NOVEL HEAT TREATMENT APPLICATIONS FOR CONCENTRICALLY BRACED FRAMES

Abstract

Concentrically braced frames (CBFs) have been widely used in seismic areas as efficient structural systems to provide both lateral stiffness and strength. They dissipate earthquake energy through the inelastic deformation of the braces in both tension and compression. While these frames are efficient in providing lateral stiffness and strength, their inelastic mechanism is not ductile when compared to other systems such as moment resisting frames (MRFs). This student proposes a new approach to enhance the ductile behavior of CBFs by locally heat treating gusset plate connections or braces. In this method, the steel is heated locally to austenitizing temperature and then cooled with the appropriate rate to achieve the desired material properties. In gusset plate connections, to permit the rotation imposed from brace buckling, the conventional approach is to use linear fold lines, which can result in overly large plates. A more compact design uses elliptical fold lines, but both designs can lead to damage to welds with surrounding components. To enhance the performance of the gusset plate connection, a yield path is defined with a locally weakened zone within a high strength steel gusset plate. The weakened zone, created through heat treatment concentrated the inelastic deformation, resulting in an efficiently sized connection in which the failure mechanism is tightly controlled. A design methodology for the heat treated gusset plate is proposed, and finite element analysis is used to analyze the behavior of the heat treated gusset plates. In conventional braces, repeated buckling leads to deterioration and low-cycle fatigue which limits the ductility capacity of the CBF, compared to MRFs. As a novel approach, heat treatment is used to increase the local yield strength in the brace. Through this method, the buckling is permitted to occur, but an enhancement in the buckling behavior is intended. Various heat treated configurations are investigated, and finite element analysis is used to compare the behavior of heat treated braces.