Zhuojin Pipe Fitting Co., LtdBearing Capacity Design Calculation for Square-Section Steel Tube Confined Concrete Flexural Members
Literature Overview
This paper by Tao Zhong and Han Linhai, published in "Journal of Harbin University of Civil Engineering and Architecture" (2000, Vol. 33, No. 3), presents design formulas for the bearing capacity of square-section steel tube confined concrete (SSC) members under combined axial compression and bending. The authors developed design equations based on extensive analytical calculations and experimental results, providing simplified formulas and tables for practical engineering application.
Design Formulas and Methodology
The paper addresses three fundamental loading cases for square SSC members:
| Loading Case | Design Formula Type | Key Design Parameters |
|---|---|---|
| Combined Axial Compression and Bending (Strength) | Combined axial compression strength bearing capacity | Concrete strength, steel tube strength, confinement ratio |
| Combined Axial Compression and Bending (Stability) | Combined axial compression stability bearing capacity | Slenderness ratio, stability coefficient |
| Pure Bending | Flexural member bearing capacity | Plastic section modulus, plastic development factor |
Combined Axial Compression Strength
The combined axial compression strength formula accounts for the interaction between axial force and bending moment. The formula incorporates the confinement effect of the steel tube on the concrete core, which is a key feature distinguishing SSC members from conventional reinforced concrete members. The confinement ratio, defined as the ratio of the steel tube wall thickness to the cross-sectional dimension, is a critical parameter that influences the bearing capacity.
Stability Bearing Capacity
For slender SSC members, the stability bearing capacity is reduced compared to the strength bearing capacity. The stability coefficient depends on the slenderness ratio of the member and the material properties. The paper provides simplified formulas and tabulated values for the stability coefficient, which can be directly used in design calculations.
Pure Bending Bearing Capacity
The pure bending formula is based on the plastic section modulus of the composite cross-section. The steel tube and concrete core contribute to the flexural resistance, with the steel tube providing significant contribution due to its high yield strength. The plastic development factor accounts for the redistribution of stresses within the cross-section as the member approaches its ultimate capacity.
Combined Axial Compression and Bending Design Formula
The main contribution of this paper is the recommended design formula for SSC flexural members under combined loading. The formula integrates the strength and stability considerations into a unified expression that can be applied to practical design situations. The formula is expressed in a form that is compatible with standard structural design procedures, making it accessible to practicing engineers.
Design Parameters and Simplified Tables
The paper provides simplified formulas and tables for three key design indicators:
- Combined axial compression strength design value: A tabulated value that accounts for the material properties and cross-sectional geometry.
- Combined axial compression stability coefficient: A function of the slenderness ratio, provided in tabular form for easy lookup.
- Plastic development factor of the member cross-section: A factor that accounts for the inelastic behavior of the cross-section under bending, provided as a function of the axial force ratio.
These simplified tables and formulas make the design process practical and efficient, reducing the need for complex iterative calculations.
Reflections and Engineering Implications
This paper is highly practical for structural engineers working on steel tube confined concrete structures. The provision of simplified formulas and tables significantly reduces the computational burden of design, making SSC members more attractive for practical applications. The paper builds on the theoretical foundation established by earlier researchers, such as Tang Guodong, and translates it into design-ready tools.
The square cross-section is particularly advantageous for SSC members because it provides better confinement efficiency than circular sections for the same amount of steel. The flat faces of the square section allow for more uniform concrete confinement, and the corners provide additional confinement through the geometric constraint. This paper provides the engineering basis for utilizing this advantage in design.
For engineers considering SSC members in structural design, the key considerations are the slenderness ratio, the confinement ratio, and the loading combination. The design formulas provided in this paper can be applied to a wide range of practical situations, from building columns to bridge piers. The paper also highlights the importance of proper detailing at connections, as the full capacity of SSC members can only be realized if the connections are designed to transfer the required forces without premature failure.