Modeling of steel circular tank - Contact between tank base and foundation/soil

[hanilouis]

Dear Colleagues,

I'm currently preparing SAP2000 model for circular steel tank with 107 meters diameter. Actually i need you advice how to model restraints for base joints knowing that tank is rested on ring beam under shell area and remaining bottom area it's rested on soil. How to calculate vertical and horizontal stiffness for vertical and horizontal springs? should i use hinged supports? How can model the friction between tank annular plate and reinforced concrete ring beam?

Thanks for your advice in advance....

 

[JStephen]

What exactly are you trying to determine with the model?

For shell analysis of larger tanks, API-650 states boundary conditions are plastic moment in the bottom plate and zero radial growth at the bottom- see 5.6.5. This assumes a 2-D model or use of Timoshenko's shell analysis, and doesn't need to include the whole tank.

At the edge of the ringwall, you may theoretically get a step or bend in the bottom plate or annular plate. This is going to be more a function of the compaction of the soil below, not the elastic properties of the soil below, and normally isn't considered in the design. Usually, the bottom is treated as a flexible membrane, bending forces are not calculated in it, and consequently, there is no limitation on deflection or bending in the bottom plate, provided it is reasonably uniform. (If not, refer to API-653).

 

[hanilouis]

Thanks a lot JStephen for your valuable feedback.....

The purpose of SAP2000 model is to check the stresses due to Aluminum dome roof reactions in shell, wind girder and bottom.

Also, there is another problem in the "already" fabricated annular plate that outer projected part is only 80mm.


Thanks again .....

 

[JStephen]

The dome roof loads in the wind girder can be analyzed to some extent using the circular ring equations from Roark's Formulas for Stress and Strain.
With sliding connections, the radial loads are usually fairly small.

You might check how the dome loads on the tank were derived- I have seen some cases where they were just an Mc/I type of distribution around the perimeter, or where only the maximum reactions were given, and in either case, there's not a lot of motivation to analyze the shell with great exactness.

 

[TLHS]

What are you actually trying to see by modelling these boundary conditions. I think you could likely simplify your analysis by not taking the bottom plate into account at all in your 3D analysis model and then maybe checking a couple of items by hand.

Your roof loads are generally going to go directly into your walls and will, in turn, be taken entirely by your ring foundation if they sit under your walls. Your bottom plate is much too flimsy to pass load very far from the bottom of your tank wall.

Unless you get uplift, or have some sort of bearing problem at the base of your wall I can't picture how your bottom plate would be materially impacted.

 

[hanilouis]

Thanks TLHS & JStephen for your feedback

Unfortunately, analysis of tank by using SAP2000 is a client request and i have to follow!!