I have a case of an underground line; it is analyzed by B31.8 with a design factor of 0.6. In a operative case with the presion and temperature of design, the system fail in almost all nodes (including the straight pipe ,Is estrange for me), I think that may be it is a mistake of generation in the underground model, because I did a no-underground model, with the same restrictions but with simples rests and guides (+Y, GUIDES) , and I consider it should be more critical , but in the final result I get a overstress just in the bends at not in the straight pipe like in the underground model.

¿Why I get overstress in the straight pipe in the underground model?

I attached the models.

Thank for you concepts.