I've built a model of an underground pipeline (ASME B31.4 code). I set the value for Fac to 1.0 and made sure that the length of my pipe is longer than the virtual anchor length.

This should be a restrained pipeline, but I'm seeing axial displacements along the length of this line.

There are no bends or anything. The model is very simple: roughly 1000 feet of pipe, a valve, and then another 1000 feet of pipe.

Could someone explain why I'm seeing axial displacements?

Since making the original model, I changed the soil restraints on both ends to rigid, axial restraints. But I am still seeing axial displacements.

That FAC shifts the stress calculation from the standard calculate-position-then-load-then-stress to a simple estimate of stress based on thermal strain and hoop stress.

If a system is "fully restrained", it is locked in place and there is no bending. No need for a bending stress calculation.

By using FAC=1, the program still does the structural analysis based on your system layout and loads but the stress calculation will be replaced with the equation from the Code: E*alpha*deltaT + (1-nu)*Hoop_Stress.

So you still see movement but the stress equation is hard wired.

Thanks for the reply Dave.

So are you saying that the stress output should be accurate but the displacements will not be accurate?

No.
No matter how you set FAC, CAESAR II will calculate displacements. Those displacements are based on your piping loads and the system and soil stiffness. I cannot say whether or not your displacements are accurate.

By setting FAC=1, you are replacing the regular (bending) stress calculation with the simple stress calculation that assumes that the pipe does not move. If the pipe is fully restrained, you do not need to use CAESAR II to calculate stress, just apply the Code formula.