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Re: The stand

Title: RE: The stand
Holey, moley, Batman....this is like Math class.............I love it!!!.....;-)
Mike Jacobs
Center for Advanced Technologies
Lakewood H.S.
St Pete, Fl  33705

Let me try some numbers, using bending stress as the limiting factor. (Don correct me on anything I get wrong.) The bending stress of the joists is lowest at the supports and highest in the center of the span. A conservative maximum allowable bending stress for wood might be 1000psf.

Eq. 1 : M = 12" * W

where M is the internal bending moment of the joist, W is the tributary load placed on the joist, and 12" is the assumed distance the tank is placed from the wall. We want to find W, but first we must find M.

Eq. 2 : f_b = M / S

where f_b is the allowable bending stress (1000 psi, above), M is the moment from Eq. 1, S is the 'section modulus' (for a 2X12 joist, S=31.64 in^3; for a 2X10, S=21.39 in^3).

so M = 1000 psi * 31.64 in^3 = 31,640 inch-lbs = 2,637 ft-lbs.

and W = 2,637 ft-lbs / 12" (= 1 ft.) = 2,637 lbs are allowable on a joist in bending, at 12" away from a vertical support.

That seems like an awful lot, so I'm guessing that at that distance from the wall, allowable shear is going to be the governing factor. As luck would have it, I don't have that figure available.  :-( Maybe tonight when I get home...