Sealed Rectangular Tubing - Stress from temp. increase

[brgriffi]

Hello,

We're designing a structural system that will have HSS 8x4x5/16 members approximately 8ft. long spanning in between two S12x31.8 beams. The tubes are water-tight welded to the edge of the structural steel; we need these tubes to be water-tight because the structure will be partially submerged and is made from galvanized steel.

A pictorial representation:
I======I

Our client is requiring that all aspects of our system meet the requirements of a 150* temperature differential (-40F cold night to 110F hot day).

Using a simple ideal gas law calc, we found an internal gauge pressure of 5.3 psi. I'd like to calculate the stress on the beam from this but I can only find spherical and rectangular stress equations for pressure vessels.

Is there a quick way of checking to make sure our sealed tubes won't blow up/ crack welds? One of our more conservative engineers is concerned that we will need to add a venting system to the structure.

Thanks,
Brandon

 

[desertfox]

Hi brgriffi

I assume your refering to the air in the HSS section? so why not multiply the pressure inside the HSS to get a point load on the beam and stress the beam in the weak axis or have I missed something.

Also what about the physical expansion and contraction of the HSS they will impose loads possibly on the beams, assuming the beams are restrained.

desertfox

 

[JStephen]

Take a slice through the tube and conservatively treat the sides as pinned beams, and calculate the bending. If that shows to be inadequate, you could look at it in more detail. If that shows to be adequate, that should take care of it.

Your galvanizer may require vent holes for galvanizing, though- check with them and see. Normally handrail pipes and things like that have to be vented.

 

[brgriffi]

Thanks for the responses. That is how I ended up treating it and I just wanted to make sure I wasn't missing something since its been a while since looking at this stuff for me. I accounted for both pressure increase and thermal expansion in the worst-case scenario and everything looked ok.

We've got a unique situation and we're working with our galvanizer to figure out the best way to both vent the tubes but make sure they are ultimately sealed water-tight.

 

[berkshire]

brgriffi (Mechanical)
To vent for galvanizing and be water tight after, I would suggest several NPT pipe bosses be welded on the structure
To be capped after galvanizing is complete. I realise of course that you may already have thought of that.
B.E.