EFFECT OF MODIFYING BRACE SLENDERNESS IN CONCENTRICALLY BRACED FRAMES

Steel moment-resisting frames are susceptible to large lateral displacements during severe earthquake ground motions leading engineers to turn to stiffer concentrically braced frames (CBF) as a way to resist earthquake loads. Braced framed systems are efficient because the lateral loads are resisted primarily by brace axial loads with little or no bending in the members. Brace behavior, however, is typically controlled by undesireable member buckling behavior. As a result, during cyclic loading, the brace hysteretic behavior is unsymmetric and there is deterioration of the building lateral load behavior. Buckling restrained braces (BRB) and partially buckling restrained braces (PBRB) have been proposed as alternatives to conventional brace design. Both use the steel section more effectively than conventional frames: BRBs achieve stable, symmetric hysteretic behavior by accommodating ductile compression yielding and thus can achieve the full theoretical capacity of the brace in both tension and compression. PBRBs, while not as efficient as BRBs, are ideal for the retrofit of existing braces since they can be built in situ with little impact on the surrounding structure. Additionally, the brace capacity is not affected, thus there is no need to retrofit the connections or subsequent elements of the force resisting system. The objective of this thesis is to assess the potential design space of PBRBs. A series of analyses of a six story CBFs is conducted. The braces are provided with a range of hysteretic behaviors varying from that of a 'slender' brace to that of a BRB. Between these extremes, 'intermediate' and 'stocky' brace behavior represents the spectra of potential PBRB behavior. It is demonstrated that structural behavior is improved as the braces progressed from slender to stocky to BRB. The incremental improvement is most significant as one improves upon slender behavior and becomes less pronounced as the brace slenderness is reduced. This result is encouraging for the use of PBRBs as a retrofit measure for slender braces