Seismic behavior of high strength square CFT beam-columns

The behavior of high strength square CFT beam-columns was experimentally investigated. The CFT specimens were 305 mm square tubes made from conventional (A500 Grade-B) or high strength (A500 Grade-80) steel with nominal width-to-thickness (b/t) ratios of 32 or 48, and filled with high strength (104 MPa) concrete. The effect of the stress-strain characteristics and nominal b/t ratio of the steel tube on the stiffness, strength and ductility of the high strength CFT specimens was investigated. Eight monotonic beam-column tests and three cyclic beam-column tests were conducted. The experimental results indicate that the concrete infill delays the local buckling of the steel tube and that the steel tube offers some confinement to the infill concrete thus increasing its compressive ductility. The monotonic curvature ductility of the CFT beam-column specimens decreases significantly with an increase in the axial load level and the b/t ratio. Cyclic loading does not have a significant influence on the flexural stiffness and moment capacity of the beam-column specimens. However, the post-peak moment resistance of the beam-column specimens decreases more rapidly under cyclic loading. This has a significant influence on the cyclic ductility of the specimens. The moment capacity of axially loaded beam-column specimens can be conservatively predicted by using the American Concrete Institute provisions for conventional strength CFT beam-columns.