The Section modulus of a beam that I compute does not match wood design tables to determine the correct size of the wood framing member. Can anyone offer some insight?(adsbygoogle = window.adsbygoogle || []).push({});

I have to size a wooden beam. The span is 24 feet with a tributary area of 10.5 feet. The dead load value is 85psf, the live load value is 125 psf, for a total value of 210psf. I was given an Fb value of 1775. It is a simple beam equally supported on both ends.

First I calculated the weight for the total load: (10.5 ft)(210) = 2205 lbs/sf

I calculated the Moment for the total load -- wL^2 / 8 = ((2205lb/ft )(24^2))/8 = 158,760 psf

Then I calculated the section modulus S=M/Fb = (158,760 lbft (12 inches)) / 1775

I am looking at a chart of wood beam sizing entitledSection Properties of Standard Dressed Sawn Lumberbut the S value on the chart highest value is 874 in^3.

Am I correct in assuming you have to take into account the tributary area to determine the Moment? If I leave the tributary area out and just use the deal/live load of 210, I calculate a 4x14 beam. But I cant imagine that that is correct.

Thanks!

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# Homework Help: Sizing a wood beam

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