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Hazelton, Kansas | Kelly,
Ok, i may not understand your question, but here goes.
Let's say you have a simple beam, with a sawhorse at each end, and a dead weight hanging from the middle. Let's further say, the beam is 2x4x1/4 rectangular box tubing, on edge.
The appropriate I value is (2x4x4x4/12) - (1.5x3.5x3.5x3.5/12) = 5.31in^4
Now, the load in the center of the span is enough to either fail the beam or cause it to deflect more than we want. We need to beef up the beam.
I say, plate the top and bottom with 2x1/4 strip. You say, plate the sides with 4x1/4 strip. Who's right?
My I = ( 2x4.5x4.5x4.5/12) - (1.5x3.5x3.5x3.5/12) = 9.82in^4
Your I = (2.5x4x4x4/12) - (1.5x3.5x3.5x3.5/12) = 7.97in^4
So...my way used half as much steel and gained more strength than yours. You don't have to believe me...but if you go to the trouble to verify this in your shop, I really like Bud Light.
Now, if the strips were operating in isolation, and not attached to or supported by the tube, your way would be right. But, when welded to the tube, my way is equivalent to thickening the flange of an I beam, while your way is equivalent to thickening the web.
Does that help? Seriously, try it with some light stuff if you aren't convinced. Just take a 2x2 and weld strips on it. Then try it both ways, by rotating it 90 degrees and applying the same load, and measuring the deflection.
FWIW
MDS
Edit...and Ben in the Basin's vote doesn't count, cause he learned that at the institution where I taught...sorry, Ben...couldn't resist...:-)
Edited by crowbar 2/23/2013 15:19
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Square vs. rectangle? Tube bridge physics.
