I'm doing an analysis of underground lines,but when I convert the model to UG model the bends are filled with restraints(I think not evenly distributed), is it normal or there is something wrong with the input?

I have attached a file of how it looks.

What you see are the active soil restraints opposing expansion in the pipe. It's quite normal. Perhaps you should read the documentation to fully understand how buried pipe is handled in the program. There are other differences compared to exposed pipe, such as buried weight set to zero which have raised questions in this forum.

I have read the CII manual also Peng's work about UG stress analysis, question is, how does CII determine the distribution of restraint on the bend? On the previous system that I've done the restraints are much scattered on the bend, on this new system it is the concentration of restraints on every bend that causes the system to fail during thermal expansion. I'm thinking that I must have made an error on my input that's why I'd like to get opinions if the concentration of restraints on bend in UG lines are normal.

Thanks,

Check the bend radius. B31.8 set the minimum limit for bend radius. See if yours is more than that.

I wouldn't be too surprised to see a failure in a bend of buried pipe. Take a look at Peng again, he even gives an example of a failing bend with numerous solutions.