Rafter Supported Roof

[Mech2325]

Hi all,
I want to ask that when the roof is a supported cone roof for example having rafters, the rafters are bolted with clips welded to the top shell course. I want to ask if there are calculations made to ensure that the tank wall thickness would be able to withstand the loads imposed on it by the rafters as they are bolted to clips welded to the shell's thin plate??

 

[JStephen]

Usually not, although that could be done. Bednar's pressure vessel handbook has an approach for line loads on a shell that could be used, although it seems overly conservative. If necessary, you could make the clips taller or change the detailing to avoid applying a moment to the tank shell in the first place.

 

[Mech2325]

Jstephen: The Bednar book is for pressure vessels (thick walled structures) while storage tank is a thinned wall structure and i wonder why does the API 650 not address this.

 

[JStephen]

API probably doesn't address this because it hasn't come up as a significant problem in the past.
Are you actually experiencing failures in that clip area? Or just pondering the possibilities?
One thing I have learned is that there is literally no end to the design and calculations that COULD be done in a given situation. If NASA got hold of that connection, they could put 50 engineers to work on it with FEM and 20 years later, they still wouldn't be done.
The derivation in the Bednar book doesn't seem to be based on thick vessel assumptions. As I mentioned, it seems overly conservative, and I would be interested in another quick approach that gave higher allowable loading.

 

[Mech2325]

Jstephen: No, i am not facing problems. Just was thinking if it might effect the tank wall but then it is uniformly supported all around the internal peripehery of top course. Plus the top course would, if bend, would bend inwards where it is supported by the rafters itself, so i dont think it will be a thing to worry about. Thanks for your comments.

 

[IFRs]

I always do a quick and basic check the tank shell capacity, and size the rafter clip accordingly.