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Concrete-filled steel tubes (CFSTs) are widely used in construction. To achieve composite action and take full advantage of the two materials, strain continuity at the steel–concrete interface is essential. When the concrete core and steel tube are not loaded simultaneously in regions such as beam or brace connections to the steel tubes of a CFST column, the steel–concrete bond plays a crucial role in load transfer. This study uses a validated finite-element model to investigate the bond-slip behavior between the steel tube and concrete in square CFST mega columns through a push-out analysis of eleven 1.2- × 1.2-m mega columns. The bond-slip behavior of CFST mega columns with and without mechanical connectors, including shear studs, rib plates, and connecting plates, is studied. The finite-element results indicate that the mechanical connectors substantially increased the maximum bond stress. Among the analyzed CFST mega columns, those with closely spaced shear studs and rib plate connectors with circular holes exhibited the highest bond stress, followed by plate connectors and widely spaced shear stud connectors. In the case of shear stud connectors, the stud diameter and spacing influenced the bond behavior more than the stud length. As the stud spacing decreased, the failure mode shifted from studs shearing off to outward buckling of the steel tube.

In this study, the fire resistance performance of steel–polymer prefabricated composite floors, which have a sandwich-type structure, was assessed via standard fire tests and analyzed using finite element analysis. This form of analysis should consider two aspects, namely the thermal and structural fields, so as to simulate complicated material properties and large deformations. As previous studies have already conducted analysis in the thermal field, this study entailed only the structural analysis based on the temperature distributions obtained from the thermal analysis. The variables of the specimens were the thicknesses of the top and bottom steel plates and polymers. According to the analysis results, the top steel plate thickness had no impact on the stability ratings, a criterion for fire resistance performance, whereas the bottom steel plate showed a linear correlation with the stability rating. An equation for the stability rating of composite floors was proposed, and an equation for fire resistance performance was devised based on the insulation ratings, which were obtained from the thermal analysis results.

Beam to column connection ductility and beam flexural stiffness are crucial parameters for seismic performance and drift control in moment-resisting frames. However, an increase in beam span and depth to obtain column-free flexible internal space affects these parameters negatively. In this paper, a new long-span deep beam-column connection detail is proposed with improved stiffness and adequate ductility for moment frames in low-to-moderate seismic zones. As panel zones can undergo plastic deformation beyond the joint rotation demand in low-to-moderate seismic zones, the beam flange and panel zone in the proposed connection are proportioned such that a stable yielding occurs in the panel zone and two plastic zones on the beam segment away from the beam-column interface. The beam flanges at the beam-column interface are thickened, widened, and tapered to control beam plastic hinge locations and mitigate distress resulting from significant panel zone plastic deformation. The performance of the proposed connection was evaluated through a full-scale cyclic load test and parametric finite element analysis on a beam-column connection with a 1200 mm deep beam. The test and finite element results showed the formation of plastic hinges at the anticipated locations. Furthermore, the connection exhibited stable hysteresis beyond the minimum rotation limits specified for intermediate moment frames (American code) and ductility class medium (Eurocode). Moreover, the connection effectively allowed balancing between beam and panel zone yielding while hindering beam flange distress associated with significant panel zone plastic deformation. Furthermore, a comparison between the proposed and welded unreinforced flange-welded web connection demonstrated improved ductility, stiffness, moment capacity, and energy dissipation capacity. Additionally, equations were proposed to quantify the improvements, and balance beam and panel zone energy dissipation.