Evaluation of embedded concrete-filled tube (CFT) column-to-foundation connections

Concrete-filled tubes (CFTs) are effective structural components. Relative to conventional reinforced concrete components, they have higher strength-to-size efficiency and facilitate rapid construction. Yet they are not used frequently in US construction. The component response is well understood from extensive experimental and analytical studies that have been conducted on CFT components themselves. There remains a fundamental challenge of implementing these components in structural systems, which is caused by the uncertainty of the connection design and performance. Relative to component research, limited research has focused on the connections of CFT columns. A recent research effort has resulted in a new embedded connection for a CFT column anchored into a reinforced concrete foundation. The connection is fully capable of transferring combined bending and axial loads and has sufficient deformability to sustain multiple inelastic deformation cycles under extreme loading. In addition, the connection has unique features that facilitate constructability and rapid construction. However, the connection design to date is solely based on experimental study, which evaluated a very limited range of design parameters. A coordinated analytical study, using high-resolution continuum models, was undertaken to investigate the unstudied parameters and develop appropriate design expressions, which is the subject of this paper. An analytical model was developed and verified using the test results. The verified model was then used to conduct a parametric study to enhance the understanding of the experimental behavior and extend the test databases. Parameters studied included the embedment depth, diameter-to-thickness (D/t) ratio, shear reinforcement ratio, strength ratio of the concrete in the footing and concrete infill, and the axial load ratio. From the analysis results, the failure mechanisms were evaluated with respect to the individual parameters. The results showed that an increase in embedment depth, D/t ratio, shear reinforcement ratio, the axial load ratio, or the ratio of the concrete strength in the footing relative to the concrete infill of CFT column increased the connection strength and could result in ductile yielding of the CFT column, which is the desired response mode. Finally, the analysis and test results were combined to develop a refined design equation for the required embedment depth.