Quasi-Static Tension Test of Square CFST Column Diaphragm Through Joints

Literature Overview

The study by Miao Jikui, Jiang Xinliang, and Chen Zhihua (2009, Journal of Tianjin University, Vol. 42, No. 3, pp. 208-213) presents an experimental investigation of the structural performance of square concrete-filled steel tube (CFST) column to steel beam joints with diaphragm-through connections under quasi-static tension loading. Supported by the Tianjin Applied Basic and Frontier Technology Research Key Project (No. 08JCZDJC19600), the New Century Excellent Talents Support Program (NCET), and the Tianjin Construction Science and Technology Development Project (No. 2008-24), this research addresses a critical connection detail used in composite steel-concrete structures. The diaphragm-through joint, where a steel diaphragm plate passes through the CFST column and is welded to both the column tube and the beam flange, is a widely used connection type in China's high-rise construction.

Experimental Program and Specimen Configuration

Eleven cruciform joint specimens were tested under quasi-static tension loading to evaluate the bearing capacity and ductility of the diaphragm-through joint. The specimens varied in several key parameters, including diaphragm thickness, pour hole diameter, steel tube width-to-thickness ratio, and diaphragm extension length. The following table summarizes the specimen parameters:

The test specimens were designed to simulate the actual joint configuration found in composite steel-concrete buildings, where the CFST column carries axial compression and the beam transmits bending moments and shear forces through the diaphragm connection. The pour hole in the diaphragm allows concrete to be placed in the column during construction, creating a continuous concrete core that enhances the joint's load transfer capacity.

Key Experimental Findings

The study identified several important findings regarding the structural performance of the diaphragm-through joint:

1. Load-bearing capacity and ductility: The diaphragm-through joint with rounded chamfer (arc-rounded corner) exhibited good bearing capacity and ductility. The rounded chamfer reduces stress concentrations at the diaphragm edges and promotes more uniform stress distribution, which is critical for preventing premature fracture under cyclic loading.
2. Primary influencing factors: The diaphragm thickness and pour hole diameter were identified as the primary factors affecting the joint's bearing capacity. Thicker diaphragms provide greater load transfer area and stiffness, while smaller pour holes increase the effective diaphragm area and improve the connection's moment resistance.
3. Secondary influencing factors: The steel tube width-to-thickness ratio and diaphragm extension length were identified as secondary factors. A lower width-to-thickness ratio improves the local stability of the column tube walls, while a longer diaphragm extension increases the effective bearing area.
4. Effect of concrete infill: Filling the steel tube with concrete improved the joint's yield bearing capacity and stiffness, but the concrete strength had limited influence on the ultimate bearing capacity. This finding suggests that the concrete primarily contributes to the initial stiffness and yield behavior but does not significantly affect the ultimate limit state.
5. Theoretical vs. experimental comparison: The theoretical formula for local tensile bearing capacity was found to be conservative for specimens with concrete infill but overestimated the capacity of empty steel tube specimens. This discrepancy highlights the need for refined analytical models that account for the actual load transfer mechanisms in the joint.

Welding and Fabrication Considerations

From a welding and fabrication perspective, the diaphragm-through joint involves several critical welded connections that must be carefully designed and executed:

• Diaphragm-to-column tube weld: This is typically a full-penetration fillet weld or groove weld that connects the diaphragm plate to the inner surface of the square CFST column tube. The weld must achieve full fusion and adequate penetration to ensure effective load transfer. The welding process should be selected based on the steel grade and wall thickness, with FCAW or SMAW being common choices for such applications.
• Diaphragm-to-beam flange weld: This connection transfers the beam bending moment to the diaphragm and is subject to high stress concentrations. Full-penetration groove welds are typically required, with weld preparation and fit-up following AWS D1.1 or ISO 3834 standards.
• Weld quality verification: All critical welds should be inspected through non-destructive testing methods including ultrasonic testing (UT) for volumetric defects, magnetic particle testing (MT) for surface defects, and visual inspection for geometric imperfections. The acceptance criteria should comply with standards such as GB/T 3323.2 or ISO 17640.
• Preheating and interpass temperature control: For steel grades with carbon equivalent values above 0.4%, preheating temperatures of 100-150°C should be applied to prevent cold cracking. The interpass temperature should be maintained below 250°C to avoid excessive grain growth and reduced toughness.

Engineering Practice Integration

The findings of this study have direct implications for the design and fabrication of diaphragm-through joints in composite steel-concrete structures:

• Diaphragm thickness selection: The diaphragm thickness should be selected based on the beam bending moment demand and the column axial load. A minimum thickness of 20-25 mm is recommended for typical high-rise applications, with thicker plates used for heavier loading conditions.
• Pour hole design: The pour hole diameter should be minimized while allowing adequate concrete placement and compaction. A diameter of 200-300 mm is typically sufficient for most applications, and the hole should be positioned to allow full access for concrete placement tools.
• Chamfer details: Rounded chamfers at the diaphragm edges should be provided with a radius of at least 1.5 times the diaphragm thickness to reduce stress concentrations and improve fatigue resistance.
• Steel tube wall thickness: The width-to-thickness ratio of the square CFST column should be limited to prevent local buckling of the tube walls. A maximum ratio of 12-15 is recommended for typical applications, with lower ratios used for seismic zones.

The theoretical formula discrepancy identified in the study—conservative for concrete-filled specimens and non-conservative for empty tube specimens—suggests that current design codes may not adequately capture the actual load transfer mechanisms in these joints. Engineers should exercise caution when using theoretical formulas for capacity verification and consider supplementing with experimental data or finite element analysis for critical applications.

Study Insights and Recommendations

This experimental study provides valuable data on the structural performance of diaphragm-through joints, which are widely used in China's composite steel-concrete construction. The identification of diaphragm thickness and pour hole diameter as primary design parameters offers clear guidance for optimizing joint design. The finding that concrete infill improves yield capacity but has limited effect on ultimate capacity suggests that the concrete primarily contributes to the initial stiffness and serviceability performance rather than the ultimate strength.

For steel tube manufacturers and fabrication shops, the key implications are clear: the quality of the diaphragm plate material, the precision of the pour hole fabrication, and the weld quality at all critical connections directly affect the joint's structural performance. Engineers should specify detailed fabrication drawings with clear weld symbols, acceptance criteria, and inspection requirements. The use of finite element analysis to supplement experimental data and theoretical formulas is recommended for complex or critical joint configurations. As China continues to develop its composite steel-concrete construction industry, the accumulation of experimental data and the refinement of design methods will be essential for ensuring the safety and reliability of these important structural connections.