Behavior Of Concrete Filled Double-Skin Steel Tubular Beam-Columns

Behavior and mechanism of Concrete filled double-skin steel tubular (CFDST) beam-columns subjected to monotonic or cyclic loading were investigated. Two kinds of CFDST section tubes were studied in this paper, that is, the outer steel tubes are circular hollow sections (CHS) or square hollow sections (SHS), while all the inner steel tubes are CHS. Based on systematic parameter analysis, simplified models were derived to predict the load-carrying capacities of the composite members, and simplified moment-curvature and lateral load-lateral deflection hysteretic models, as well as ductility coefficient were suggested.The main achievements were summarized in detail as follows:(1) The CFDST tubular beams, columns and beam-columns with CHS outer steel tubes were tested systematically. The main experimental parameters were tube diameter-to-thickness ratio, hollow section ratio, eccentricity and slenderness ratio.(2) Load-deformation relationship curves of CFDST subjected to compression and bending were analyzed by finite element method and fiber element method. The predicted load versus deformation relationship curves are in good agreement with those of tests. Stress distributions of steel and concrete, interactions between steel and concrete and failure modes of inner and outer steel tubes were analyzed by finite element method. The influences of hollow section ratio on the mechanism of CFDST beams, columns and beam-columns were analyzed. The influences of different loading paths on the bearing capacities of the CFDST beam-columns were also discussed. Parametric analysis was performed on the influences of bearing capacities of the composite members by fiber element method. The selected parameters were hollow section ratio, steel ratio, strength of the materials, etc. Simplified models were derived for the member capacities of the composite beam-columns.(3) The CFDST beam-columns were tested under constant compressive axial load and cyclic lateral load. The main parameters varied in the tests are axial load ratio and hollow section ratio.(4) Moment-curvature hysteretic curves, load-displacement hysteretic curves and their skeleton curves were calculated by fiber element method. Moment-curvature relationship skeleton curves and load-displacement relationship skeleton curves were also calculated by finite element method. The predicted curves are in good agreement with those of tests. Based on the fiber element method, parametric analysis was performed on the behaviors of moment-curvature and lateral load-lateral displacement relationships for the composite beam-columns. The selected parameters were the axial load level, slenderness ratio, steel ratio, strength of the materials, etc. Finally, simplified models for the moment-curvature and the lateral load-lateral displacement hysteretic relationships, as well as simplified formula for calculating ductility coefficient were suggested.