Modeling of a Tank supported by a ring girder sitting on a tower (Von-Mises Stress)

[Bridge_Man]

Hi,

I am struggling with this one. I have this very old tank (100+ yr) that is still functioning, so we know its OK. I need to perform some assessments on it. Am modeling it using SAP2000

The model consists of the tank modeled as shell plates, a riser that goes up to the tank (also shell plates) a ring girder that surrounds the tank at a certain elevation (frame members connected perpendicularly to the tank through rigid links), and a tower that supports the ring girder and the tank along with it.

Global analysis and reaction all made sense and were within a small % of expected reactions from Water, Dead, wind , etc...

Here is my problem, according to one code am following, I do need to use the Von-Mises stresses and compare them to the allowable stress, the problem is my steel is 30 ksi and the stresses am getting are relatively high (goes up to 60 ksi close to concentration locations where the tower legs meet the ring girder and where the tank bottom meets the riser) I know these stresses are not realistic, otherwise, this tank wouldn't be here today. how should I be looking at this.

Appreciate the help.

 

[NRP99]

If the local peak stresses are limited to 1-2 layers of element then no need to worry. May be these are stress concentration effects due to the abrupt change in area and local yielding will redistribute the stresses and structural integrity will be maintained. This may also cause due to bad quality mesh, refine the mesh in these local zones but most probably its change in geometry effect.
Do a elastic-plastic analysis run to see whether there is yielding (plastic strain going above 0.2%) of broad region which would indicate the amount of material yielded. Assuming here that the tank is not subjected to variable loads which is not at all usual for storage tanks.

But I would still worry about the Tank's life of 100 years. Is the design life of the tank over? Then may be thorough assessments like NDT and other stuffs need to do to support FEA calculations. Basically establish fitness for service of tank for further use.

 

[FEA way]

Check Enterfea's blog and YouTube videos (especially the one titled "Linear FEA in stress design"). There you can find very interesting explanations of the differences between linear and nonlinear analyses. Basically, if you account for nonlinear material behavior (plasticity in this case), you will likely see just a small local yielding that doesn't indicate failure of the structure. Linear static analyses can be very misleading in such cases.