Has anyone had this problem before?

I'm using CAESAR II 2014, 7.00.01.1600, (Build 141003).

I first built a model with 1 temperature and 1 pressure and then ran the model with the default load cases.

L1: W+T1+P1 OPE
L2: W+P1 SUS
L3: L1-L2 EXP

The system passed, but I noticed some supports were lifting off. I went back to the model, and without changing anything else, I went to the Load Case Editor. I added two cases to account for the Hot Sustained case and now my load cases look like this:

L1: W+T1+P1 OPE
L2: W+P1 SUS
L3: T1 OPE
L4: L1-L2 EXP
L5: L1-L3 SUS

For some reason, the EXP case (L4 now) is now failing. The calculated stresses are about 30% higher than before. What would cause this stress to change?

As far as I can see, nothing in cases 1, 2 or 4 should have changed from the first run, correct?

Any help is appreciated.

I could see the ALLOWABLE stress for expansion changing - CAESAR II uses the largest SL in (1b) and the new (hot) sustained case may reduce the expansion allowable limit.
On the calculated stress side of the equation, at this point, I agree that cases 1, 2 & 4 should show nothing new.

Thanks Dave. You were right. I saw that the % of allowable had changed so I assumed the calculated stress had changed.

But this does raise another question. To my knowledge, there are two ways of handling the Hot Sustained case. One is by adding the cases that I did above. The other is to simply note the supports that are lifting off, create a different model, and then remove those supports from that model.

Since the way I used above changes the allowable stresses in cases that I wasn't evaluating, would that mean that it is not a proper way to analyze for hot sustained stress?

If I create a separate model and remove supports, it shouldn't change the expansion allowable stress, correct?

The two model approach will not provide both sets of "stress due to sustained loads" for a proper calculation of the allowed expansion stress range according to (1b) [what we have called the liberal allowable].
If you create a separate model holding only those restraints that are active in the operating case, the calculated expansion stress range from that model will not necessarily reflect the "installed" response influenced by those now-missing supports.
You should take a look at CAESAR II 2016 where we introduced the Alt-SUS load case. We run the standard cases you have come to expect but we added a new load case where the sustained load vector is pushed through the same (converged) stiffness matrix that produced the operating response.