Gap restraints are those restraints which do not touch the pipe upon installation (and therefore are not active), but are close enough to the pipe that they may become active during the normal movement of the pipe. The gaps may have been installed intentionally (such as in a limit stop), unintentionally (such as when a structural steel support is not fit up closely to the pipe), or simply by chance (such as when a pipe runs through a wall penetration). Examples of gap restraints are shown in Figure 3-103.
Gap restraints have two states — closed and active or open and inactive. Analysis of these restraints require an iterative solution similar to that of one-way restraints: the analysis is initially done without the restraint. The pipe's movement at that location is checked — if it is insufficient (or in the wrong direction) to close the gap, the analysis is complete. If it is sufficient to close the gap, an imposed displacement is put on the pipe to close the gap, the restraint is reinserted into the model, and the analysis is redone. The results are then rechecked — if the load on the restraint is in the right direction (tending to further close the closed gap), the analysis is complete. If it is in the direction where it is trying to reopen the gap, the imposed displacement and the restraint are removed and the system is reanalyzed. This process is repeated until the status of all gaps converge.
Gap restraints may be entered with friction, which further complicates the non-linear solution (since the friction will act at the gap restraint once the gap at the closes and the restraint begins to work, but not otherwise). In the event that gaps are unequal on either side of the pipe (as in the penetration in Figure 3-103), the gap restraints may be entered as multiple one-way restraints, each with unique gaps.
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