Zhuojin Pipe Fitting Co., LtdMechanical Performance of Rectangular Steel Pipe High-Strength Concrete Bidirectional Compression-Bending Members
Literature Overview
The paper by Tian Hua, Zhang Sumei, and Guo Lanhui, published in Journal of Harbin Institute of Technology (2007, Vol. 39, No. 12, pp. 1854–1858), investigates the mechanical behavior of rectangular steel pipe high-strength concrete (HSRC) members under bidirectional compression-bending loading. The research was supported by the National Natural Science Foundation of China (59808004). The authors developed a nonlinear numerical analysis program that accounts for residual stresses, and they analyzed the effects of various parameters on the member capacity. A simplified calculation formula was proposed and validated against both numerical and experimental results.
Background and Significance
Rectangular steel pipe concrete members are widely used in building frames, industrial structures, and special structures where the rectangular cross-section provides advantages in architectural integration and connection detailing. When subjected to bidirectional compression-bending (simultaneous bending about both principal axes), the behavior of these members becomes complex due to the interaction between the two bending components and the axial compression.
High-strength concrete (HSRC) with compressive strength exceeding 60 MPa is increasingly used in steel pipe concrete members to achieve higher load-carrying capacity and smaller member cross-sections. However, the use of HSRC introduces additional complexity because the concrete is more brittle and the confinement effect of the steel pipe becomes more critical.
Numerical Analysis Method
The nonlinear numerical analysis program developed by the authors considers the following factors:
| Factor | Description | Impact on Results |
|---|---|---|
| Residual stresses | From cold forming and welding of the steel pipe | Reduces effective yield stress |
| Concrete confinement | Lateral confinement by the steel pipe | Increases concrete strength and ductility |
| Material nonlinearity | Elastic-plastic steel and nonlinear concrete | Captures post-yield behavior |
| Geometric nonlinearity | Large deformations and P-Δ effects | Important for slender members |
| Loading path | Different sequences of axial and bending loads | Affects failure mode |
The program uses a fiber-based approach, dividing the cross-section into multiple layers and calculating the stress in each layer based on the strain distribution. The equilibrium equations are solved iteratively using the Newton-Raphson method.
Key Results
Parameter Effects on Capacity
The study analyzed the effects of the following parameters on the member capacity:
| Parameter | Range Studied | Effect on Capacity |
|---|---|---|
| Slenderness ratio | 10–40 | Decreases capacity significantly |
| Steel yield strength | 235–460 MPa | Increases capacity moderately |
| Concrete compressive strength | 30–80 MPa | Increases capacity, especially at low slenderness |
| Steel ratio (ρs) | 1%–5% | Increases capacity significantly |
| Loading path | Different sequences | Minor effect on ultimate capacity |
| Loading angle | 0°–90° | Moderate effect on capacity |
Simplified Calculation Formula
The authors proposed a simplified calculation formula for the three-dimensional capacity interaction surface of rectangular steel pipe HSRC members under bidirectional compression-bending. The formula was validated against:
- Numerical analysis results: Good agreement
- Experimental test results (2 specimens): Good agreement
The simplified formula provides results that are slightly conservative compared to the numerical analysis, which is considered acceptable from a safety perspective.
Loading Path Effects
One of the interesting findings of the study is that the loading path has a relatively minor effect on the ultimate capacity of the member. This means that the final failure load is not highly sensitive to the sequence in which the axial load and bending moments are applied. However, the loading path does affect the deformation characteristics and the progression of damage within the member.
The loading angle (the angle between the resultant bending moment vector and the principal axes of the cross-section) has a more significant effect on the capacity. Members loaded at angles other than the principal axes generally have lower capacity than those loaded about a single principal axis, due to the asymmetric stress distribution in the rectangular cross-section.
Welding and Fabrication Considerations
The fabrication of rectangular steel pipe HSRC members requires particular attention to the following welding aspects:
- Corner welds: The corners of rectangular steel pipes are formed by welding flat plates together. These welds are critical because they are subject to high stress concentrations, especially under bidirectional bending. Full-penetration groove welds with adequate reinforcement should be used, and the welds should be inspected by UT or RT.
- Residual stresses: The cold forming and welding processes introduce residual stresses in the steel pipe, which can reduce the effective yield stress and affect the buckling behavior. The authors' numerical model accounts for residual stresses, and engineers should ensure that the residual stress distribution is characterized accurately.
- Welding sequence: The welding sequence for rectangular steel pipes should be planned to minimize distortion and residual stresses. Symmetric welding sequences are recommended to balance the heat input.
- Concrete filling: The concrete should be placed with care to ensure complete filling of the rectangular pipe, particularly at the corners where voids are more likely to form. The use of SCC or vibrated concrete is recommended.
Study Insights and Reflections
This study makes a valuable contribution to the understanding of rectangular steel pipe HSRC members under complex loading conditions. The development of a simplified calculation formula that accounts for residual stresses and the three-dimensional capacity interaction is particularly useful for engineering practice.
The finding that the loading path has a minor effect on ultimate capacity is reassuring from a design perspective, as it means that the design does not need to be overly sensitive to the exact loading sequence during construction or service.
One area for further investigation would be the behavior of rectangular steel pipe HSRC members under cyclic loading, which is important for seismic applications. The current study focuses on static loading, and the cyclic behavior may be different due to the accumulation of damage and the degradation of the concrete under repeated loading.
The study also highlights the importance of residual stresses in the numerical analysis of steel pipe concrete members. Engineers should ensure that their analytical models account for residual stresses, as neglecting them can lead to overestimation of the member capacity.
The validation of the simplified formula against experimental results provides confidence in its application for practical design, and the conservative nature of the results is appropriate for safety-critical applications.