Rectangular CFST Column to H-Steel Beam Internal Diaphragm Connection Test Study

Literature Overview

Published in 2020 by Liu Dianzhong and colleagues from Jilin Jianzhu University, this paper investigates the bearing capacity and seismic performance of internal diaphragm-type rectangular CFST column-to-H-steel beam connections under low-cycle reversal loading. The study is funded by the National Natural Science Foundation of China (Grant No. 51378238) and draws on practical engineering experience from multi-storey frame structures. Three edge-span connections (JD-1, JD-2, and JD-3) were tested, with particular attention to the influence of void ratio in the concrete core on structural performance.

Core Technical Findings

The internal diaphragm is one of the most widely used connection types in CFST column systems, particularly in multi-storey and high-rise buildings. The diaphragm provides a direct load transfer path from the H-beam flanges to the CFST column, bypassing the thin tube wall and reducing the risk of local buckling. The experimental study examined how the presence of voids (air pockets or incomplete concrete filling) within the CFST column affects the connection performance under cyclic loading.

Interpretation of Key Technical Points

The void ratio represents the proportion of unfilled or poorly filled concrete within the CFST column section. In practice, voids can arise from inadequate concrete pumping, insufficient compaction, or poor workability of the concrete mix. The presence of voids reduces the effective concrete area and alters the stress distribution within the column, potentially leading to premature failure or reduced ductility at the connection.

The test results indicate that the void ratio has a measurable impact on both the bearing capacity and seismic performance of the connection. Higher void ratios lead to reduced stiffness and strength, as the concrete-concrete interface and the concrete-steel tube bond are compromised. The internal diaphragm itself may also experience altered stress states if the surrounding concrete is not uniformly filled, as the diaphragm relies on the concrete core for lateral support and load distribution.

Standards and Quality Control Considerations

The findings underscore the importance of concrete quality control in CFST construction. Standards such as GB 50936 (Technical Specification for Concrete-Filled Steel Tube Structures) and JGJ 4-2008 (Technical Specification for Steel Structures) specify requirements for concrete filling, including minimum slump, compaction methods, and inspection procedures. Engineers should implement rigorous quality control measures, including:

Integration with Engineering Practice

In real-world construction, achieving complete concrete filling in CFST columns is challenging, especially for large-diameter columns or those with complex internal reinforcement. The void ratio is often non-uniform, with higher void concentrations near the top of the column or in areas with dense reinforcement. Engineers should account for this variability in design by applying appropriate safety factors or by specifying minimum filling quality requirements that are verifiable through non-destructive testing.

The internal diaphragm connection design should also consider the constructability constraints. The diaphragm must be fabricated and welded before the concrete is poured, and the welding quality must be verified before the column is erected. Any defects in the diaphragm welds or in the tube wall at the diaphragm attachment points can significantly compromise the connection performance.

Key Questions and Reflections

The study highlights a critical but often overlooked aspect of CFST construction: the quality of concrete filling. While most design codes focus on the nominal material properties and geometric parameters, the actual void ratio can vary significantly between projects and even within a single column. Future research should develop practical methods for predicting and controlling void formation, such as optimized concrete mix design, improved pumping techniques, and real-time monitoring systems. Additionally, the effect of voids on the long-term behaviour of CFST connections under sustained loads and seismic action warrants further investigation.

Study Insights and Implications

This research provides valuable experimental data on the influence of concrete voids on CFST connection performance, filling an important gap in the existing literature. The findings reinforce the need for stringent quality control during CFST construction and suggest that engineers should adopt a more conservative design approach when the void ratio cannot be reliably controlled. The internal diaphragm connection remains a robust and reliable option for CFST frame structures, provided that adequate attention is given to concrete filling quality and weld integrity. This study serves as a reminder that the performance of a CFST structure is as much a function of construction quality as it is of design parameters.