Concrete Filled Tube Braced Frame Testing

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Concrete filled tubes (CFT) have been used as columns for seismic resistant construction, since they offer significant advantages over either steel or reinforced concrete. The concrete fill provides compressive strength and stiffness to the steel, and it reduces the potential for local buckling. The steel tube provides formwork to minimize the cost of the concrete placement and reinforces the concrete for axial tension, bending and shear as well as enhancing its ductility. CFT columns are particularly well suited for braced frames, since the large axial strength and stiffness are needed, and braced frames constitute the majority of the buildings constructed with CFT columns.While CFT columns are desirable for braced frames all research to date has focused on moment frame connections, combined loading of CFT members and bond stress in CFT elements. Braced frame-CFT column connections are fundamentally different than moment frame connections, since they transfer relatively large axial forces to the column, which must be distributed between the steel and concrete. The ductility demands are different in braced frames because dissipation is achieved through brace buckling rather than flexural yielding of the beams. This study will focus on the design of brace-beam-CFT column connections. Several prototype connections, which offer economic and performance advantages, will be identified, and detailed nonlinear; computer analyses of the connections will be performed. These analyses will evaluate expected ductility and fracture potential, identify critical locations, and identify concerns with each prototype connection. The most promising connection alternatives will be selected for experimental evaluation. Design methods will be developed for these selected connections, and an experimental study will be used to evaluate the design proposals and verify ductility and seismic performance. Three brace-beam-column connection subassemblages will be tested under inelastic cyclic loading. The specimens will be of moderately large scale so that they provide full scale simulation of connection in buildings of 3 to 6 stories and moderately large scale models of connections for tall buildings.