CONNECTIONS TOCONCRETE-FILLED STEEL TUBESByYOUSEF MOHAMMED ALOSTAZandSTEPHEN PATRICK SCHNEIDERA Report on Research Sponsored by theNATIONAL SCIENCE FOUNDATIONNSF CMS 93-00682DEPARTMENT OF CIVIL ENGINEERINGUNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGNOCTOBER 1996

ABSTRACTCONNECTIONS TO CONCRETE-FILLED STEEL TUBESThe objective of this research program was to study connections to concrete-filled steel tubes. Thisresearch focused on the connection to circular steel tubes, because this shape presents more detailingdifficulties compared to the square counterpart. To accomplish the goals of this study, the research wasdivided into three phases: prototype frame design and analysis, a finite element analysis of severalconnections, and an experimental study on six large-scale connection specimens. In the first phase, twoprototype moment-resisting steel frames were designed to satisfy the 1991 NEHRP Provisions. The inelasticperformance of these frames was studied, and the connections with the highest ductility demand wereisolated. The second phase of the research was devoted to the inelastic fmite element analysis of severalconnection details. A 3-D finite element model was developed for each connection detail. The parametersused in the analytical study included: the diameter-ta-tube waIl thickness ratio, the applied axial load on thecolumn, the moment-ta-shear ratio of the girder, and the yield strength of the steel tube. In the third phaseof the research, six 2/3rd-scale specimens were fabricated and tested with the quasi-static method. Theflexural strength of the connection was the primary concern in this study. The analytical models and eachexperimental specimen were aT-shape configuration. A monotonic load was applied to the analytical finiteelement 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 tubewall exhibit large distortion of the tube wall in the connection vicinity, thus preventing the development ofthe 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 improvedwhen part of the girder forces was transferred to the concrete core. However, the improvement in behaviordepended on the connection detail. Continuing the girder through the concrete-filled steel tube was the mosteffective method to develop the full plastic hinge in the connected girder.Lll