Type a large value such as 1E10 if the torsional spring rate is unknown. This produces conservative results. These results are conservative with respect to loads and non-conservative with respect to displacements. It is very common to rate the bellows allowed torsion by the amount of rotation that it experiences. Large torsional stiffnesses result in small, seemingly satisfactory rotations. When results from a piping analysis are communicated back to the expansion joint manufacturer, it is important to report both the rotation and the stiffness used to produce that rotation. For more information, see Expansion Joints.