Dear all,
When must I model my tower or vertical vessel as a pipe not rigid?
When is it recommended to do that?
Regards,

You shouldn't ever model a tower as a rigid element. First it's not rigid. Second, a rigid won't deflect under wind load, while your tower will.

Really Thanks for your guidance.
But I found towers in our company's CAESAR II files of projects as rigid.
What made me ask this question was that I saw a tower in a plant moving in the wind, So I thaught It is wrong to model it with rigid elements.
You mean all of the files I have seen before are wrong?
or there is a limit for height?
In fact i'm novice and need guidance

by not allowing the tower to move you (by using Rigid elements) you under report the actual loads at some of the boundary conditions, WAM wind anchor motion may be a significant load.

Are they wrong... well if I was checking the files they wouldn't be rigid elements when my checking was complete, my comments would be to make them agree with the vessels calculated wind displacements.

Rigid elements are often used model thermal growth of piping connections such as pump nozzles and even vessel nozzles.

If, however, other loads (such as wind) are included in the piping evaluation, then, identifying the vessel as "rigid" may be inaccurate for the reasons noted in this thread.

I can't say much about the accuracy of the vessel response as the effective stiffness of the entire assembly will incorporate all connected piping and, in the case of wind loads, the load will be a function of the total wind area - including platforms and the like. I doubt that is a typical approach to modeling vessels though.