I have a question about SUS stresses in Alt-SUS cases.
For example, vertical movement at a support point is +1" in an operating condition.
Case 1: +Y (no -Y)at the location
Case 2: Y
Case 3: +Y & -Y w/gap 0.5" < 1"
Case 4: +Y & -Y w/gap 1.5" > 1"
Case 5: -Y w/gap 0.5" (no +Y)

Caesar considers the support inactive in the calculation of Alt-Sus stresses for Case 1 & 4, which seems very clear.
And Caesar considers the support active for case 2, 3 and 5. It seems that case 5 does not make sense at all while I have no idea about Case 2 & 3.

If anybody understands Caesar's approaches for case 2, 3 and 5, please advise me about this matter?

I do not know the composition of your 5 load cases, nor do I understand your notes.

Can you provide more information?

If I'm understanding this correctly -

With +1" vertical movement in OPE

Case 2: Y
-> Vertical movement is constrained by support, therefore is considered active.

Case 3: +Y & -Y w/gap 0.5" < 1"
-> -Y support is engaged after it rises 0.5" in which case it becomes active. Your +Y support should show as inactive.

Case 4: +Y & -Y w/gap 1.5" > 1"
-> 1" vertical movement isn't enough to close the gap, support considered inactive.

Case 5: -Y w/gap 0.5" (no +Y)
Same as Case 3, -Y is engaged and becomes active after vertical rise of pipe.

So, your cases 1-5 are not CAESAR II load cases but, instead, five different support configurations at a point which "wants" to move up 1 inch (in the operating case)
Case 1: +Y restraint allows node to move up, support is disengaged, therefore, the restraint is inactive in the Alt-SUS case.
Case 2: double-acting Y support has this point is restrained in the operating case, therefore, restrained in Y in the Alt-SUS case.
Case 3: Y restraint with a half inch gap is engaged in the operating state, therefore, restrained in Y in the Alt-SUS case.
Case 4: Y restraint with a 1.5 inch gap does not close in the operating case, therefore, no Y restraint applied in the Alt-SUS case.
Case 5: the -Y restraint with a half inch gap is active in the operating case (restraint allows pipe to freely move in -Y and has a half inch gap above the pipe), therefore, the restraint is active in the Alt-SUS case.

For any case (regardless of which one you choose), if your pipe is expected to lift off a support for a significant duration of time, you must run a sustained case with sustained allowable stresses demonstrating the lacking of that support.

With that said, there's a "legalese" disconnect, here. If you could arrange your support locations wherever you wanted, you could slide these lift-off points all the way until they provide 0.00001 N of support, you get away with using the higher allowable (though one might argue that you shouldn't).

Case 1: +Y (no -Y)at the location

No question, you need to run an alternative sustained case.

Case 2: Y

Through legal loophole, you probably don't have to (but as a best practice, you probably should).

Case 3: +Y & -Y w/gap 0.5" < 1"

If you believe actual real-world conditions will only have the +Y active or the -Y active for significant periods of time, this is practically the same as Case 2. If you believe actual real-world conditions may actually have the pipe hovering above the support, say 0.25", then you must run the alternative sustained case. Either way, the best practice would be to check it.

Case 4: +Y & -Y w/gap 1.5" > 1"

No question. This is exactly like case 1.

Case 5: -Y w/gap 0.5" (no +Y)

No need to run an alternative case, because your default sustained case will capture the lack of +Y.

After all said and done, I believe there is a disconnect in the wording of the code and actual practice. If you have a guide in your system that remains activated 99.9% of the life of the pipe due to thermal expansion, then that, too, would count as a long-term effect and should be considered a sustained load. However, it's not common practice to do so, despite lifting off of supports does count.

Thank you all.

I could understand how CAESAR II considers lift-off supports in Alt-SUS cases but my question is whether or not CAESAR II approach is reasonable. As long as I understand Alt-SUS case is added to evaluate SUS stress (mainly due to weight) with lift off supports taken off from the system. But CAESAR II considers support active in Alt-Sus case if it is active in OPE case regardless of load direction. Case 5 (Nobody will not use this type of support) shows absolutely different result from what I expect. There should be no support considered, in my opinion, even in Alt-SUS case as there is nothing to support weight but it is considered active. The same suspect can be extended to Case 2 or 3. Each case has double acting restraint but it is actually -Y (w/inactive +Y) during operation so that it neither supports weight. It means it shall be considered inactive in Alt-sus case, in my opinion.
In short, It seems that sus stress considering support scenario during operation cannot be correctly evaluated by Alt-SUS case for Case 2, 3 and 5 including -Y restraint.

Please let me know if my understanding is wrong and advise about CAESAR II approaches that I don't recognize.

You are correct that in Case 5 it won't support any weight loads, but there will be a downward force on the pipe as it closes the gap. This force will be in addition to any weight loads.

Loads in Case 5 should be greater than the weight-only loads in Case 1.

Parky,

I agree with your point that nobody would likely intentionally design for a case 5, but I wouldn't rule out the need for it in a forensic analysis.

Mamacint,

We agree, but the question is whether we would need to base allowable stresses for those loads in the sustained case or expansion case.

Hi All,
Please see attached sample file in which;
case 1 : node no.120~180(+Y)
case 2 : node no.220~280(+Y & -Y)
case 3 : node no.320~380(+Y & -Y w/gap smaller than movement)
case 4 : node no.420~480(+Y & -Y w/gap larger than movement)
case 5 : node no.520~580(-Y w/gap smaller than movement)

Results are;
SUS stresses at elbow (140, 240, 340, 440, 540)
SUS displacement at support (160, 260, 360, 460, 560)

Case 1
Alt-SUS 1328.7psi at 140 & -0.0845"
SUS 27.8psi at 140 & 0"

Case 2
Alt-SUS 27.8psi & 0"
SUS 27.8psi & 0"

Case 3
Alt-SUS 27.8psi & 0"
SUS 27.8psi & 0"

Case 4
Alt-SUS 1328.7psi & -0.0845"
SUS 27.8psi & 0"

Case 5
Alt-SUS 27.8psi & 0"
SUS 1328.7psi & -0.0845"

From the above results, SUS stress without support case cannot be calculated by CAESAR II for Case 2 & 3 in which Y(actually -Y) is active at hot condition.

Question: For case 2 and 3, does the above result meet code requirement? Don't we need to check sus stress of the case without support for Case 2 & 3 configuration?

Do not get lost in the concepts of weight and +Y lift-off. The Code states that primary stress must be evaluated in ALL support conditions.

This means that if in one operating scenario a support engages or disengages, that state must be evaluated for primary stresses. It doesn't matter whether the support is a +Y, a -Y, a +X, or a -Z - that fact is there is a change in the boundary condition state of the model. That state is represented by the converged stiffness matrix of that operating scenario.

There is a webinar on this website that explains these concepts in detail. This would be good to view.