Hello,

I have a system and out of curiosity, I added a "Hot Sustained" combination on my load cases. I already checked and found out that I have no lift-off on my supports, but then continued to find out if there would be significant differences between the "Sustaned" and "Hot Sustained" load cases.

My load combinations are like this:

L1: W1+T1+P1 (OPE)
L2: T1 (EXP)
L3: W1+P1 (SUS)
L4: L1-L3 (EXP)
L5: L1-L2 (SUS)

Upon reading the results, I found out that the my system failed on the "Hot Sustained" stress case (L5), but the stresses on the Sustained case (L3) are very low.

If I am not mistaken, there should only be a slight difference between the two stresses, Sustaned vs "Hot sustained", since there is no lift off of supports.

Has anyone experienced this before? Or Can anyone give their thoughts regarding this matter?

Thank you in advance.

Leo,

L3 will be equal to L5. If there is no non-linear restraints in your system.

one more check you can do, If your system has only linear restraints then L2 also equal to L4

If L2 NOT EQUAL TO L4 means ==>>
1. non-linear restraints effect
2. all supports are not resting properly.

Hi Durga,

Thank you for your reply.

Yes, my supports are non-linear, and I also checked the displacements during OPE case (L1) and they have 0 movements on the +Y supports.

One thing that I am unsure about, is if I still need to satisy the "Hot sustained" case even though there is no support lifitng and all supports are taking load during operating case.

Since Casar II is showing that it has a failure on the "hot sustained" load case (L5), do I still need to make changes to reduce the stresses within the allowable?

Thanks,

Hi Leo,

Depends on project and your client requirement.

If your client satisfy with liftup check then it is ok otherwise if he wants hot sustained check then we have to reduce the stresses in hot sus stress. We also faced the same problem.

and one more point I want to add.

Even though CAESAR-II shows zero. if it is 0.0002 like that displacement also it doesnt take proper load. thats why better compare OPE loading with sus then we will get a idea.

Hi Leo,

I am having the opinion that you should satisfy the hot sustained stresses. Since these stresses are the sustained stresses in pipe operating condition. If you are getting higher stresses (than allowable) then you should try to find out the solution.

Regards,
R.K.

Dear Durga,

If the actual displacement is 0.0002, like the example you stated, it will still not be considered as a lift off in actual installation since the support attached to the piping will have its own stiffness/weight, which is not considered in the caesar analysis. Thus, it may counter the said displacement and may cause the support to "sit" in actual installation. However, I am just curious how Caesar is producing this result, like the normal sustained from 55% ratio becomes 110% when the "hot sustained" case is added.

Regardless, you are right that I should consult with client and hear what they have to say about this.

I thank you Durga and RK for your reply.

Regards,

Leo,

What you said is true, but I tried to explained a possibility ways in coming the difference b/w sustained and hot sustained stress in CAESAR-II, not the pratical one.

Even in the most of the projects the client limits a displacement 3 mm. In actual case it will sit but still we have to consider as lift up case, It all depends on what client wants.

"Hot Sustained" is not a Code term. In CAESAR II we use hot sustained to estimate the changing sustained load distribution as the pipe moves from one state to another - specifically - from operating back to "installed".
Think of the load at that resting support. Even though the pipe is still engaged with the support, the load at that support changes from perhaps a very large number to almost zero. That load went somewhere else and may have increased the calculated sustained stress there as well.

We are introducing a new style of sustained stress calculation in CAESAR II 2016. We're calling it alternative sustained stress or Alt-SUS. After running an operating state, you can request that CAESAR II uses that operating support configuration as the boundary conditions for a (linear) sustained stress calculation. The intent is to address the B31.3 statement to calculate sustained stresses for the various support configurations that may present in a system with nonlinear restraints.

Sorry for reopening this thread.
The Caesar 2016 is yet to be installed at our work place and we are still working with dilemmas of hot sustained.
One question: is L1-L4 any better than L1-L2 when checking hot sustained case.

In my opinion, no. I do not believe L1-L4 provides any additional information.
When you say L1-L4, the program will subtract the position from L4 from the position of L1. L4 is defined as L1=L1-L3. So, L1-L4=L1-(L1-L3)=L3.
I suggest that your L1-L4 case (using the load cases above) would duplicate the installed support configuration shown in L3.