Comparison of Uniaxial Compression-Bending Bearing Capacity Calculation Methods for Rectangular Steel Tube Concrete Columns

Literature Overview

The paper by Fu Bo, Cheng Zhishu, Jia Shuhua, Sun Hao, and Tong Genxu, published in Progress in Steel Building Structures in 2022, presents a comprehensive comparison of three calculation methods for the uniaxial compression-bending bearing capacity of rectangular steel tube concrete (SRC) columns as provided by three different Chinese design codes. This research was supported by the National Key R&D Program of China (Grant No. 2016YFC0701201). The three codes examined are GB 50936-2014 (Technical Specification for Steel Tube Concrete Structures), CECS 159:2004 (Technical Regulations for Rectangular Steel Tube Concrete Structures), and JGJ 138-2016 (Design Code for Composite Structures). The authors identify significant discrepancies among the three methods and propose a new calculation formula based on structural stability theory. The paper falls under classification TU398 and addresses a practical challenge faced by structural engineers: which calculation method should be used for the design of rectangular SRC columns under combined axial compression and bending?

Core Technical Content and Key Findings

The study compares the three calculation methods across several parameters, including the axial compression strength capacity, the axial compression stability capacity, the strong-axis and weak-axis flexural capacities, and the strong-axis and weak-axis uniaxial compression-bending capacities. The comparison is conducted for different cross-sectional aspect ratios and confinement coefficients, and the calculated values are compared against experimental data.

The following table summarizes the key findings of the comparison:

The most significant finding is that GB 50936-2014 produces large deviations from experimental values for the weak-axis compression-bending capacity of columns with cross-sectional aspect ratios greater than 2.0. This is a critical issue because rectangular SRC columns with high aspect ratios are commonly used in practice, and an inaccurate prediction of the weak-axis capacity could lead to unsafe designs. The authors attribute this discrepancy to the simplified assumptions in the GB 50936-2014 calculation method, which does not adequately account for the non-uniform stress distribution in the concrete core under combined loading.

The second significant finding is that CECS 159:2004 produces unconservative results for the weak-axis compression-bending capacity of columns with slenderness ratios greater than 50. This is particularly concerning because slender columns are more susceptible to buckling, and an unconservative prediction could lead to catastrophic failure. The authors suggest that the CECS 159:2004 method may not adequately account for the second-order effects (P-delta effects) that become significant at high slenderness ratios.

The third finding is that JGJ 138-2016 produces overly conservative results for the strong-axis compression-bending capacity. While conservatism is generally acceptable from a safety standpoint, excessive conservatism leads to inefficient designs that increase structural weight and cost. The authors suggest that the JGJ 138-2016 method may be overly conservative because it does not fully account for the confinement effect of the steel tube on the concrete core.

Proposed New Calculation Formula

Based on structural stability theory, the authors derive a new calculation formula for the uniaxial compression-bending bearing capacity of rectangular SRC columns. The new formula is derived by considering the equilibrium of the composite section under combined axial compression and bending, taking into account the nonlinear stress-strain relationships of both the steel tube and the confined concrete. The key improvement over the existing methods is the explicit consideration of the non-uniform stress distribution in the concrete core, which is particularly important for rectangular cross-sections with high aspect ratios.

The new formula incorporates the following key parameters:

The authors provide recommendations for the applicable range of each of the three existing methods, which can serve as a practical guide for design engineers to select the most appropriate code for their specific design situation.

Engineering Practice Implications

The discrepancies identified in this study have significant implications for the practice of structural engineering. In China, where multiple design codes coexist, engineers often face the challenge of selecting the most appropriate code for a given design situation. The findings of this study suggest that no single code is universally applicable, and that the selection of the appropriate code depends on the specific geometric and loading conditions of the column.

From a fabrication and quality control perspective, the accuracy of the bearing capacity calculation depends on the accuracy of the input parameters, which include the concrete strength, the steel tube material properties, and the geometric dimensions of the cross-section. Any deviation in these parameters from their assumed values will affect the predicted bearing capacity. Therefore, the quality control plan for rectangular SRC columns should include rigorous testing of the concrete strength, the steel tube material properties, and the tube geometric dimensions.

In particular, the wall thickness of the steel tube is a critical parameter that affects both the confinement effect and the geometric properties of the cross-section. Variations in wall thickness due to manufacturing tolerances or corrosion can significantly affect the bearing capacity of the column. Therefore, ultrasonic thickness gauging should be performed on all steel tubes used in SRC columns, and any tube with wall thickness below the specified minimum should be rejected.

In conclusion, this paper provides a valuable comparative analysis of three calculation methods for the compression-bending bearing capacity of rectangular SRC columns, and the proposed new formula based on structural stability theory offers a more accurate and reliable alternative. The recommendations for the applicable range of each existing method provide practical guidance for design engineers, and the emphasis on the importance of accurate input parameters underscores the critical role of quality control in ensuring the safety and reliability of SRC structures.