Hi all,

Please give me an advice I have a support neglected in the calculation..

To explain futher im doing analysis in hot sustain case with steam line with temp of 470deg C. I have a +Y support at a node with "NO GAP" with coef. fric. .3. Now I try calculate it and WHALA! My support not considered....... Because at the support location I have a sag of 31mm . and a zero vertical force on it! Now i wonder where did it go????

Please HELP!

Regards!

You mention it is a hot sustained case. I assume this is a combination load case. Combination load cases can have displacements that appear to violate your restraints because it is the difference between two other load cases and not a real displacement as measured from the neutral position.

Hi Sir Loren,

I think I did not violate any of caesar control parameter. This Hot sustain case is discuss in Tec Ref. I didnt understand why it crosses the boundary that id set.

L1=WW+HP(HYD)
L2=W+T1+P1+H(OPE)
L3=W+T2+P1+H(OPE)
L4=W+T3+P1+H(OPE)
L5=W+P1+H(SUS)
L6=W+P2+H(SUS)
L7=T1(EXP)
L8=T2(EXP)
L9=L2-L5(EXP)
L10=L3-L5(EXP)
L11=L4-L5(EXP)
L12=L2-L7(SUS)
L13=L3-L8(SUS)

This are my load cases. I see no wrong with it. Please give you expert advice on this subject.

Thank you!

Regards!

The displacements that you see are a mathematic construct only and should not be looked upon as "real".

Luf Sir,

I really dont understand why my boundary is crossed. In the result I find sag in the +y support node and without force acting on it and yet I DID NOT PUT ANY GAP of my restraint.

As for my own understanding "This is thoeriticaly imposible". It seems like theres a failure of calculation that occur. Again this is a hot sustain condition...
Please guide me!

Thank you and Regards!

This is one of the reasons I do not agree with this method. You must understand the displacements you see are mathematical in nature and not real.

In the movie the matrix the character Neo is told by a young boy who is mentally causing a spoon to bend that there is no spoon. These are not real displacements.

Here is an example. Let's take your L2 case and your L7 case. If I look in the displacements for these two load cases they may both be lifting off the support. If L2 lifts 0.5 inches upwards and L7 lifts 1.0 inches upwards then your result of L12 = L2-L7 will be 0.5-1.0 = -0.5 inches.

There is your negative displacement even though a +Y support is there. This is why Mr. Luf is telling you the displacement is not real. It is the difference in displacement between your two load cases, namely L2 and L7 in this example.

So you did not violate anything. But you should not be looking at displacements, nor forces and moments in this load case or any other combination load case (there are a few exceptions to this, but that is another topic).

Your hot sustained condition in and of itself is not real either. In other words, you will never see this condition in your piping system. But from a stress standpoint the delta-displacement method that we use is correct (well, there is some contention there too, but again that is another topic).

Look at your load case construction: L2-L7. You are simply asking CAESAR II to subtract the displacements from the thermal change case (L7) from the operating position of the piping. At this point, for displacements, there is no piping model to satisfy; there is no +Y support. You are just subtracting one number from another. But these are useful numbers.

There was a B31.3 Interpretation a few years back that went something like this: If my pipe lifts off a support, must I evaluate my sustained stresses with that restraint removed from the analysis? Answer: Thou shalt evaluate sustained stress in all support configurations.

Try this. Look at your operating case results and note all the liftoff supports. (I am ignoring all other nonlinear situations like friction and gaps on guides here.) Now build a second model where the liftoff supports are removed from the model and run a sustained analysis on that new model. I claim that the -Y displcement you see at these supports are "real" and they are the same as your L2-L7. L2-L7 is what's called the "Hot Sustained" analysis as it evaluates sustained stress based on the hot support configuration. It works.

One final note: I used the term sustained stress, that's not what the Code calls it. The Code calls it longitudinal stress due to sustained load. The load is sustained, the stress can change due to the changing support configuration.

Thank you Sir's for the effort of replying my queston.

Regards!

you should use spring in this location.