BRBM Frames: An Improved Approach to Seismic-Resistant Design Using Buckling-Restrained Braces

Buckling-restrained braces (BRBs) are an important addition to the structural engineer’s seismic design toolkit and are in widespread use in high seismic regions throughout the United States. Recently, designers and researchers have begun considering alternative bracing configurations to capture the benefits provided by BRBs in a fundamentally different way. The buckling-restrained braced mast (BRBM) frame is one such alternative configuration. The BRBM frame consists of yielding BRBs in series with a stiff elastic wide-flange mast frame that is able to pivot about its base. This mast redistributes brace forces between stories, fundamentally changing the kinematics of the system, producing a more uniform distribution of interstory drift, and eliminating the possibility of inelastic weak-story mechanisms. The BRBM system inherently provides for increased redundancy, reducing the number of frames required and providing enhanced seismic performance and better architectural compatibility at a lower cost than conventional brace-frame systems. Introduction Over the previous decade, the development of buckling-restrained braces (BRBs) has been an important addition to the structural engineer’s seismic design toolkit. These proprietary devices, which are now in relatively widespread use, represent a major improvement in terms of reliability and ductility over conventional braced-frame systems that use wide-flange or tubular-steel diagonal members. The main advantage of BRBs is their ductility and stable response under reversed seismic loading. As a result, the building code allows BRB systems to be designed for reduced seismic loads. While there are economies in designing for lower seismic forces, the redundancy provisions of the building code can still result in numerous bays of bracing, which in turn require a large number of BRBs, adversely affecting cost and architectural programming. Lastly, research and analysis has shown that the relatively soft BRB systems are susceptible to weak-story mechanisms. More recently, designers and researchers have been considering alternative bracing configurations that promise to not only capture the benefits provided by BRBs but also provide for improved system performance and greater design flexibility at a ATC & SEI 2015 632