Seismic Performance of Square CFST Column-H Section Beam Connection Nodes

Literature Overview

Published in 2015 in World Earthquake Engineering (世界地震工程), Volume 31, Issue 1, this paper by Gao Zhanyang, Li Hongquan, and Wang Zhaocun from Shougang International Engineering Technology and Beijing University of Technology investigates the structural behavior of large-scale square concrete-filled steel tube (CFST) column to H-section steel beam connections under cyclic loading. The study is grounded in a real engineering project and examines two connection configurations: internal diaphragm (内隔板) connections and through-type diaphragm (贯通式隔板) connections, totaling four test specimens.

Experimental Configuration and Test Program

The experimental program was designed to evaluate both the load-bearing capacity and energy dissipation characteristics of the connections. Each specimen was subjected to low-cycle reversed loading, which simulates the cyclic nature of seismic forces. The primary measured parameters include elastic limit load and displacement, yield load and displacement, ultimate load and displacement, and hysteresis behavior indicators.

The internal diaphragm connection features a steel plate welded inside the CFST column that provides bearing and shear transfer between the column and the beam. The through-type diaphragm connection uses a plate that penetrates through the column cross-section, with additional welding on both faces. Both configurations rely on welded connections between the diaphragm plate and the column wall, as well as between the diaphragm plate and the beam flange.

Key Findings and Technical Analysis

The most significant finding is that the internal diaphragm connection outperforms the through-type diaphragm connection in both load-bearing capacity and energy dissipation. This result may initially seem counterintuitive, as through-type connections typically provide more direct load paths. However, the internal diaphragm configuration likely benefits from better confinement interaction — the concrete core in a square CFST column interacts more effectively with an internal plate, creating a composite action that enhances both strength and ductility.

The paper reports that welds did not crack during testing, indicating that the flux-cored wire welding (药芯焊) used for the primary structural welds provided adequate strength and toughness. This is a critical observation for seismic design, where weld integrity under cyclic loading is a common failure mode. The use of flux-cored arc welding (FCAW) for these connections is consistent with industry practice for heavy structural applications, where the process offers good penetration, high deposition rates, and acceptable toughness in thick sections.

Hysteresis and Energy Dissipation

The hysteresis curves and skeleton curves obtained from the tests provide quantitative measures of ductility and energy dissipation capacity. The internal diaphragm specimens exhibited fuller hysteresis loops, indicating better energy dissipation. In seismic design, energy dissipation is as important as peak strength — a connection that can dissipate seismic energy through controlled inelastic deformation without sudden failure is preferred over one that is stronger but more brittle.

Engineering Practice Implications

For engineers designing seismic-resistant structures with CFST columns, this study provides valuable guidance on connection selection. The preference for internal diaphragm connections aligns with the principle of "strong column, weak beam" — the connection should yield in a ductile manner before the column or beam reaches its capacity. The square CFST column is particularly challenging because the flat faces of the square section are susceptible to local buckling under concentrated beam flange forces, and the connection design must address this vulnerability.

From a welding quality control perspective, the absence of weld cracking is encouraging but does not eliminate concerns. The paper does not report non-destructive testing results (such as ultrasonic testing or magnetic particle inspection) for the welds, nor does it discuss weld preparation details such as groove geometry, root gap, or weld sequence. In practice, weld sequence is critical for minimizing residual stress and distortion in thick-section connections, and the welding procedure specification (WPS) parameters would significantly influence fatigue and seismic performance.

The connection details should be evaluated against applicable codes such as GB 50011 (Seismic Design Code for Buildings) and GB 50017 (Design Standard for Steel Structures), which specify requirements for connection ductility, weld quality grades, and detailing provisions for special structural elements. The internal diaphragm connection, with its demonstrated superior seismic performance, should be given priority consideration in new CFST structural designs, particularly for buildings in high seismic zones.

Study Insights and Reflections

The study's limitation lies in the relatively small number of test specimens — only four, with two for each connection type. This sample size is insufficient to establish statistical confidence in the comparison, and the results may be influenced by specimen-specific factors such as steel grade variations, concrete quality differences, or welding execution quality. Future research should expand the specimen matrix to include variations in column wall thickness, concrete strength, beam-column size ratio, and welding procedure to establish more robust design guidelines.