How do you apply platform loads to a self supporting cone/umbrella? 1

[LeonEarle]

I have a 20' diameter tank that currently has a self-supporting cone roof with 3.65" in 12" pitch. I need to install a 12' x 15' platform above the roof and need to know how to apply the platform loads for designing the roof thickness. Platform will be 17.25 psf dead weight and API 650 says platform is to be designed with 1,000# moving concentrated load. We are planning to support with 9 supports from platform to roof.

Roof design dead load is weight of roof plate. Rood design live load is 20 psf in accordance with API 650. To apply the roof loads, do I distribute the loads over the area of the platform or calculate the loads as 9 concentrated loads?

Is there a reference for how to do this?

Thanks!!!

 

[JoeTank]

The platform will apply nine concentrated loads to the roof plate. API has no procedures for designing a roof with such loads therefore you are on your own design-wise. I recommend that you configure the roof platform so that the loads are positioned at the perimeter of the roof, directly over the shell plate.

Joe Tank

 

[JStephen]

I would also design the platform to be supported only at the perimeter of the tank. Even if the platform is smaller than the roof, just run the beams on across to the other side.

You have two issues. One is a lack of design methods for analyzing the roof under concentrated loads. (Come to think of it, you could treat it as a cylinder with radius equal to the cone roof normal radius, and use WRC 297 on it, but that would be a pain.)

Second issue is that there should be a big difference in rigidity between the shell and the roof, so that supports distributed across both would not be equally supported by both.

 

[chicopee]

Whatever point loads you have on top of tank wall, you will need internal bracing at each point load to resist lateral thrust. Depending on wall thickness, wall could also buckle at point loads, therefore, 9 external columns to support point loads may be necessary.

 

[JoeTank]

Chicopee,
I concur that stiffening of the tank wall is likley required. I normally put the vertical shell stiffeners on the outside of the shell.

Joe Tank

 

[JStephen]

In a lot of cases, you won't need lateral support at the support points due to the stiffness of the roof itself. Vertical stiffeners would depend on how high the loads were.

 

[chicopee]

JoeTank- vertical stiffeners would be a good solution obviously length would have to be investigated. About side loading, would the roof to shell ring be able to cope with wind loads on the platform and from eccentric loads between columns and tank wall? JStephen does not think additional bracing would be necessary.

 

[JoeTank]

I have never seen a case where wind loads of the platform would require any special efforts design-wise. The eccentricity of the vertical load must be accounted for in the design of the vertical shell stiffeners. In general, the length of the stiffener ends up being longer due to the eccentricity.

Joe Tank