Seismic resistance of concrete-filled square steel hollow structuralsection beam-columns.

Abstract

This thesis describes a research program designed to investigate the seismic performance of square concrete-filled tubular (CFT) columns subjected to a combination of axial load and biaxial bending moment. The square CFT columns included normal strength and high-strength concrete-filled square steel hollow structural section columns with two ratios of tube wall thickness to tube dimension. An extensive experimental program included testing twenty-four, concrete filled, square steel hollow structural section column specimens under constant axial compressive load and cyclic horizontal load with horizontal displacement control. This experimental investigation included strength capacity, ductility and flexural stiffness of CFT beam-columns for 203 x 203 x 9.5 HSS (Class 1, compact sections) and 203 x 203 x 4.8 (Class 4, slender sections) hollow structural section (HSS) filled with either normal strength or high strength concrete. Class 1 (compact) and Class 4 (slender) HSS CFT columns subjected to constant axial load and cyclic shear displacements exhibited excellent hysteresis behavior using either normal strength concrete or high strength concrete. No premature failure, local buckling before yielding, was observed for either Class 1 or Class 4 HSS CFT columns. The drift ratios exceeded 7%. The moment capacities of HSS CFT columns loaded with the plane of bending across the diagonal were only slightly less than those loaded with the plane of bending parallel to the square side. This reduces the need for elaborate procedures to calculate the biaxial moment capacities at orientations between 0 and 45°. The strength capacity of square HSS CFT columns subjected to constant axial compressive load and biaxial cyclic bending moment can be conservatively calculated by column section analysis using fully plastic behavior for both concrete and steel. Strength modification factors alpha1 and alpha 2 accommodate confinement and seismic loading, fast concrete strain rates, effects on the CFT column sections and softening of the columns caused by large axial load. Flexural stiffness of square HSS CFT columns can be calculated using the ACI 318-99 flexural stiffness formula with modification to the coefficient of the flexural stiffness contributed by the concrete.