Push-Over Analysis of Square Concrete-Filled Steel Tube Frame Structures

Literature Overview

This paper by Nie Jian-guo, Qin Kai, and Xiao Yan (2005), published in Industrial Construction, presents a push-over analysis of a 10-story square concrete-filled steel tube (CFST) frame structure. The authors conducted a comparative study with a reinforced concrete (RC) frame structure to evaluate the seismic performance and dynamic characteristics of both structural systems. The research was supported by the National Science Fund for Distinguished Young Scholars Overseas Youth Scholar Cooperation Fund (Grant No. 50128807).

Core Technical Findings

The push-over analysis provides a static incremental loading procedure that simulates the nonlinear response of structures under seismic loading. The study compares the following parameters between the CFST frame and the RC frame:

Parameter	CFST Frame	RC Frame	Relative Performance
Lateral stiffness	Higher	Lower	CFST superior
Yield displacement	Larger	Smaller	CFST superior
Ultimate displacement	Larger	Smaller	CFST superior
Ductility capacity	Higher	Lower	CFST superior
Dynamic characteristics	Favorable	Less favorable	CFST superior

The push-over curves demonstrate that the CFST frame exhibits superior seismic performance compared to the RC frame, with greater lateral stiffness, larger yield and ultimate displacements, and better ductility capacity. The dynamic characteristics comparison further confirms the advantages of the CFST frame system.

Technical Interpretation

The superior seismic performance of the CFST frame can be attributed to the composite action between the steel tube and the concrete core. The steel tube provides tensile strength and ductility, while the concrete core provides compressive strength and enhances the stability of the steel tube against local buckling. The confinement effect of the steel tube on the concrete core leads to a more ductile failure mode, which is essential for seismic resistance.

From a welding perspective, the fabrication of square CFST columns involves longitudinal and transverse welds that must maintain the integrity of the composite section. The welding process for square tubes typically involves SAW (Submerged Arc Welding) for the longitudinal seam and GTAW (Gas Tungsten Arc Welding) or FCAW (Flux-Cored Arc Welding) for the transverse welds. The quality of these welds directly affects the load transfer between the steel tube and concrete, and any weld defects can compromise the composite action that provides the superior seismic performance.

Engineering Practice Implications

The push-over analysis methodology provides a practical tool for evaluating the seismic performance of CFST structures. Engineers can use this approach to verify that the designed structure meets the required performance objectives under different seismic intensity levels. The comparison with RC frames provides valuable data for structural system selection in seismic zones.

Design Parameter	CFST Frame Recommendation	RC Frame Comparison
Story drift limit	Can be more stringent	Standard limits apply
Ductility demand	Lower demand due to higher capacity	Higher demand
Connection design	Welded or bolted with moment frames	Typically moment frames
Foundation design	Similar to RC frame	Similar

The study also provides reference data for the dynamic characteristics of CFST frames, which is essential for accurate seismic analysis. The fundamental period and mode shapes of CFST frames differ from those of RC frames, and these differences must be accounted for in the design of seismic isolation systems and damping devices.

Study Insights and Reflections

This research contributes valuable comparative data for the seismic design of CFST frame structures. The push-over analysis methodology is well-suited for evaluating the nonlinear behavior of composite structures and provides a more realistic assessment of seismic performance than linear elastic analysis. However, the study is limited to a single 10-story structure, and further research is needed to validate the findings for different structural configurations, such as moment-resisting frames, braced frames, and shear wall systems. The interaction between the welding quality of the CFST columns and the seismic performance is an important area for future investigation, as weld defects can significantly reduce the ductility capacity that is essential for seismic resistance.