Static Tensile Testing of Square CFST Column Diaphragm-Through Joints

Overview of the Study

This research, conducted by Miao Jikui and colleagues from Tianjin University and Shandong Jianzhu University, presents experimental and theoretical investigations on the static tensile behavior of diaphragm-through joints in square CFST columns connected to steel beams. The study was supported by the Tianjin Key Project of Applied Basic and Frontier Technology Research (08JCZDJC19600) and published in the Journal of Tianjin University in 2009.

Experimental Program and Test Results

Eleven cruciform joint specimens were designed and tested under quasi-static tensile loading to evaluate the load-bearing capacity, ductility, and failure modes of diaphragm-through joints. The diaphragm-through joint is a connection detail in which a steel diaphragm plate passes through the CFST column, providing a direct load path for beam flange forces into the column wall.

Specimen Configuration and Parameters

Parameter	Description	Range
Diaphragm thickness	Key variable	Varying
Casting hole diameter	Key variable	Varying
Steel tube width-to-thickness ratio	Secondary variable	Varying
Diaphragm extension length	Secondary variable	Varying
Concrete fill	Filled vs. empty tubes	Two conditions

Failure Modes Observed

Two primary failure modes were identified:

Diaphragm-type failure: Local yielding and rupture of the diaphragm plate near the casting hole, characterized by visible plastic deformation of the diaphragm with limited column wall involvement.
Column wall-type failure: Local buckling or yielding of the square steel tube wall at the diaphragm interface, with the diaphragm remaining largely elastic.

Theoretical Analysis and Comparison

The authors developed a local tensile bearing capacity formula for diaphragm-through joints, considering the effective bearing area of the diaphragm, the resistance of the column wall, and the contribution of concrete infill. The theoretical predictions were compared against experimental results.

Comparison of Theory and Experiment

Specimen Type	Theory vs. Experiment	Assessment
Concrete-filled specimens	Theory is conservative (underestimates capacity)	Acceptable for design
Empty steel tube specimens	Theory overestimates capacity	Not conservative; requires revision

This discrepancy is significant from a design safety perspective. The theoretical model appears to overestimate the confinement effect of concrete on the column wall, leading to non-conservative predictions for empty tubes where no concrete confinement exists.

Engineering Practice Implications

Key Design Recommendations

The diaphragm thickness and casting hole diameter are the dominant parameters controlling joint capacity; engineers should prioritize optimizing these two variables during design.
The steel tube width-to-thickness ratio and diaphragm extension length have secondary effects; moderate variations in these parameters produce relatively small changes in capacity.
Concrete filling enhances yield capacity and stiffness but has limited influence on ultimate bearing capacity, suggesting that concrete primarily contributes to elastic behavior rather than ultimate strength.
The rounded chamfer (arc-fillet) diaphragm geometry demonstrates superior load-bearing capacity and ductility compared to sharp-edged diaphragms, owing to reduced stress concentration at the casting hole.

Design Formula Considerations

The theoretical formula should be modified for empty steel tube applications. A practical approach is to apply a reduction factor of approximately 0.85–0.90 to the theoretical capacity when no concrete infill is present, based on the observed overestimation. For concrete-filled joints, the current formula provides a conservative estimate, which is acceptable for design but may lead to over-conservatism and unnecessary material usage.

Critical Reflections and Study Insights

The study highlights an important gap between theoretical models and experimental reality, particularly for empty steel tube joints. The overestimation of capacity in empty tubes suggests that the theoretical model implicitly assumes a level of confinement that does not exist without concrete. This finding has direct implications for the design of steel-only diaphragm-through joints, which may be used in applications where concrete placement is impractical.

From a manufacturing perspective, the emphasis on diaphragm thickness and casting hole diameter underscores the importance of precision fabrication. The casting hole diameter must be carefully controlled to ensure proper fit-up with the CFST column while maintaining adequate diaphragm cross-section. Welding quality at the diaphragm-column interface is also critical, as the load transfer mechanism relies heavily on the integrity of these welds.