One-way restraints are restraints which are active only along half of the range of a specified degree of freedom. The most common application of these are hangers and supports, which prevent the pipe from moving down, but permit it to move up. Another example of a one-way restraint is a limit stop bumper, where a pipe expands axially until it bumps up against a restraint. Another example may be an accidental restraint — such as where the pipe may have been installed touching a column or penetration on one side, but not the other.
One-way restraints obviously have two states — active or not active. The restraint is active if it is acting on the pipe, and inactive if the pipe has moved away from it. Analysis of one-way restraints require the following type of iterative solution: the analysis is done with the restraint active. If the restraint load is in the direction of the one-way restraint, the analysis is completed. If the load is in the opposite direction of the restraint, the restraint is removed from the model, and the analysis is redone. Then the pipe's movement at that location is checked — if it is away from the restraint, the analysis is complete. If it is towards the restraint, the restraint is re-inserted into the model and the process continues. Again, if there are more than one one-way restraints, it may be a lengthy process.
One-way restraints are coded in CAESAR II by placing a sign (+ or -) in front of the degree of freedom being restrained. The sign convention is that direction in which the restraint acts on the piping system (i.e., a +Y support acts upward on the pipe, so the pipe cannot move down).