Gypsum board - bending strength 2

[RFreund]

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!

EIT

http://www.HowToEngineer.com

 

[Splitrings]

Modulus of Rupture is calculated assuming a linear stress distribution. The material is tested in the lab to determine the ultimate moment. Then they back calculate the modulus of rupture assuming the linear stress distribution (modulus of rupture = Mc/I). So, there is no safety factor. You'd have to apply one of your choice.

 

[Splitrings]

If the material doesn't have a linear stress distribution then the modulus of rupture value is going to be higher that the materials actual ultimate bending stress. This is why some times the modulus of rupture is referred to as a fictitious stress.

 

[a2mfk]

Are you sure its gypsum board and not something like Tectum? Do you have pictures?

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/...erature/securock-roof-board-submittal-RF5.pdf

 

[RFreund]

Thanks for the information.

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/getfil...70-98dd-f9e4f61b875e&file=WP_20140131_007.jpg

http://files.engineering.com/getfil...eb-886e-d4f95029218e&file=WP_20140131_019.jpg

http://files.engineering.com/getfil...69-b2ea-9d34e6a01ea2&file=WP_20140131_025.jpg

http://files.engineering.com/getfil...82fc-136a0104ba53&file=WP_20140131_025_MU.jpg

EIT

http://www.HowToEngineer.com

 

[Splitrings]

It looks similar to what I have seen called exterior gypsum sheathing. Its used on our fire station for exterior wall sheathing.

 

[RFreund]

@Splitrings
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

http://www.HowToEngineer.com

 

[Splitrings]

ASTM C473 specifies two test methods for testing bending strength; Method A and Method B. Both test a sheet supported by its long sides and its short sides. I'll pull the ASTM and look closer.

 

[dhengr]

RFreund:
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.

 

[Splitrings]

ASTM C473 uses a 12" by 16" specimen. Its tested both directions. Method A is loaded at a uniform rate and Method B the head of the loading device moves?? I am not so clear on what they are referring to.

 

[RFreund]

Splitrings - thanks I appreciate the info, unfortunately I do not have these references.

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

http://www.HowToEngineer.com

 

[a2mfk]

It is blowing my mind anyone would use 4 layers of any kind of gypsum board for a roof deck. It doesn't even make sense economically. Are you sure there is no other roof decking and this wasn't installed for some odd reason, maybe fire protection?

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...

 

[RFreund]

@a2mfk - could be a different product. We will see how far they want to go and as of now we are not using the deck to support any load. They are already aware of the condition as they told me this is what to expect; it's just that I really didn't really expect them to be correct. I thought (and there is still a chance) that it is some other sort of system. It is somewhat old - 1960's - 70's is the guess.

EIT

http://www.HowToEngineer.com

 

[dhengr]

Rfreund:
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.

 

[StructuralEd]

Around here, poured gypsum roof decks were very popular in the 60s and were generally placed on fiberglas insulation board or gypsum board, both of which were used as forms for the poured material.

 

[wannabeSE]

I have seen a few poured gypsum roofs on penthouses. But, the bulb tees have all been at 24" OC. A few years ago, JRGse uploaded an old but good design document for poured gypsum roofs. See

http://www.eng-tips.com/viewthread.cfm?qid=208425

 

[RFreund]

Thanks for the info!

EIT

http://www.HowToEngineer.com

 

[a2mfk]

I looked at the PDF, thanks wannabe. I would say that looks like the same system. The table seems to indicate (3) layers of gypsum board for up to 48" spans if I am reading that correctly. That would explain the 4 sheets of gyp you encountered.

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.

 

[charliealphabravo]

I would contact technical support at the Gypsum Association. They might recognize the system and give you some background. It's been a few years since I had to talk to them but I remember them being very helpful.

 

[TDIengineer]

Gypsum Struct 1.

Good stuff fall through it all the time.