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Beam strength question Question
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Posted by: mattw

06/14/2003, 18:07:40

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I am trying to span a 20-foot horizontal length with vertical supports only at the ends. Load will be uniform and minimal (pedestrian traffic). Can somebody help me figure out what material to use so that I get minimal deflection? Early design stages so open to opinions and ideas. I have been considering Angle Steel or perhaps Aluminum I-Beam. Thanks!

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Re: Beam strength question
Re: Beam strength question -- mattw Post Reply Top of thread Forum
Posted by: jmt

06/16/2003, 13:23:26

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You need to establish a maximum deflection that you can handle. A general rule of thumb for deflection is length/360 (in this case = 0.67").  You also need to know your uniform load. Calculate deflection by the following formula: 5*w*L^4/(384EI) where w = uniform load in pounds per inch. L = beam length = 12 * 20 = 240 inches. E = Youngs modulus (psi) and I = moment of inertia (in^4).

I would go with a steel I-beam.  E = 30e6 psi. Solve for required I and go to the steel handbook and pick a section that meets the minimum required I. 

Don't forget to include self weight of the beam in the deflection calc. It could be a significant portion of the total uniform load.

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