Seismic design, performance, and behavior of composite-moment frames with steel beam-to-concrete filled tube column connections

Concrete filled steel tube (CFT) columns have been widely used in composite-moment frames (C-MFs) both in non-seismic and in high seismic zones. The objective of this research is to develop a design methodology of such moment resisting frame structures designed with CFT columns in achieving ductile behavior and high strength. These composite-moment frames mostly constructed around the perimeter of the building provide the enough stiffness to withstand the lateral displacement due to wind or seismic loads. In this research, three sets of prototype composite frame models were designed on the basis of the proposed design examples as 3-, 9-, and 20-story post-Northridge SAC buildings with composite-special moment frame (C-SMF) systems designed for the western US area. The exact moment-rotational behavior of steel beam-to-CFT column connections including the strength degradation was simulated using the 2D joint model with the rigid boundary element. Nonlinear pushover analyses were conducted on the numerical frame models so as to evaluate the over-strength, inelastic deformation, and P-Delta effect for the entire structure. The statistical investigation was introduced to nonlinear dynamic analyses under 40 SAC ground motions corresponding to a seismic hazard level of 2% probability of exceedence in 50 years in order to efficiently examine seismic performance and behavior of entire composite frame structures. All frame models meet the allowable limit for safe designs. In addition, the entire frame design becomes conservative as the number of stories increases. The distribution of interstory drift ratios (ISDRs) as well as the over-strength ratio also demonstrates this conservative design of low to high-rise CMF structures.