Bending Capacity of Non-Equal Wall Thickness Rectangular Steel Tube Concrete Beams

Literature Overview

This paper by Lu Fangwei, Yang Weixing, and Wang Xun from Wuhan Municipal Engineering Design and Research Institute, published in Industrial Construction (Vol. 40, No. 10, 2010), investigates the bending bearing capacity of non-equal wall thickness rectangular steel tube concrete (CFT) structural members. The work was supported by the National Natural Science Foundation of China (Grant No. 50478020). The authors derive a theoretical formula for bending capacity based on established assumptions and validate it through degradation analysis and comparison with experimental results.

Core Technical Content

The fundamental concept behind non-equal wall thickness rectangular CFT beams is cross-sectional optimization. In conventional rectangular CFT columns or beams, all four walls share the same thickness. However, when such a member is used as a beam subjected to bending, the top and bottom walls (parallel to the bending axis) experience significantly different stress states compared to the lateral walls. The non-equal wall thickness design assigns a thicker wall to the flange region and a thinner wall to the web region, thereby reducing material usage while maintaining or improving structural performance.

Theoretical Derivation

The theoretical formula derivation follows the classical plastic hinge approach adapted for the composite section. Key assumptions include:

• Plane sections remain plane after deformation.
• The steel tube and concrete are fully bonded with no slip at the interface.
• The constitutive models for steel and concrete follow the standard stress-strain relationships defined in relevant Chinese codes (GB 50017 and GB 50010).
• The neutral axis location is determined by force equilibrium considering the non-uniform wall thickness distribution.

The bending moment capacity $M_u$ is computed as the sum of contributions from the top flange wall, bottom flange wall, two web walls, and the concrete core, each integrated over their respective stress distributions.

Degradation Analysis and Validation

The authors performed a degradation analysis to verify the correctness of the proposed formula. By setting the top and bottom wall thicknesses equal, the formula should reduce to the known solution for equal-wall-thickness rectangular CFT beams. The successful reduction confirms the mathematical consistency of the derivation.

Engineering Practice Implications

From a manufacturing perspective, producing non-equal wall thickness rectangular tubes presents distinct challenges compared to standard equal-wall profiles. The forming process must accommodate differential thickness in a single continuous operation, which typically requires either:

• Roll forming with variable reduction stations: Each station in the roll forming line is designed to achieve the target thickness profile progressively.
• Hydroforming: A hydraulic forming process that can produce complex variable-thickness profiles in a single operation.
• Welded construction: Fabricating the beam from individually rolled plates welded together, which offers maximum flexibility but increases labor and welding inspection requirements.

For welded fabrication, the weld quality at the junction between thick flange plates and thin web plates becomes critical. The differing thermal mass at the weld zone can lead to uneven cooling rates, potentially causing residual stress concentrations and microstructural heterogeneity in the heat-affected zone. Preheating, controlled welding sequence, and post-weld stress relief should be specified to mitigate these risks.

Connection with Standards

The design should comply with relevant provisions in GB 51248 (Technical Code for Concrete-Filled Steel Tubular Structures) and GB 50017 (Standard for Design of Steel Structures). The interaction between steel tube and concrete, including the confinement effect, must be accounted for in the section properties. The non-equal wall thickness configuration requires careful attention to local buckling checks for the thinner web walls under compressive stress.

Key Questions and Reflections

One critical question that arises from this research is: what is the optimal wall thickness ratio that minimizes the total section weight while satisfying all strength and stability requirements? The paper provides a theoretical framework, but the optimization problem involves multiple interacting constraints including global bending capacity, local buckling of individual walls, shear capacity, and serviceability deflection limits.

Another consideration is the constructability of such members. While the theoretical gains are clear, the practical feasibility depends on the availability of manufacturing processes capable of producing the variable-thickness profiles at acceptable quality levels and reasonable cost. The welding approach, though flexible, introduces additional quality control demands that may offset the material savings.

Summary and Outlook

This research provides a sound theoretical foundation for the design of non-equal wall thickness rectangular CFT beams, with the proposed formula validated through both analytical degradation and experimental comparison. The approach represents a meaningful step toward cross-sectional optimization in composite structures. For engineering practice, the next logical steps include developing standardized manufacturing specifications for variable-thickness rectangular tubes, establishing detailed welding procedures for the fabrication approach, and conducting full-scale structural tests under cyclic loading to evaluate the seismic performance of such optimized sections.