# Rigid frame deflection and material selection

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.

• Clarify your dimensions, as numbers only are not clear. Commented Nov 5, 2023 at 18:00
• 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. Commented Nov 5, 2023 at 20:54
• 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.
– r13
Commented Dec 5, 2023 at 23:32

$$\sigma = \frac{M}{s}= \frac{2250*21}{4 *0.12953}=91,569lbs >>36000lbs$$
• $$\sigma = stress<3600 \text{ A35 steel yield}$$