I am modeling a pipe running across a series of pipe supports and restraints; most of the supports are simple +Y. Sometimes, the pipe lifts off under loading, and since these are +Y pipe supports, I would expect to see 0 as an Fy reaction force at that support. Instead, I see a positive (upthrust) force. Displacements report shows that the pipe is indeed lifted off the support. Any idea what is going on here? (I run Caesar 5.20)

Oh wait, I discovered the answer.

The loadcase in question, where I was seeing vertical reaction force, was the expansion case. This case is the algebraic difference between the operating and operating no thermal cases. Even though the expansion loadcase may show positive force, that's just one component of the realworld operating case; the +Y support is not somehow pulling down on the pipe.