Zhuojin Pipe Fitting Co., LtdLongitudinal Allowable Stress of Steel Tube in Steel Tube Concrete
Literature Overview
The paper by Liu Yongjian, Jiang Lei, and Zhang Ning from the National Engineering Laboratory of Bridge Structural Safety Technology, Chang'an University, published in Journal of Architecture and Civil Engineering (2015, Vol. 32, No. 6, pp. 1-7), addresses the multi-axial stress state of the steel tube and core concrete at the ultimate load stage of steel tube concrete (SRC) short columns under axial compression. The study introduces a steel tube longitudinal allowable stress reduction factor and a concrete compressive strength improvement factor to provide a more accurate theoretical analysis of the stress state and axial load-bearing capacity of SRC members.
Theoretical Framework and Stress State Analysis
In steel tube concrete members, the steel tube and core concrete are not independent load-bearing elements; they interact through lateral confining pressure and frictional shear forces at the interface. At the ultimate load stage, the steel tube is subjected to a biaxial stress state consisting of longitudinal compressive stress and circumferential tensile stress, while the concrete core is in a triaxial compressive stress state. This interaction leads to a composite strength that exceeds the sum of the individual component strengths.
| Stress Component | Steel Tube | Core Concrete | Interaction Mechanism |
|---|---|---|---|
| Longitudinal | Compressive (σ_z) | Compressive (σ_c) | Load sharing through bond and friction |
| Circumferential | Tensile (σ_θ) | Compressive (σ_r) | Confinement through Poisson effect |
| Radial | Negligible | Compressive (σ_r) | Interface pressure transfer |
| Shear | Interfacial shear (τ) | Interfacial shear (τ) | Friction and mechanical interlock |
The introduction of the steel tube longitudinal allowable stress reduction factor (φ) accounts for the fact that the steel tube cannot fully utilize its longitudinal yield strength because the circumferential tensile stress, which develops as the concrete core expands laterally, consumes part of the steel tube's yield capacity through the von Mises or Tresca yield criterion. The concrete compressive strength improvement factor (η) quantifies the enhancement of concrete strength due to the lateral confinement provided by the steel tube.
Calculation Method and Verification
The proposed calculation method introduces two key coefficients that are determined based on the geometric and material properties of the SRC member. The reduction factor φ is calculated as a function of the steel tube's stress state and the confinement level, while the improvement factor η depends on the confining pressure and the concrete's triaxial strength characteristics.
| Parameter | Definition | Typical Range | Calculation Basis |
|---|---|---|---|
| Steel tube longitudinal stress reduction factor (φ) | Ratio of actual longitudinal stress to yield strength | 0.75 ~ 0.95 | Von Mises yield criterion with circumferential stress |
| Concrete strength improvement factor (η) | Ratio of confined concrete strength to unconfined strength | 1.10 ~ 1.60 | Confining pressure and concrete triaxial behavior |
| Confinement coefficient (ξ) | Ratio of steel tube confining stress to concrete strength | 0.10 ~ 0.60 | Steel tube thickness, diameter, and yield strength |
| Load-bearing capacity ratio | Ratio of SRC capacity to sum of individual capacities | 1.10 ~ 1.30 | Composite action efficiency |
The comparison between calculated and experimental values showed good agreement, validating the proposed method for the commonly used range of design parameters. This method provides engineers with a more rational basis for SRC member design compared to the traditional superposition method, which simply adds the steel tube and concrete capacities without accounting for the composite interaction effects.
Engineering Design Implications
For practical engineering design, the proposed method has several important implications. The steel tube longitudinal allowable stress reduction factor should be incorporated into the design calculations to avoid overestimating the load-bearing capacity of SRC members. The concrete strength improvement factor can be used to optimize the concrete strength grade selection, potentially allowing the use of lower-strength concrete while maintaining the required structural capacity through the confinement effect.
| Design Scenario | Traditional Method | Proposed Method | Difference |
|---|---|---|---|
| Axial load capacity calculation | N_u = f_y·A_s + f_c·A_c | N_u = φ·f_y·A_s + η·f_c·A_c | More accurate, accounts for interaction |
| Concrete grade selection | Based on unconfined strength | Based on confined strength | Allows lower concrete grade |
| Steel tube thickness design | Based on buckling only | Based on buckling and composite action | More efficient design |
| Safety factor application | Uniform safety factor | Differentiated by component | More rational safety margin |
The confinement coefficient range of 0.10 to 0.60 covers most practical SRC column designs, where the steel tube thickness-to-diameter ratio and the steel yield strength determine the level of confinement. For high-confinement applications, such as those requiring enhanced ductility in seismic design, the confinement coefficient can be increased by using thicker steel tubes or higher-strength steel grades.
Study Insights and Implications
This research contributes to a deeper understanding of the composite action between steel tube and concrete in SRC members. The introduction of the longitudinal allowable stress reduction factor is a conceptually important advancement, as it acknowledges that the steel tube's effective longitudinal strength is reduced by the circumferential tensile stress developed during the composite loading process. This insight is directly applicable to the design of SRC columns, beams, and nodes in bridge and building structures, where the accurate assessment of load-bearing capacity is essential for both safety and economy. Engineers should adopt this refined calculation method in their design practice, particularly for SRC structures where the steel tube and concrete interaction is significant, and where the traditional superposition method may lead to unconservative designs. The method also provides a framework for future research into more complex loading conditions, such as combined axial and bending loads, where the stress state of the steel tube becomes even more complex and the composite interaction effects are more pronounced.