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RE: Wood Rafters, UBC tables

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Lew Midlam wrote:
But isn't gypsum board/drywall also considered a brittle material requiring
the l/360 criteria??

Reply
I agree whole-heartedly. The first area to crack is at the joint. In most
cases a paper tape is used to seal the Gypsum joints and in cases of longer
spans (such as a thirty foot clear span ceiling designed for L/240, the
joints will undoubtedly crack in time. I found that by using a nylon tape,
you can minimize the amount of cracking, but longer spans within code limits
are still very taxing to gypsum board seams. I would suggest strongly that
the deflection be considered for the worst condition of L/240 or L/360 and
that if the deflection is more than 1" allowable, the stiffness be increased
in area's of longer spans (excess of 20 feet) to prevent unsightly cracks.

Dennis Wish PE