Connections to Concrete-Filled Steel Tubes

CONNECTIONS TO CONCRETE-FILLED STEEL TUBES The objective of this research program was to study connections to concrete-filled steel tubes. This research focused on the connection to circular steel tubes, because this shape presents more detailing difficulties compared to the square counterpart. To accomplish the goals of this study, the research was divided into three phases: prototype frame design and analysis, a finite element analysis of several connections, and an experimental study on six large-scale connection specimens. In the first phase, two prototype moment-resisting steel frames were designed to satisfy the 1991 NEHRP Provisions. The inelastic performance of these frames was studied, and the connections with the highest ductility demand were isolated. The second phase of the research was devoted to the inelastic fmite element analysis of several connection details. A 3-D finite element model was developed for each connection detail. The parameters used in the analytical study included: the diameter-ta-tube waIl thickness ratio, the applied axial load on the column, the moment-ta-shear ratio of the girder, and the yield strength of the steel tube. In the third phase of the research, six 2/3rd-scale specimens were fabricated and tested with the quasi-static method. The flexural strength of the connection was the primary concern in this study. The analytical models and each experimental specimen were aT-shape configuration. A monotonic load was applied to the analytical finite element models. For the test specimens, a predetermined cyclic displacement was imposed at the girder tip, and the specimens were tested to destruction. Analytical and experimental results suggest that connections which attach exclusively to the tube wall exhibit large distortion of the tube wall in the connection vicinity, thus preventing the development of the girder flexural strength. Using external diaphragms improved the simple connection behavior, however, the performance was susceptible to the geometry of the diaphragm. The behavior was significantly improved when part of the girder forces was transferred to the concrete core. However, the improvement in behavior depended on the connection detail. Continuing the girder through the concrete-filled steel tube was the most effective method to develop the full plastic hinge in the connected girder.

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