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Re: gyp ceiling diaphragms

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Dan Merrick wrote:

"Look at ICBO ER-1874

It has shear values for gyp walls with screws and values for gyp 
ceilings! I've never heard of such a thing. Rarer than polka-dot paint."



Dan, if you look closely at the values given in Table 1 of ER-1874, they
must be reduced by 50% in UBC Seismic Zones 3 & 4 and by 50% in IBC Seismic
Design Category D, E and F for diaphrams(Footnote 1, Table 1) [resulting in
loads capacity of 35 to 45 lbs/ft.].  For shear walls (vertical diaphrams)
the above limitations apply except that they may not be used in Seismic
Design Categories E and F (Footnote 1, Table 2)[resulting in load capacity
per foot of 35 to 71 lbs/ft, where usuable].  The resulting low values make
the use of Gyp Board, except in relatively minor structures, not really
feasible. Most architects I know think the ideal building is all windows <G>
resulting in high loads per foot. I think it remains an allowable option by
ER Report as the result of the desire of gypsum manufacturers to think they
provide a wind/seismic product, but with limited usefulness.

I don't know the impact on the lower force zones, but here in California
(UBC zones 3 & 4), it makes little sense to rely on gyp. board shear walls
or diaphragms.

Regards,
Bill Cain, SE
Oakland CA

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