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[SEAOC] Re: Gypsum concrete roof

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This type of gypsum roof system was quite popular in the 60's,
especially for industrial building.  I had seismically upgraded
a building of this type, built in the 60's, the roof diaphragm is
2-1/2 inch gypsum with wire mesh formed on formboard spanning
between subpurlins( bulb-tee or truss tee) supported by open
web steel joists and beams.  The gypsum roof you have, can be
either class A or B (see UBC Table 26-H, or AFM 88-3 chapter 5).
We did some compression and shear tests on the gypsum concrete
in order to find out the existing capacities and properties of
the gypsum roof system and apply the results on a 3-D FEM analysis
to do the seismic upgrade design.

My conclusion was that this type of roof system is very brittle
and weak.  If you expose the roofing, as we did, you will
probably find cracks all over the roof, depending on the history
of the traffic at the roof.  Your prime concerns shall be the shear
transfer connections from the gypsum roof to the subpurlins and
to the rest of the structure.  I also conclude that this type of
roof system should not be spanned more than 100 feet for seismic
purposes.  Be aware of the wall to diaphragm connections also,
they usually don't behave that well for past earthquakes.

I hope that this will help you, good luck!

From:  Tom Chiu, SE
       Dames & Moore, Inc.