To my knowledge NFPA does not provide a limitation on the total hanger rod length connected to a horizontally loaded pipe. Hangers are intended to only be loaded axially (along the rod). Additional hangers or restraints could be required due to additional lateral loads on the pipe depending on the pitch of the pipe. Handbook Section 126.96.36.199 states that the rods must not be formed or bent and the rod must be axially loaded to the structure. Axial loading is the stresses via both tension and compression vertically in both directions on the element (or rod in this case). Thus, NFPA only requires that the hanger assembly and its components be designed to carry those axial loads based on the point load weights/structural attachment/and any seismic requirements. Any transverse (non-axial) loading on the pipe is addressed in NFPA 13 188.8.131.52. The big issue I see with multiple rod lengths connected via rod couplings is dependent on (1) any system water hammer in change of direction due to velocities in the pipe and (2) the system type. A dry or deluge system with a large amount of water pressure could cause the pipe to sway significantly when tripped if not secured adequately. Also it is important to ensure the shear and tension strengths of the structural fasteners are not beyond their limitations causing a failure. This would have to be evaluated by a structural PE. Sometimes its best to shorten the hanger support distances, add retaining straps, add additional lateral supports, etc. or have a structural PE sign off on the hanger design just to be safe.
Practically, rod stifferner required for all tread rod over 1.8 meter in lenght.
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