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angle iron deflection
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Posted by: macd

06/28/2007, 00:40:45

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Can someone provide the equation for deflection of an angle iron with the load parallel to one of the arms? Any common conditions would do, but to be specific, say, supported at ends and uniform load. The underlying question realy is how much a 2.5 x 2.5 x 0.25 in. angle iron will deflect relative to a fir 2x4 (to determine if a header in a small doorway could be replaced with an angle iron I happen to have. I have found equations to compare steel I-beams to wood 2x4s but the asymmetry of the angle iron complicates the issue and I am not sure of the procedure for finding the center of area.
The alternative question would be to ask for a we source that provides wood-steel comparisons that might include angle irons. Yes, I realize that there are issues about how one supports the ends, but I am hoping that the steel (if necessary, I can get a larger piece) will be comparable to the wood that similar mounting can be used. Thank you







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Re: angle iron deflection
Re: angle iron deflection -- macd Post Reply Top of thread Forum
Posted by: swearingen

06/28/2007, 09:33:20

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It turns out that the equation for deflection of the piece of steel angle is the same as the equation for the 2x4. The difference is the numbers you use in the equation. Any of the various equations (they change by load and support situation as you mentioned) will have the terms E and I in them, the modulus of elasticity and moment of inertia, respectively.

You'll need to check, but I'll take 1.5 MMpsi for the E of your fir and 29 MMpsi for the E of the steel. The I of a 2x4 laid on its side is 1.56 while the I of a 2.5 x 2.5 x 0.25 angle with one leg vertical is 0.346. Since the equations I mentioned above are linear with respect to E and I, and deflection is inversely proportional to E and I, we can get a comparative factor for these shapes using only these two figures to get an idea of which one is better. The EI of the 2x4 (dropping insignificant 0's and units) is 1.5*1.56 = 2.34. The EI of the steel is 29*.346 = 10.0. Since these numbers would go on the bottom of the deflection equation, and the steel's number is bigger, that means the steel's deflection would be smaller, regardless of loading or support condition.

In the real world, you must pay attention to those support conditions, however, but you mentioned that you understood this fact.

Just for grins, if you stand the 2x4 up on its edge, you get an EI of 9.38 - it becomes almost as stiff as the angle...








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