Strength of bent tubing

[jaydigs]

I'm wanting to use a piece of bent round tubing to support a TV. The tubing would be anchored to an I beam with 2 "U" bolts and the bend would be 90 degrees. Does anyone know how to analyze this system to make sure the tube I spec is sufficient? This is probably an elementary question but all the problems I remember from school were standard beam problems. Thanks in advance!

 

[SnTMan]

jaydigs, any reasonably good strength of materials / mechanical design textbook should have a section on curved beams. See for example "Mechanical Engineering Design" by Shigley & Mischke.

Regards,

Mike

 

[ornerynorsk]

Whatever your calcs spell out as being sufficient, double the diameter if at all practical. School environment, need I say more?

 

[Stezza]

The effect of the curved beam is to increase the stress on the outside fibre and reduce it on the inside fibre.
One way this is taken into account is to use a factor to increase the stress compared to a straight beam.
For tubes "Analysis of Flight Vehicle Structures" by Bhrun states that the factor, K, to increase your stress varies from:
3.28 when the radius of curvature/tube outer radius = 1.2
1.57 when the radius of curvature/tube outer radius = 2
1.10 when the radius of curvature/tube outer radius = 8

Remember to resolve your forces and moments into the local axis system for the critical section of the tube - probably the fixed end if its a canterlever - before you calculate your stresses.

 

[Tmoose]

Have you inspected actual bends?
How close to round is the tubing going to be in the bend?

The failure mode would almost certainly be from bent tube in the "corner" buckling, with the tubing going oval, then kinking.
If it was my beloved TV, I'd be inclined to fill the tube bend with expanding foam or plaster

 

[strokersix]

Stezza, I think you must have mis-typed. Inside fiber sees higher stress.

 

[Bribyk]

Unless this is mounted very high up, I would design for a child hanging on this mount as well. Even so, a label clearly stating that it must be mounted at least X meters above floor or nearest perch level would be prudent. I can see a child hanging from this (I would've) and would hate to see the TV and mount come down on top of them.

 

[Stezza]

Yes my mistake - the higest stress is on the inside fibre.
Also I should have stated that the factor K applies to the calculated bending stress.

 

[trainguy]

Guys,

I suggest a sketch. It's not clear at all from the OP that this is a "curved beam".

tg

 

[racookpe1978]

Late response, hope this finds you (and your TV!) still safely supported.

First,consider the weight of the TV, and its value, plus the risk of a child teenager, adult putting some or all of their weight onto the TV stand. From your description of the materials (bent tube, U-bolts, I-beam) I gather you are making this for your home use, not for commercial production.

If so, then you are going to be limited by how you choose to bend the tube: this limits the diameter you can bend, the radius of the bend, and the material that you are bending. My own rolling bender, for example, cannot bend greater than 1/2 x 2 flat bar, but it can bend any practical radius. The 220Volt rolling bender at a friend's wrought iron shop can roll 3x3x1/4 tube steel or a C6 channel the hard way and never look back. A conduit bender that you will find at a Home depot or hardware store has specific radii and preset (conduit-sized) diameters of the tubing that will limit you in what you can try.

A TV and dumb-load hanging from the stand could be easily 200 lbs. If you can bend a tube by hand with only hand tools or a hardware store bender, then the weight of the TV will likely bend the TV stand you have just bent.

Try a welded bracket across the tube you have bent - It will be more reliable.