Date of Award

January 2015

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

First Advisor

Iraj H. Mamaghani

Abstract

Extensive investigations have verified that frame systems consist of concrete-filled steel tubular (CFST) columns have more benefits than ordinary reinforced concrete and steel systems. The CFST column increases earthquake resistant capabilities due to the concrete filling inside the steel tubes and are ideal for buildings subjected to large compressive stress. The use of CFST columns is drawing attention due to their strength and quake-proof advantages. Local buckling of the steel tube is delayed by the restraint of the concrete, and the strength of concrete is increased by the confining effect of the steel tube.

This thesis deals with the local buckling restraining behavior of thin-walled CFST columns under seismic loads by conducting a bidirectional cyclic loading numerical analysis. The CFST columns are modeled and analyzed, by the commercial computer program ABAQUS, to calculate the responses of the CFST columns under bidirectional cyclic load. The obtained results from analysis indicate that the buckling deformation should be slowed for the reduction in compressive force on buckled part due to shifting of compressive force from steel tube to the in-filled concrete. In addition, under a cyclic load applied after the occurrence of local buckling, the opening and closing of major horizontal cracks and dilation occur in the in-filled concrete. As a result, a predominant tensile axial force will act repeatedly on the buckled part of the outer steel tube. This tensile force restrains or restores the local buckling deformations by stretching them. The magnitude of the tensile force could be enhanced by installing diaphragms on the steel tube at the upper surface of the in-filled concrete. The ratio between the residual sway displacement δr and the maximum response sway displacement δm, defined as δr/δm for partially CFST columns, is smaller than that for hollow columns because of the enhanced strength and ductility of CFST columns. An extensive study will be carried out to derive seismic design equations for Concrete Filled Steel Tubular Columns.

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