API 650 Live Loads and Roof Frangibility 1

[jeffberube]

Do increased live loads affect roof frangibility?

Looking at the equation for the cross sectional area (A = W / (1390 tanθ)) of the roof-to-shell connection, it doesn't seem to take into account the weight of the roof as the W equals total weight of the shell and any framing (but not roof plates) supported by the shell and roof (N).

Wouldn't having a snow load on the roof affect the frangibility of the joint? Or am I looking at this wrong?

 

[JStephen]

In API-650, frangibility is considered for the fire-exposure case, assuming the tank is engulfed by fire, and boiling off vapor faster than it can vent it.
I think the assumption is that in that case, there won't be any snow on the roof, and there won't be any workers up there working.
(Presumably, if you DID have snow on the roof in that case, you could re-condense a lot of that boiled vapor onto the roof plate, though).
A frangible roof might also be desirable in the event of internal combustion or explosion, but that is not the design condition being evaluated in that equation.

Informative video here:

https://www.youtube.com/watch?v=DECyAxDk88U

 

[jeffberube]

@JStephen Makes sense. Thank you.