In caesar II application guide it tells that bellows can be modelled with Zero or a finite legth.

Can anyone tell me what is meant by a zero length Expansion joint.


Thanks in advance

Go through the Gimbal & Hinged Expansion Joint, you can find the solution for your question.

Many years ago, the typical way of modelling an expansion joint was to code right to the center of the joint and add the expansion joint terms (stiffnesses) at the center. For example, if I have an expansion joint running from 100 to 110, I might run a rigid element from 100 to the midpoint at 105 and then another rigid element from 106 to the other end at 110 and specify the "zero-length" xj terms between 105 and 106.

Many xj manufacturers list bending "flexibility" rather than bending stiffness in their catalog. This bending flexibility is what you enter for a "zero length" xj in a beam element program like CAESAR II rather than bending stiffness. Use bending stiffness for "finite length" xj's. If you enter axial and lateral stiffness for finite length joints in CAESAR II, CAESAR II program will calculate the appropriate number. But you have to have the correct flexible length of the joint specified.

As I am using these terms - bending stiffness is four times bending flexibilty.

I cannot remember seeing any manufacturer to give bending stiffness. I would say those who give are giving bending spring rate, which is bending flexibility. To confuse more there is working spring rate and initial spring rate. Working has to be tested the other can be calculated. Working is lower of the two, maybe some 30%.

Dave mentions correct length is important. The issue is discussed in the documentation. Correct length is the convoluted length and nothing more.

Sometimes it looks like zero length means to some designers very short. Next time when anybody includes compensator into their pipe design make sure you allow sufficient length for it. And do not specify low spring rate, be realistic.