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E. Kansas | In addition to the explanations above by Alberta Farmer and exit, you might read about the definition of "neutral axis". It is a rather simple concept but must be understood when learning about loading of beams/etc. Moment of inertia needs to be understood as well.
For example, a given I beam could be loaded on the top of the flange, or it could have the same load applied to the side of the flanges (50% load to each flange side, for simplicity). In each case, there is a neutral axis, and it is across a totally different part of the I beam. For flange loading on the top, the neutral axis is in the I beam web, which is the x-axis in Alberta Farmer's I beam chart. If loaded on the sides, the neutral axis would be through both flanges and the web, which is the y-axis in the chart. Because of the moments of inertia of the I beam material from a neutral axis, the given I beam will hold more when loaded on the top of the flange than on the sides of the flange.
The neutral axis, moments of inertia, etc. concepts apply to other shapes as well (such as tubing), as well as to non-symmetrical shapes. Those concepts also apply when analyzing more complicated loading (uneven/multi-directional load application, torsional loading, etc and combinations of them).
For your question about the square or rectangular tubing with the loading described, the square tubing will have higher load capacity.
Edited by billw 2/23/2013 04:07
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Square vs. rectangle? Tube bridge physics.
