This has been a problem that has been nagging me for years. Every once in a while I run into a renovation project where the existing building uses Gypcrete for roofing decking/diaphragm transfer to the LFRS. Are there any published values of diaphragm strength for such systems?
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Gypcrete Shear Values
- Thread starter SteelPE
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I wasn't under the impression that gypcrete could be used as a structural diaphragm. I'm curious, how do you know that was done with the existing buildings that you mentioned? Is it listed as such in the design criteria listed on the plans?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
Every old gypsum roof system I've ever seen has explicit steel truss framing around the perimeter (in the horizontal plane) to create a steel frame that doesn't need a diaphragm. Usually these horizontal trusses take up the full perimeter framing bays. So the older gypsum poured roof systems didn't count on any diaphragm behavior.
Not sure specifically about Gypcrete. If you go to the Maxxon site ( there's no mention of "diaphragm" there.
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Not sure specifically about Gypcrete. If you go to the Maxxon site ( there's no mention of "diaphragm" there.
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The last project with poured gyp that I worked on, I found the "Design Data for Poured Gypsum Roof Decks" published by the Gypsum Roof Deck Foundation, maybe in 1973.
They say, "gypsum roof decks have a high ability to resist lateral forces. Based upon extensive test data, horizontal diaphragm shear values in excess of 1100 lb per ft are allowed by each of the following regulatory jurisdications..." they list IBC, LA, San Fran, and Army/Navy/Air force.
They also say, "typical details of deck construction and methods of shear transfer to vertical elements are shown on p13:.
(this is my first time uploading a file to the site, let me know if it didn't work)
They say, "gypsum roof decks have a high ability to resist lateral forces. Based upon extensive test data, horizontal diaphragm shear values in excess of 1100 lb per ft are allowed by each of the following regulatory jurisdications..." they list IBC, LA, San Fran, and Army/Navy/Air force.
They also say, "typical details of deck construction and methods of shear transfer to vertical elements are shown on p13:.
(this is my first time uploading a file to the site, let me know if it didn't work)
Once20036 - not IBC but ICBO. The IBC (as published by the ICC) didn't happen until the IBC 2000 edition.
Despite what the document says, I would have a hard time depending on a gypsum deck system in a high seismic area. It just seems to me that a gypsum anything would have very little ductility and post-elastic (i.e. no elastic) capacity. Just based on my gut feel, though.
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Despite what the document says, I would have a hard time depending on a gypsum deck system in a high seismic area. It just seems to me that a gypsum anything would have very little ductility and post-elastic (i.e. no elastic) capacity. Just based on my gut feel, though.
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