All of you are looking at the wrong field. The documentation says you should enter a very small "Allowable Travel" -- not "Rigid Support Displacement Criteria". That would be the following field, the "Max. Allowed Travel Limit". Any hanger for which the expected thermal travel exceeds this value will be designed as a Constant Spring. The "Rigid Support Displacement Criteria" is used to force a rigid rod hanger (for any hanger location where the thermal movement is less than that value).