Default Stiffness Value

Richard/Dave,

We are currently trying to change our CAESAR default translational stiffness value in order to avoid overdesign of foundations/structures. We picked 1E06 lb/in since standard structures/racks/individual supports rarely exhibit a stiffness greater than that number particularly in the horizontal direction.

But I have some questions if you could please help me answer them:

1) If we change the translational stiffness default to 1E06 lb/in, to what restraints will this apply to? Is it only XYZ and LIM restraints or does it also apply it to anchors? How about CNODE'd restraint nodes? Note that we will not change the rotational stiffness values and will leave those fully rigid as we don't see great benefit in changing it.

2) Will CAESAR fill in the default value in the Restraint Stiffness cell or will it just show it as blank like it currently does for 1E12? I assume it won't show a number, which means we will have to be very careful during input to fill in higher numbers for anything rigid like vessel connections and such.

3) Do you personally see any issues with doing this?

I appreciate your help. Thanks in advance!

My view:
1) If no stiffness value is entered for a translational boundary condition, the program will apply the default value set in the configuration file. This will be those you list and also GUIDE, ANC, the bilinear group (e.g., X2) and the rod restraints (e.g., +YROD). This also applies to entered translational displacements where no explicit restraint is defined.
2) The cell remains blank. Be sure to include the configuration file as part of your archival documentation.
3) More accurate analysis is better but stiffer is more conservative when evaluating strain. There are trade offs...

Thank you, Dave.

After studying this a bit more, we might back down form implementing this change. I would rather just train my guys to be more aware of realistic stiffness numbers and where/when to use them.

I also think is better to use more realistic stiffness numbers for supports instead altering the global stiffness. We had the same request from Structural Dept to lower the loads and one idea was to change globally the stiffness. Note that Autopipe use 10E+10 instead 10E+12. For big sizes it makes a difference. You will see some displacements into the supports you deem rigid, but most important you will see relative displacements between Node and Cnode.

While it's great if you can input realistic stiffness values, it's typically not feasible to do so.

When we plug in stiffness values, it's a linear approximation for a stiffness that is quadratic or of higher order, which is further complicated by the fact that a given support is host to other piping loads, as well.

To that end, any successful stress analyst needs to know not to have supports directly resisting thermal growth.

By the way, for relatively small pipe size and loads there is only marginal effect by using reduced stiffness.

Have you ever used the structural module? At what extent? I understand that structure can be imported from other programs but is it working fine?