A lot depends on the size of the pressure vessel. If a fuel tank is
25 by 25 inches that is 625 square inches. At 5 psi that is a force
of over 3000 pounds. We are talking about 4 inches by 14 inches
or 56 square inches or 280 pounds for the square tube plenum. The next
thing you need to take into consideration is the shape of the pressure
vessel. A sphere is the strongest followed by a cylinder with a flat
rectangular box (such as a fuel tank) the worst. A thick wall
square tube is not that much worse than a round tube or cylinder.

Paul Lamar

The easiest way to think of this problem is that the "square" corners become extra stiff, so you have a 4 inch span that is fixed at both sides. Nearly the
best possible sceneario, short of a 4" cube or sphere. Russell make that soda can 1/8 wall and then go to -10 psi. I doubt you could measure the deflection
with a micrometer.
Bill Jepson

 
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