I am currently do stress analysis for reaction loops for a chimical plant. Bases on their existing plant arrangement, spring supports have to be put under a pump to absorb thermal expansion caused by the vertical discharge pipe. It is obvious that those springs under the pump have been compressed by the pump at installation stage. That's to say they are preloaded. I use some rigids to model the pump and put some springs under the given points. But what can I do to tell CAESAR that those springs have been compressed in respect to the pump weight because I found CAESAR just told there are 0.3mm displacement at cold installation? It is apparent that something was doing worngly.

Regards and thanks

To properly define a spring support in CAESAR II you must provide both spring rate and preload. It sounds to me as if you are entering only a spring rate. Check the Hanger box rather than the Restraint box to define the spring. At the bottom of the input window you can specify both the existing spring rate and the "theoretical cold load". Be sure to include the spring preload (+H) in the analysis by either clicking on "recommended" loads or typing in H in the proper load cases.

Dave, thanks a lot. I think I should describe my modeling for the pump.
I used rigids to represent the whole pump as following:
1. A zero weight rigid for the suction flange face (node 170) to pump body center (node 180).
2. A zero weight rigid for the pump body center (node 180) to discharge flange face (node 190).
3. A zero weight rigid for the pump body center (node 180) to motor center (node 2180).
4. Two rigids with weight of avergaed 4100N from the pump center and motor center to the base respectively.
This model could roughly simulate the physical pump. Since I put averaged weight of pump and motor to two rigids described in item 4, I thought CAESAR could treat them as theoretical cold loads.

I revised as what you commented and found the springs can act as what I expect at cold installation stage. Could you advise again if I can delete the weight of rigids described in item 4? It looks that I should still keep them.

By the way, the pump base is not only supported by some springs, there are some springs pressed from the top of the base respective to those supporting springs, so colud you advise if CAESAR can simulate this series case.

The spring preloads balance the system deadweight. If you have one, you need the other. No, do not remove the rigid weights modeling the pump and motor.

You should be able to model the top springs by adjusting the overall spring rate used at each location.