When i check the results of water hammer analysis，i found the value of mass participation factor to be possitive or negative。What's the meaning for the negative value ？

This is not the classic "participation factor", rather this is a mode influence coefficient. This value also includes the effect of the applied spectrum (magnitude and direction).

The sign of the value really isn't as important as the magnitude, relative to the other values for the same mode. The higher the value, the more influence this "mode+spectrum" has on the response of the model.

Thank you,Richard!

I'm very appreciated that you give a hand. There is anthoer question that when i want to check the results such as stress of element, i find that the same node maybe have two values of stress which can come from two nodes definiting the element.But how i can check the stress distribution with node?
And restraints load may have the same situation.

Hoping for your help,Thanks a lot!

Think of the various elements as free bodies, with their associated forces and moments. Stresses are computed based on those forces and moments (and the applicable Piping Code). If the forces and moments differ between two elements that share a node, or the cross section changes, the stresses will be different.

I have got your ideas,but i have never do jobs like that.How can i make it that caculate the stress of element? what about the stress associated with node given in the results ?Is there any examples for to deal with results ?

As we all know,caesar2 can build structual model,which is taken into conderation in pipe system, is there any anasys for structual models ?

Can I mail to you ?

Hoping for your help,Thanks a lot!

Recall what you did in your "Statics" classes with "portal frames". The nodes must be in equilibrium, but if the cross-sections differ, the stresses will be different.