Fatigue and Eccentric Loading on Columns

[CDayton]

I am trying to quantify the reliability of a stand that my company uses to hold a 90,000 lb Boiler Feedwater tank. The stand has 4 columns with horizontal beams between them, and the tank is welded to a steel plate that sits on top of the columns.

Under static conditions, the stand is fine. We recently changed some piping, and now some entering steam "knocks" the piping and the tank when some of it flashes off. This causes the entire rigid structure to sway periodically.


My main concern is the columns. When the tank sways, the columns are deflected, as well as being axially loaded. Does this cause eccentrity? Should I treat the main distrubted load as an eccentric load as well? Can you point me in the right direction for the governing principles and equations to help quantify the stresses involved?

I have attached a picture for clarification.

Thanks you

 

[CDayton]

I just realized that on the drawing I posted I accidentally showed that the force in the x-direction (the one governing the swaying) is denoted by F = Asin(Bx). This is incorrect as the x should be a t. But really it's irrelevant. That is just to show that there is a non-static force acting at the top of the columns in the x-direction.

 

[dhengr]

Cdayton:
Did you try bringing up the picture you posted, hopefully as a jpg or pdf file, there is nothing there. Just brace the stand and its legs, that should settle them down. But, this new forcing system may still raise hell on the piping or the tank where it is connected to the legs.

 

[CDayton]

I'm not sure why the link isn't working. I'll try and repost it.

And yes that's what we are currently adding. It does seem to be working. We are adding cross bracing from the top of the columns down to an anchored baseplate near the adjacent column. This seems to have minimized the swaying of the structure, but the bracing has a pretty nice wobble in it.

The piping isn't connected directly to the legs, technically. But it is connected to the tank. The tank bottom, however, was corroded recently, so we cut off the bottom of it and welded it directly to the plate at the top of the stand. So now the tank and stand are basically one big entitiy.

So far the piping hasn't leaked or broken, but we will see what happens in the future.