Gypsum board - bending strength
Gypsum board - bending strength
(OP)
Is there somewhere I can find information regarding the strength of gypboard for out of plane bending strength.
Have a situation where the roof appears to have been constructed of bulb tee's and gypboard panels ((4) 1/2" sheets). The sheets are supported by bulb tees (@ 39" oc) on 2 edges and joists (@48" oc) on the other 2 edges. They want to put PV arrays on the roof with ballast blocks. Not sure it is possible but I've been on the roof and there is some strength there. I'd like to see if we can quantify that. Also lets set aside the fact that wet gypboard would obviously be not good.
I have found some properties here at the gypsum association but I'm not sure how to apply the Modulus of rupture to find an allowable bending stress (i.e. what factor of safety or even if an elastic stress distribution is reasonable to assume).
Any thoughts??
Thanks!
Have a situation where the roof appears to have been constructed of bulb tee's and gypboard panels ((4) 1/2" sheets). The sheets are supported by bulb tees (@ 39" oc) on 2 edges and joists (@48" oc) on the other 2 edges. They want to put PV arrays on the roof with ballast blocks. Not sure it is possible but I've been on the roof and there is some strength there. I'd like to see if we can quantify that. Also lets set aside the fact that wet gypboard would obviously be not good.
I have found some properties here at the gypsum association but I'm not sure how to apply the Modulus of rupture to find an allowable bending stress (i.e. what factor of safety or even if an elastic stress distribution is reasonable to assume).
Any thoughts??
Thanks!






RE: Gypsum board - bending strength
RE: Gypsum board - bending strength
RE: Gypsum board - bending strength
http://www.tectum.com/roofdeck.htm
Or perhaps something more like this- a fiber-gypsum composite board? But I don't think this product is meant to span, rather it is a substrate (cover board) for laying over metal deck or a similar structure in a built-up roof assembly.
http://www.usg.com/content/dam/USG/United%20States...
RE: Gypsum board - bending strength
I originally thought that it would be some sort of Tectum deck or something poured in place but it really appears to be individual sheets. Here are some pictures (not very great unfortunately). I could barely slip my fingers in the gap and feel that there appears to be individual sheets.
http://files.engineering.com/getfile.aspx?folder=1...
http://files.engineering.com/getfile.aspx?folder=3...
http://files.engineering.com/getfile.aspx?folder=5...
http://files.engineering.com/getfile.aspx?folder=9...
EIT
www.HowToEngineer.com
RE: Gypsum board - bending strength
RE: Gypsum board - bending strength
Thanks for the info. I found this with a quick search. It lists what appears to be a point load value for the flexural strength (4' weak direction) and lists the following ASTM tests.
3. Tested in accordance with ASTM C 473
4. Minimum requirements for ASTM C 1177
Are you (or anyone for that matter) familiar with how these tests are applied? i.e. is this a 4'x4' piece spanning one direction with a single concentrated load at center span?
Thanks again!
EIT
www.HowToEngineer.com
RE: Gypsum board - bending strength
RE: Gypsum board - bending strength
I would be leery of using that roof system for any concentrated PV panel loads, either up or down. I would want my loads to land on a stl. jst. or bridge to several of the bulb tees if I could figure out what they are made out of and good for. There was another system which wasn’t bulb tees, but rather a 2" high little steel truss shape; two round bars for the top chord, continuous bent round bar for the truss diags, and two light angles (deck support shelves) for the bot. chords; all resistance/spot welded together. I don’t remember off-hand what the infill matr’l. was, but the layout was about what you describe. I would be concerned about creep and punch through with concentrated long term loads. And remember, if it is really four layers of .5" gyp. bd., it isn’t appreciable stronger than 4 times the strength of a single sheet. It would have to be glued together at the three faying surfaces, in some way, for it to really act as a thicker composite section. Properties probably even vary from one manufacturer to another, and would even be dependant upon the orientation of the board and the grain orientation of the outer paper cover, and the strength of that paper.
RE: Gypsum board - bending strength
RE: Gypsum board - bending strength
DH - thanks for the comments. And yes if they had glued them together that would be great but I am treating them as separate sheets. I agree, I don't think we will be able to apply the point loads on the gypboard. There just is not a lot of capacity and also seems to be a lot of unknown. However I've come this far so I am curious to know what kind of strength they do have.
EIT
www.HowToEngineer.com
RE: Gypsum board - bending strength
The building is fairly old based on that style of bar joist, so this may be some manufactured roofing product that is no longer around.
Regardless, I would go with dhengr's advice and land your point loads on the main structure.
If you do some further testing or exploration and it is for sure gypsum board, you have a duty to let the owner and building department know about this condition. I don't see what kind of live load this would support, or what kind of diaphragm you could get from this...
RE: Gypsum board - bending strength
EIT
www.HowToEngineer.com
RE: Gypsum board - bending strength
You are a glutton for punishment if you pursue this very far, there are just too many unknowns and variable to ever put numbers on it, other than the original manufacturer’s testing and tabulated values, if you could find those. 60's & early 70's was the vintage of the system I was describing, and the infill would have been more like Tectum than sht. rk., I’m sure. As for a bit more investigation.... take a utility knife (a chisel or gouge) or some such to the saw cut corner in your photos, their red circle. The saw cut and hole for the duct work has done as much damage as you can do. I don’t know that exterior grade sht. rk. even existed back then. But, as for using current values for that, I would be real careful there too. Some Arch’s. use it around here to improve the fire rating on ext. walls, but it’s no good structurally as sheathing. And, I’ve seen some of it fall right off the wall when we took bad siding of the walls. It had gotten wet and didn’t dry well, it just turned to mush.
RE: Gypsum board - bending strength
RE: Gypsum board - bending strength
RE: Gypsum board - bending strength
EIT
www.HowToEngineer.com
RE: Gypsum board - bending strength
This seems like one of those systems pushed really hard by the gypsum industry, huh? Seems like steel roof deck would be just as cheap, stronger, faster and easier to install, etc.
RE: Gypsum board - bending strength
RE: Gypsum board - bending strength
Good stuff fall through it all the time.
RE: Gypsum board - bending strength