An Experimental Evaluation of High-Strength Square CFT Columns

Abstract

The behavior of concrete filled tube (CFT) columns made from high strength materials was investigated experimentally. The effects of the width-to-thickness (b/t) ratio, steel tube stress-strain characteristics, and axial load on the stiffness, strength, and ductility of CFT beam-columns and stub columns were studied. Twelve experiments, which included four stub tests (monotonic axial load) and eight beam-column tests (constant axial and monotonic axial load) and eight beam-column tests (constant axial and monotonic flexural load) were conducted. The CFT specimens were 305 mm square tubes, made from either conventional (A500 Grade-B) or high strength (A500 Grade-80) steel with nominal b/t ratios of 32 and 48. The CFT specimens were filled with high strength (104 MPa) concrete. Experimental results indicate that the concrete infill delays, the local buckling of the steel tube, and that for lower levels of axial load and smaller b/t ratios the steel confines the infill concrete, thus increasing its ductility. Comparison of the experimental results with predictions based on current code provisions indicates that the axial load capacity of the high strength CFT stub column specimens can be predicted with reasonable accuracy by superposition of the yield strength of the steel tube and 85% of the compressive strength of the concrete infill. The moment capacity of the high strength CFT beam-column specimens can be conservatively estimated using American Concrete Institute provisions for conventional strength CFT beam-columns. The initial and serviceability-level section flexural stiffness of these specimens was predicted with reasonable accuracy using the uncracked transformed and cracked transformed section properties, respectively. The experimental results indicate that the curvature ductility of a high strength CFT beam-column decreases significantly with an increase in the axial load or the b/t ratio of the steel tube.