When are threaded rod stiffeners required?

[rbhall]

Looking for some help on inspecting threaded rod. When is a threaded rod stiffener required? I inspect sprinkler, water piping & HVAC duct. I offten see the threaded rod size increase but never rod stiffeners added. Is there a rod size when a stiffeners are not required? Example- I may see sprinkler piping supported 8' from the structure with a 1/2" threaded rod or duct with 3/8" threaded rod 10'. I have read every code I can get my hands on and can't seem to find a maximum lenght for a threaded rod or when stiffeners are required. I am located in a Seismic C area and have searched for some answers in ASCE 7-05 but haven't really came across an answer. Would appreciate any help.

 

[MiketheEngineer]

IBC 2006 - 09 talks about the forces that need to be accounted for in such seismic conditions. If you can keep the object from moving too much - are stiffeners required??

 

[wannabeSE]

Stiffeners are often (almost always) used on threaded rod hangers at seismic brace locations to prevent buckling of hangers in compression. Stiffeners are not needed when the compression force is less the compression capacity of the threaded rod alone.

In high seismic areas, one rule of thumb is that stiffeners are required at every brace location unless the hanger is really short (1' to 2' depending on hanger diameter and loads). Several manufactures have seismic bracing guides for pipes or ducts (Mason, Unistrut, Tolco, B-line, Powerstrut etcetera).