Use of Cast Modular Components for Concentrically Braced Steel Frames

Abstract

Cast modular components have been under development for earthquake resistant steel structures. These concepts take advantage of the versatility in geometry afforded with the casting process to create components specifically configured for ductile behavior. Two systems were developed as part of this dissertation research: (1) the Cast Modular Ductile Bracing system (CMDB); (2) the Floating Brace system (FB).The CMDB system makes use of cast components introduced at the ends and the center of the brace to produce a special bracing detail with reliable strength, stiffness and deformation capacity. The system takes advantage of the versatility in geometry offered by the casting process to create configurations that eliminate non-ductile failure modes in favor of stable inelastic deformation capacity. This thesis presents analytical research performed to determine the buckling strength and buckling direction of the bracing element based on the geometries of the cast components. Limiting geometries are determined for the cast components to control the buckling direction. Design formulas for buckling strength are proposed. The Floating Brace system is a new lateral bracing concept developed for steel special concentric braced frames. The concept uses a set of special plate details at the end of the brace to create a stiff, strong and ductile lateral bracing system. The plates are arranged such that some provide direct axial support for high initial stiffness and elimination of fatigue issues for daily service wind loads. The remaining plates are oriented transverse to the brace and thus provide ductile bending response for the rare earthquake event, in which the axial plates become sacrificial. The main bracing member and cast pieces remain elastic or nearly elastic. Thus, following the seismic event, the plates can be replaced. In this thesis, analytical studies using nonlinear finite element analysis are performed to determine the optimum: (a) relative strength of the end connection to the brace; and (b) ratio of strength between axial and transverse plates. Design equations are provided. Prototypes for each concept were developed. Castings were created. Large scale laboratory physical testing was performed as experimental verification (proof of concept) for the two systems.