Seismic evaluation of stiffened CFST column to RC beam frames

Abstract

A stiffened concrete-filled steel tubular (CFST) member offers improved composite action over the unstiffened counterpart in terms of delaying local buckling of the square steel tube and enhancing confinement to the infilled concrete. The diagonal ribs, made of steel plates with circular openings and welded to the steel tube, were found particularly effective among various stiffening forms in improving the structural performance of thin-walled square CFST members. To better accommodate this new type of composite columns into engineering practice, the corresponding joints with reinforced concrete (RC) beams were proposed and were found to possess good axial and cyclic behavior. Consequently, composite frames composed of diagonal rib stiffened CFST columns and RC beams are expected to have superior performance and to be widely used in practice. However, the previous research mainly focused on structural members, and no study at the system-level was conducted. In addition, the guidance on seismic design of the stiffened CFST frames in mainstream design codes is lacking. To fill this research gap, this work investigated the seismic behavior of diagonal rib stiffened CFST column to RC beam frames numerically, and fiber-based finite element (FE) models were developed in software OpenSees and were validated extensively by the experimental results. The research parameters included the column-to-beam strength ratio, bond-slip of beam reinforcing bars (rebars) within the joint zone, and shear deformation in ultra-short columns. Besides, unstiffened CFST column to RC beam frames were also designed for comparison. The pushover analysis was then carried out to investigate the effect of different parameters on the strength, deformation mode, and yielding mechanism. In addition, the incremental dynamic analysis (IDA) was also conducted to study the behavior in the elastic stage, elastic-plastic stage and collapse stage. Finally, the design considerations for stiffened CFST frames were proposed. These results will facilitate the use of this efficient form of composite structures.