I'm building a frame that will receive a point load mid span (likely it won't be but want to design for worse case).

The frame is 1.5 x 0.095 square tubing, and it will be skinned with 14ga on one side. Span is 21" and frame is welded.

My question is how do you determine deflection when a beam is skinned with metal? Shouldn't this increase its strength?

I want to make it stronger than needed, but not unnecessarily heavy. I was originally debating between 0.095 and 0.065 wall tubing and 12 vs 14 GA sheeting.

I'm using 2250# for my point loading figure at center of 21" span.

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  • 1
    $\begingroup$ Clarify your dimensions, as numbers only are not clear. $\endgroup$
    – Solar Mike
    Nov 5, 2023 at 18:00
  • $\begingroup$ You can estimate a new moment of inertia for the skinned beam using the parallel axis theorem which is in any structures textbook, or wiki, no doubt. $\endgroup$ Nov 5, 2023 at 20:54
  • $\begingroup$ The sheet metal buckles easily, and given the way it is attached, it usually does not contribute much in stiffness. You won't regret by ignoring it. $\endgroup$
    – r13
    Dec 5, 2023 at 23:32

1 Answer 1


This a very rough calculation not considering the dynamic load factor of the application of the random load which could be up to three,

$$\sigma = \frac{M}{s}= \frac{2250*21}{4 *0.12953}=91,569lbs >>36000lbs$$

  • $\sigma = stress<3600 \text{ A35 steel yield}$

  • M = moment inch.lbs

  • S = 1.5x1.5 square tube section modulus Because the member is significantly under-designed we do not need to investigate the effect of galvanization. It is not going to be adding enogh strength to make the member 2.8 times mor strong.


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