First off, I am a relative newbie at this all, but I haven't been able to clarify my confusion about the following through the help, the CII documentation, or other discussion groups.

How does the representation of a spring support account for whether the spring is initially in tension or compression in the installation (cold) condition? I am attempting to model an existing piping system containing a few spring supports, all of which are spring supports sitting below the pipe. Example:

Support 'A': Cold load = 2400 lbs, Hot load = 2800 lbs, Travel = 0.50" DOWN
Support 'B': Cold load = 3500 lbs, Hot load = 3100 lbs, Travel = 0.50" DOWN

i.e. During thermal expansion, spring 'A' is moves down and the spring force increases. This indicates that the spring is in compression at the cold state, and the downward travel further compresses the spring, increasing the spring force. Conversely, spring force 'B' decreases with downward travel, which suggests that the spring was initially in tension and therefore the downward travel returns the spring towards it's "no load" length, so the load decreases.

Modelling these supports as a YSPR (or a predefined hanger, for that matter), my only inputs are stiffness and cold load. A subcontractor modelled all of these supports identically: Positive cold loads and stiffnesses. Clearly the opposite spring forces will affect the result and need to be accounted for...

My instinct on how to model this was to use a positive stiffness value when the change in load and the travel have the same sign (i.e. both +ve or both -ve, like example 'B'), and use a negative stiffness value in the opposite case (A).

However, I now discover during error checking that CII won't accept a negative spring stiffness. What then is the proper method to distinguish between springs A and B in my model? Am I meant to enter the cold load as negative (as hinted in the help for the spring "F" field)? If so, what is the sign convention then? Are spring loads assumed to always increase positively for downward travel? i.e. Does case A, being the more normal situation, correspond to a +ve cold load in CII, and case B a -ve cold load?

Intuitively, I would've expected the opposite to be true, based on positive and negative inputs into dF=k*dx...

CHW,

Welcome to the group. There's a problem here. The way variable springs work, whether they are hanger type or base (F) type, the load on the pipe will increase under downward movement, and the load will decrease under upward movement.

What you have shown in your example is impossible. You cannot have support B as you have indicated. If the point moves down in operation, the spring force will increase.

What is the source of this information? I would suspect that your hot and cold loads were written down incorrectly on whatever source your are refering to for the B spring.

Thanks for the reply, Edward. I've confirmed that the hot & cold loads spec'd on the support drawing for 'B' are indeed flipped.

Cheers.