# 1" or 1 1/2" gypcrete



## McEngr (Aug 29, 2011)

I am worrying too much about a project. I designed a light framed construction (wood) building and the 2nd floor contained gypcrete floors over the top of 3/4" decking. I understand that all diaphragms are semi-rigid even if plywood or metal roof deck is used, but one could argue that gypcrete makes it absolutely rigid. I've already submitted the job with a flexible diaphragm analysis, but thought I'd get a discussion going.


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## *Ananda* (Aug 30, 2011)

McEngr said:


> I am worrying too much about a project. I designed a light framed construction (wood) building and the 2nd floor contained gypcrete floors over the top of 3/4" decking. I understand that all diaphragms are semi-rigid even if plywood or metal roof deck is used, but one could argue that gypcrete makes it absolutely rigid. I've already submitted the job with a flexible diaphragm analysis, but thought I'd get a discussion going.


The code (IBC or ASCE7) will typically permit you to idealize (pretend) the diaphragm as flexible. In reality, unless your walls have very few openings, the diaphragm, even untopped is far stiffer and acts more like a rigid body. With topping, no question it will behave more rigid. The basis for my opinion is familarity with wood shear wall and diaphragm testing, and shake table test results from the CUREE woodframe project with bare (untopped) diaphragms. Measured diaphragm deflections are usally very small compared to shear wall deflections. The flexible (tributary) method is allowed for simplicity and good historical performance that has resulted from using the assumption. You are allowed to make a more labor intensive analysis compared to the flexible analysis permitted if you think it prudent.


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## McEngr (Aug 30, 2011)

Thank you Ananda. I have a diaphragm deflection spreadsheet that I used assuming it flexible. I was able to prove (whether you agree or not) with the numbers that the diaphragm deflected more than twice the shearwalls (the shearwalls were "squatty").

Thanks again.


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