Experimental and Finite Element Study on Square Hollow Sandwich CFST Compression-Torsion Members

Literature Overview

This study, published in Industrial Construction (2015, Vol. 45, No. 7, pp. 148-152), investigates the behavior of square hollow sandwich steel tube concrete (CFST) members under combined compression and torsion. The authors are Guo Lixiang from Jiangxi Hangxiao Steel Structure Co., Ltd., Li Ting and Huang Hong from East China Jiaotong University, and Yang Jian from Qingdao University of Technology (Linyi). The research is supported by multiple funding sources including the National Natural Science Foundation (Nos. 51008122 and 51378206), Jiangxi Provincial Young Scientist Training Plan (No. 20133BCB23015), and Jiangxi Provincial Education Department Science and Technology Project (Gan Cai Jiao (2011) 243).

Core Technical Content

Test Configuration

The study involves 7 square hollow sandwich CFST specimens and 1 square solid CFST specimen subjected to combined compression and torsion loading. The key parameters varied include:

Key Experimental Results

The experimental findings reveal several important behaviors:

• All specimens exhibit good ductility under combined compression and torsion loading
• Interlayer concrete develops approximately 45-degree diagonal cracks
• Concrete does not exhibit crushing failure
• When axial compression ratio is small (n ≤ 0.3): higher axial compression ratio leads to higher torsional capacity
• When axial compression ratio is large (n > 0.3): higher axial compression ratio leads to lower torsional capacity

Finite Element Validation

Finite element analysis was employed to simulate the torque-angle relationship curves. The FEM results show good agreement with experimental measurements, validating the analytical approach for predicting the behavior of these complex members.

Technical Analysis and Discussion

The Critical Axial Compression Ratio Threshold

The most significant finding is the existence of a critical axial compression ratio (n = 0.3) beyond which the interaction between axial compression and torsional capacity changes fundamentally. Below this threshold, axial compression enhances torsional capacity through confinement effects. Above this threshold, the compressive state reduces the concrete's ability to resist diagonal tension from torsion.

This observation has direct implications for design:

1. Members designed for combined compression and torsion should be checked at the critical axial compression ratio
2. The interaction curve between axial load and torsional capacity is non-monotonic
3. The hollow sandwich configuration provides additional confinement through the interlayer concrete

Implications for Steel Pipe Fabrication

The hollow sandwich CFST configuration requires careful fabrication:

• The outer steel tube must be precisely formed to maintain uniform wall thickness
• Internal concrete placement must ensure complete filling of the sandwich layer
• Welding of any internal connections must not compromise the concrete-steel bond
• The hollow space may require internal bracing during construction that must be removed before concrete placement

From a welding quality perspective, the diagonal crack pattern at approximately 45 degrees indicates that the failure mechanism is governed by principal tensile stress in the concrete. Weld defects that create stress concentrations could initiate premature failure at these critical locations. Non-destructive testing (NDT) protocols should include:

• Magnetic particle testing (MT) for surface cracks near welds
• Ultrasonic testing (UT) for volumetric defects
• Visual inspection of weld geometry and profile

Engineering Practice Connections

The hollow sandwich CFST configuration offers advantages in specific applications:

• Weight reduction: The hollow space reduces self-weight while maintaining structural capacity
• Serviceability: The hollow space can accommodate utilities or provide thermal insulation
• Manufacturing efficiency: The sandwich configuration allows for staged concrete placement

However, the fabrication challenges are significant. The study's findings on ductility and crack patterns provide valuable data for developing fabrication quality control procedures. The 45-degree crack orientation aligns with theoretical predictions for pure torsion failure, suggesting that the axial compression component does not significantly alter the failure mechanism in the interlayer concrete.

Study Insights

This research demonstrates that hollow sandwich CFST members offer a promising structural configuration with good ductility and predictable failure behavior. The identification of the critical axial compression ratio (n = 0.3) provides a clear design criterion for engineers. The finite element validation confirms that analytical models can reliably predict member behavior, supporting the use of FEM in design verification.

The practical significance lies in the demonstration that weight reduction through hollow configurations does not compromise ductility or predictability of failure. For steel pipe manufacturers, this opens opportunities for developing specialized hollow sandwich pipe systems with integrated features. The study also highlights the importance of interlayer concrete quality, as the crack pattern and failure behavior are directly governed by concrete properties in the sandwich layer.