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A child's wagon: I-Beams or Rectangular Tubes? Question
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Posted by: godavesec

10/30/2003, 03:08:37

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I'm looking for a reference source that would describe in layman's terms the relative strength of metal I-Beams vs Rectangular Tubes of roughly the same size.

My current project will involve stress originating at one end of the beam or tube. Imagine the stress on the handle of a child's loaded wagon as she's pushing it uphill...should the handle be an I-beam or rectangular tube to prevent deflection?

Any suggestions for sources would be most helpful. Thanks, Dave






Modified by godavesec at Thu, Oct 30, 2003, 11:49:51

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Re: A child's wagon: I-Beams or Rectangular Tubes?
Re: A child's wagon: I-Beams or Rectangular Tubes? -- godavesec Post Reply Top of thread Forum
Posted by: daves

10/31/2003, 23:46:56

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The factor you are looking for is called "I", the moment of inertia. The following is greatly simplified, but so is your case. Don't use in people-supporting equipment without a thorough background.

See:

http://www.srl.gatech.edu/education/ME3110/design-reports/RSVP/DR4/catalog/Interactive/modulus_calc_new.html

To calculate a tube, just subtract the inside "I" from the outside "I".

Generally, the farther apart the average top and bottom materials are from each other the less the material needed.

In some cases the web or tube wall that joins the top and bottom material can be very thin. This can lead to the case where the web or wall will wrinkle and allow the top and bottom to come closer - buckling failure.

For weight I-beams are usually best - It's why I-beams are so popular in construction. If the web is divided in 2 and pushed to the sides to form a tube, it is half the web thickness and more susceptible to damage which leads to buckling.

On the other hand, if there is any twisting, the tube is much more resistant to such deflections -like 10 times or more for similar amounts of material (been 20 years since I've calculated that, so please don't gripe if I'm off a little)







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