Can I use the “12 inch rule” for ductwork?
ANSWER:
The “12 inch rule” exemption has been misapplied in many cases. The only way the 12″ exemption rule can apply is if the supports are designed to prohibit significant bending for all hangers on a run of duct. One method is to select hangers that will not permanently deform when exposed to seismic design lateral loads. Another method uses vibration isolators or swivels to provide a swing connection at the top of the hanger support.