Expansion stress range

Hello to everyone,

I'm having a problem on the expansion stress allowable on one of my calculation. I'm using CAESAR v. 5.3. When I make two calculation files, 1) With hot sustained and 2) Without hot sustained.

Between the two files, I noticed that the expansion stress range or allowable is not the same on some nodes, but the code stress values are totally the same.

The expansion stress range is higher when there is no hot sustained load case in the calculation. That is why when I consider hot sustained load case, the thermal stress is failing.

Can somebody help me on this matter, please clarify if you've experienced this problem before.

Thank you and regards,

Clyde

I guess that you are using the 'liberal' expansion stress equation which allows unused (allowed) sustained stress to be assigned as additional allowed expansion stress. If you are failing under this regime you probably have quite serious thermal problems.

Hello, both files are using liberal stress allowable. So I presume that the basic expansion stress would be the same or a little difference at both files. I just can't explain why the expansion allowable stress decreases if I will make a hot sustained case.

I'm going to disagree. The "Sustained" stress can be much different from the "hot Sustained" stress if you have non-linear boundary conditions, i.e. a +Y that lifts off. In this instance, the "Sustained" case has the +Y active carrying load. In the "hot Sustained" case, that support is (numerically) not there, hence there will be an increase in the sustained stress on the pipe element adjacent to this support.

As SL changes, so does the Code stress and the Expansion Allowable.

Thanks Richard for replying,i got your point when I checked the caesar output again. I mistakenly interpret it by taking the Sl from cold sustained stress, it should be from hot sustained stress;) right?

Regarding the Hot sustained stress, I understand that the lift-off restraints will be deactivated during the hot sustained stress analysis, but I don't understand why caesar II deactivates the support even if its not lift-off, I don't know how CAESAR II calculates or how does the software knows if its lifting off.

My previous experience is that we manually remove the inactive support from the system during operating (W+T+P) condition and checked the sustained stress (W+P). When I compared to hot sustained stress (W+T1+P)-T1, again its totally different. Knowing that (W+P) is not equal to (W+T1+P)-T1, the stresses should be the same because we know that the support will be there when the pipe travels to ambient temperature. Correct me if I'm wrong. Thanks.

Your manual method does not fully account for the non-linearities in the system.



c2 doesn't know the support lifts off. When you run the "T1+D1" case, you obtain a set of displacements. In this case, maybe a support lifted off, or maybe not. It really doesn't matter to c2 because you then setup the difference case ["W+P1+T1+D1" - "T1+D1"] with the net result being the "hot sustained. If a support lifted off, then the effect of the difference "W+P1" is computed without the support.

So CAESAR II assumed the pipe to lift off first without the presence of W & P. And most of the support point will lift off because of the thermal expansion only and that expansion is used for stress calculation; I think that CAESAR II will calculate the hot sustained by assuming the pipe will sag back down to its original position or support after expansion, well I’m a little bit convinced by that at the moment. In my analysis, the pipe should expand with the presence of W & P while CAESAR II the pipe expands without W & P; I have a problem in understanding that and whether there is a problem on the non-linearity of the system.