Acceptable external pressure for square tube

[billhowcroft]

Not a true piping question but I have been asked to look at a small fabrication in 50mm square by 3mm wall structural hollow section that is to be immersed to about 1500 ft seawater ie assumed 50 bar hydrostatic pressure.

If the calcs can show the tubing will resist hydrostatic collapse with a reasonable margin we don't have to put drain holes in and the inside won't corrode as fast. I can find information on round tubes and a round tube 50 o/d by 3 wall is OK.

Can someone point me to a source of information for calculating the square tube collapse load?

 

[C2it]

Looks like a simple 2D finite element job to me.

 

[BigInch]

"Roark's Formulas for Stresses and Strains" covers that one.
If you're in the business for the long haul, you must get a copy of that book.

Independent events are seldomly independent.

 

[billhowcroft]

Not sure simple 2-D FE will take on this problem which is one of instability rather than stress level. Have Roark 7th and couldn't find square tube in the chapter 15 on instability.

Found a thread on internal pressures in rectangular tubing (thread 794-123788). The piece on vehicle frame air tanks reminded me of a colleague's story of how testing the internal storage tanks in the structure of a large earth-moving machine led to a fatality when the tester used the wrong airline pressure for leak testing - good place to use a barometric leg.

Any last advice on the external pressure calculation before I get the draffy to redraw the frame in round tubing?

 

[BigInch]

Sorry. thought for sure I saw it there.

Yes. O-beams are the most efficient anyway. Do it.

Independent events are seldomly independent.