincial codes.

 

[DST148]

@LowLax: I Googled and found the following info on Structural Building Components Association web site

http://www.sbcindustry.com.

For drywall assemblies it is desirable to limit deflection to L/240 and to never exceed L/120.

 

[Ron]

shobroco... l/360 is the typical limit for rigid ceilings such as plaster. Drywall is usually not limited to that criterion as noted in the OBC

 

[woodman88]

The attached link will get you to some span tables for Gypsum board walls with L/120, L/240 and L/360 allowable deflections.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[wannabeSE]

Chapter 2 in USG's Gypsum Construction Handbook states: "For drywall assemblies it is desirable to limit deflection to L/240(L = length of the span in inches) and to never exceed L/120 (L/180 in some codes). The preferred limit for veneer assemblies is L/360 and should not exceed L/240."

http://www.usg.com/resource-center/gypsum-construction-handbook.html

 

[msquared48]

I usually hold to L/240 for total loads and L/360 for live loads such as wind or seismic, sometimes even higher if it is a high end structure. The same standards I would also apply to the ceiling structure. I would also consider an exterior stucco finish to be brittle.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com