In mechanical, electrical, and plumbing distribution systems, hanger rods are used to provide the vertical (dead) load support, making the primary force imposed on them a tension force. However, when cables or rigid brace members are used to provide restraint against seismic forces, the hanger rods at the brace locations can be loaded with a net compression force, which, if large enough, can cause the hanger rod to fail. This guide will discuss how the net compression load is developed because of the seismic bracing device, as well provide a simple table to help determine when rod stiffeners are necessary when seismic bracing is utilized.
