box tube strength

[knucklehead]

Looking for the amount of weight for a 2x2x1/4 box tube 20' long vertical with a 3'x2x2x1/4 box extension on top and base is 12" 2x2x1/4 box with 6x6 gusset on 2 sides of top extension and base, noticed alot of flex in the tubing because of the length but was wandering how much would it hold before bending or breaking ??

 

[corus]

I would look at buckling of the column as the first failure mode. Treat it as a complete column first and use Euler's formulae to give you the buckling load.

 

[Lutfi]

Tube 2X2X1/4 is very slender member for the 20 foot height. I would never use suck a slender member. That may explain why it is so flexible.

I would be concerned with buckling more than bending as corus explained above. If you must use the same tube size, consider reducing its unsupported length/height.

Good luck.

 

[lutein]

I agree w/ Corus and Lutfi - 2x2 and 20' in height? Even it would carry the load w/o failure, the servicibility of it will be a problem. The deflection would be huge and the vibration (bouncing) will be a problem as well.

Anyway, if you are to use it, here;s a list of limit states whatI think you should check:
(1) Local Buckling of Plates in the tube, i.e. b/t ratio
(2) Overall Column Buckling