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1. Senior Member
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May 2009
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Thanks for the tip on Northern Tools guys. Proper plans are always a good starting point.

As to the original question, if I recall (30+ years ago), strength of a beam is proportional to width x height cubed. So, double the width, get double the strength, double the height, get 8 times the strength.

Using the original scenario, a 2 x 2 with 3/16" wall gives a "factor" of 1.5.

Using a 2 x 3 with 1/8" wall gives a factor of 3.375, or 2.25X stronger. The added advantage is the 2 x 3 will actually way less (83%) of the original design. I also suspect the higher tube will translate into a stiffer structure with respect to twisting as well, but I have no idea how to calculate that.

2. Originally Posted by TripleTrophy
strength of a beam is proportional to width x height cubed. So, double the width, get double the strength, double the height, get 8 times the strength.
This is only true for solid rectangular shapes.

Since hollow sections already concentrate their cross sectional area on the outside, the advantage is somewhat less. The larger size is not being filled in with additional solid core, if you follow.

For the two examples you gave, HSS 2x2x3/16 has a section moment of inertia of 0.640 in^4 while HSS 3x2x1/8 has a section moment of inertia of 1.30 (in^4), making it almost exactly 2 times as stiff.

A little less of a change than you'd see with a solid section.

http://www.steeltubeinstitute.org/pd...n_brochure.pdf

3. Senior Member
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May 2009
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Thanks for the link bodybagger, that is good information. Three is a reason I am not a civil engineer! I was aware that the hollow section can make a difference, but not how much. After all, that is the reason some structures have cast/drilled holes in them.

4. Join Date
Sep 2008
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Bodybagger your my hero I've been looking for that type of info in a nice neat format for sometime now.

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