Is gypsum board acceptable as diaphragm material?

[mmintich]

I am designing a new condo building and have minimal shear wall area on the exterior face of the building. The building has a center corridor which are shear walls and I want to transfer the roof forces back through the ceiling to the corridor walls. Is this acceptable? I am looking at the IBC code which gives design values for walls but I don't see anything about diaphragms.

Thanks.

 

[ccvasquez46]

This is a bit dated, however, the 1996 SBCCI "Standard for Hurricane Resistant Residential Construction" gives shear values for 1/2" gyp board used in horizontal diaphragm assemblies: Table 304c2 gives a recommended shear capacity of 70 pounds per linear foot.

This is based on 1/2" thick material with 5d cooler nails or 1-1/4" drywall screws at 7"spacing at the edges and 10" spacing at intermediate supports.

 

[craigory28]

For what it is worth I would not use drywall for a diaphragm (or a shear wall for that matter) in a damp climate. The humidity makes it very soft where you can't hold a picture up with a nail.

Call it over convservative but that is my two cents.

 

[DaveAtkins]

I typically don't DESIGN using GWB as a horizontal diaphragm, but I do RECOGNIZE that it is probably a pretty good one.

How else does the connection between the top of an end wall and the gable end truss above it work? Unless you brace the end wall up to the roof diaphragm, using diagonal members, the GWB ceiling is what stabilizes this joint.

DaveAtkins

 

[boo1]

In my area the nailing pattern is be hard to obtain. We have used the ceiling 5/8" gyp interior diagrams at gables, but it takes extra efforts.

If interior gyp is getting damp, consider getting a new roofer.

 

[ArneG]

Add bracing in the trusses to transfer the forces down to the interior wall, or better yet take care of it at the exterior face of the building. If your loads are minimal it shouldn't be hard to take care of it the normal way.

Its always better to take the standard way out, especially in a litigious building group like condos.

 

[STR04]

why not sheath your wall with plywood and cover with 1/4" to 1/2" gyp.

 

[samdamon]

Mmin- Are your roof trusses light gauge or wood? Strictly speaking for either material I don't think IBC recognizes using gyp in a diaphragm. The version I have (2000) lists only wood structural panels or diagonal sheathing as acceptable as a diaphragm for wood construction. IBC doesn't say anything about diaphragms for light gauge framing(Section 22). (Might this be an oversight? I think it is.) However HUD has published test data on OSB and plywood diaphragms in light gauge construction, see

http://www.toolbase.org/docs/MainNav/SteelFraming/4289_horidiaa.pdf

IBc does publish allowables for gyp-clad shear walls (table 2211.1(2). However I think it would be a stretch to extrapolate these tables for use in designing a horizontal diaphragm. The shear wall allowables are probably based on actual tests, and it would be a gamble to try and forecast what changing the orientation of the gyp sheets might do to the allowable values.

Suggest you consider using a wood structural panel for your diaphragm or the bracing suggested by ArnieG. Regards

 

[mmintich]

My original design used a limited area available on the exterior face for a beefed up shear wall segment. A site inspection however revealed that there was a large hole cut in these panels for process air for a heat pump unit installed behind it leaving the shear panel useless. I am left with designing field modifications to correct this issue. My hope is that by increasing the number of screws to the 5/8" gypsum I can provide a "simple" fix.

The trusses are wooden. I have considered bracing from the ridge to the walls but the trusses have a truss at the ceiling level combined with the roof truss members. I am not sure about fitting braces through the members to attach at the wall plate. I have also thought about cable or strap "X" bracing in the ceiling plane but again I'm not sure about if this is installable.

Thanks - I appreciate all of your ideas.

mmintich

 

[mmintich]

FYI

This topic is addressed in the 2003 version of the IBC - Section 2508.5 There is an evaluation report by the Gypsum Association at

http://www.icc-es.org/reports/pdf_files/ICC-ES/1338.pdf

mmintich

 

[Lutfi]

I would not use it in my area which is Florida. During hurricanes and in many instances roof membranes are breached. This will create water leaks. The concern is if there is ever any water intrusion on the gyp board, then you will lose any rigidity.

Regards,
Lutfi

 

[boo1]

mmintich did not indicate what loading he was designing for wind, snow, hurricane, sesmic. As Lutfi indicates materials perform differently in the field when they are wet, OSB included. Lutfi consider using roof membranes or taping the joints to keep leaks out (many insurance companies in the Gulf of Mexico area are giving discounts if they are used)

"AF&PA's Wood Frame Construction Manual (WFCM) for One- and Two-Family Dwellings, 2001 Edition outlines a procedure for calculating diaphragm capacity to brace gable endwalls against wind loads. The tabulated gypsum diaphragm capacity is 70plf, with a note that it can be increased to 90 plf when ceiling framing members are spaced 16" o.c., per ICBO Report No. 1874-89.

Table 3.15 of the WFCM actually tabulates minimum attic floor/ceiling lengths for various wind loads and building geometries. WFCM Table 2.6 outlines lateral diaphragm loads that are used in Table 3.15. The WFCM Commentary provides background calculations for all these tabulated values."