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Determine Roof Live Load

Determine Roof Live Load

Determine Roof Live Load

A coating contractor requested information on loads for an API tank roof that needs to be painted. The tank was designed to API 650 App F, so I used F.6.1 to determine the load based on information I have. Can y'all let me know if I went about this correctly, please? I basically used the tmin calculation for internal pressure, but used the external pressure instead.

Info used:
Pext = 0.036psig
Theta = 3.58°
tnom = 0.1875"
Sd = 20000
E=0.65 since the joints are lapped
Deadload = 7.656 lb/ft²
Snow Load = 0

tmin= (Pext+Deadload+Liveload)*Radius/Cos(90°-theta)*Sd*E

Live load of 20 lb/ft² gives me a tmin of 0.059"
Live load of 90 lb/ft² gives me a tmin of 0.185" (with tnom being 0.1875")

So would it be safe to say that this tank could handle a live load of 50 lb/ft²?

Is there another equation I should use instead?

RE: Determine Roof Live Load

A roof slope of 0.75":12" (3.58°) indicates this is a supported cone roof.

It looks like you're in Louisiana, so I'd be surprised if the design live load would be much more than the Code minimum of 20 psf (5.2.1.f).  Some customers ask for 25 psf.  The value should be reported on the tank design drawings.  The limiting factor will be the strength of the rafters or centre column, not the roof plate.

The equation in F6.1 is for the roof plate thickness needed to resist internal pressure, and won't be of interest to your painting contractor.

RE: Determine Roof Live Load

For a self-supported roof, the roof will fail by buckling at a lower stress level than allowed in tension. So reversing the internal pressure design procedure doesn't work. See the sections for self-supporting roofs in the main body of the code.
For supported roofs, the load will generally be limited by rafter design.
In either case, the code doesn't address how to handle concentrated loads, those loads are assumed to be uniform.

RE: Determine Roof Live Load

JStephen's post is spot-on. Star for him.
API 650 5.2.1.f allows a minimum of 15 psf live load. Given the small tank diameter, I would not be surprised if this was a self-suppored roof but the OP needs to determine what if any structure is under the roof plate.

RE: Determine Roof Live Load

Thank you all for your help. I gave the coating contractors 20 psf for the live load.

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