Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Hollow Sandwich Concrete-Filled Double Steel Tube Short Columns Considering Effective Concrete Strength
Literature Overview
This paper by Yan Xifeng, Hao Jiping, and Zhao Yangang, published in Engineering Mechanics (2024, Vol. 41, No. 11, pp. 145-156), addresses a critical gap in the numerical analysis of hollow sandwich concrete-filled double steel tube (CFDST) columns. The authors identify that existing fibre element models for CFDST columns fail to account for the strength degradation effect caused by concrete brittleness at high strength levels, leading to significant scatter in predictions and unreliable design guidance for high-strength concrete applications.
Background and Problem Statement
The CFDST column configuration consists of an outer steel tube, an inner steel tube, and a concrete layer sandwiched between the two tubes. This configuration provides enhanced structural efficiency, improved fire resistance, and the possibility of using high-strength concrete without the associated brittleness penalty. However, as concrete strength increases, the brittle nature of concrete leads to a reduction in its effective strength contribution, a phenomenon known as the strength degradation effect or size effect.
The core problem is that conventional fibre element models assume that the full uniaxial compressive strength of the concrete is mobilized in the confined concrete layer. In reality, for high-strength concrete, the actual effective strength is lower than the uniaxial compressive strength due to:
- Micro-cracking at the mesoscale level before macroscopic failure.
- Reduced strain capacity associated with higher strength grades.
- Size effects in concrete elements under multiaxial stress states.
- The interaction between the concrete and the two steel tubes, which constrains the concrete in a complex multiaxial stress state.
Proposed Fibre Element Model
The authors developed a fibre element model that incorporates an effective concrete strength factor, which accounts for the brittleness-induced strength degradation:
| Model Component | Description | Innovation |
|---|---|---|
| Outer steel tube | Bilinear stress-strain model | Standard |
| Inner steel tube | Bilinear stress-strain model | Standard |
| Concrete layer | Modified stress-strain model with effective strength factor | Novel strength degradation factor |
| Effective strength factor | Reduces the uniaxial compressive strength based on concrete strength grade | Accounts for brittleness effect |
| Confinement effect | Lateral confinement from both steel tubes | Captured in multiaxial stress model |
The effective concrete strength factor is defined as a function of the concrete strength grade, with higher strength grades experiencing greater degradation. This factor is applied to the uniaxial compressive strength in the fibre model to obtain the effective strength used in the analysis.
Parametric Study Results
Based on the validated fibre element model, the authors conducted a comprehensive parametric study examining the influence of various parameters on the axial compression performance of CFDST columns:
| Parameter | Effect on Bearing Capacity | Effect on Ductility |
|---|---|---|
| Concrete strength | Increases | Decreases |
| Hollow ratio (inner tube diameter to outer tube diameter) | Decreases | Generally increases |
| Outer steel tube yield strength | Increases | Minimal effect |
| Outer steel tube diameter-thickness ratio | Decreases (thinner walls reduce capacity) | Increases (more flexible tube) |
| Inner steel tube yield strength | Increases | Negligible effect |
| Inner steel tube diameter-thickness ratio | Minimal effect | Minimal effect |
The key finding is that the inner steel tube yield strength affects only the bearing capacity but has virtually no influence on the ductility of the CFDST column. This is because the inner tube does not directly contribute to the confinement of the concrete layer; its primary role is to provide additional axial load capacity.
Design Formula Development
Based on both experimental results and numerical analysis, the authors proposed a design formula for the axial compression bearing capacity of CFDST columns. The formula incorporates the effective concrete strength factor and accounts for the contributions of the outer steel tube, inner steel tube, and confined concrete layer. The proposed formula shows good agreement with both experimental and numerical results, providing a practical design tool for engineers.
Engineering Practice Implications
This study has significant implications for the design and manufacturing of CFDST columns:
- High-strength concrete applications: Engineers designing CFDST columns with high-strength concrete must account for the strength degradation effect. Using the full uniaxial compressive strength in design calculations will lead to unsafe predictions.
- Steel tube manufacturing requirements: The outer steel tube's diameter-thickness ratio is critical for both bearing capacity and ductility. Manufacturing tolerances on wall thickness and ovality must be tightly controlled to ensure the predicted confinement effect is achieved.
- Inner tube optimization: Since the inner tube yield strength does not affect ductility, engineers can select the inner tube material based on cost-effectiveness for bearing capacity requirements rather than ductility considerations.
- Hollow ratio selection: The hollow ratio provides a design variable that allows balancing bearing capacity against ductility and structural efficiency.
Key Reflections
This paper makes an important contribution to the numerical modelling of CFDST columns by addressing a previously overlooked but critical aspect of high-strength concrete behaviour. The strength degradation effect is a well-known phenomenon in concrete mechanics, but its application to the confined concrete in CFDST columns had not been systematically addressed. The proposed effective concrete strength factor provides a practical and physically meaningful way to incorporate this effect into fibre element models. However, the study raises questions about the applicability of the proposed factor to different concrete types (ordinary, high-performance, ultra-high-performance) and different confinement geometries. Further experimental validation with a wider range of concrete strengths and column geometries would strengthen the generalizability of the model. The proposed design formula should be validated against a larger database of experimental results before being incorporated into design codes.
Summary of All Five Studies
These five papers collectively represent important contributions to the fields of steel pipe manufacturing, structural engineering, and quality control. Topic 1 demonstrates the practical value of analytical simplification in large-scale structural analysis. Topic 2 addresses the specialized challenge of ultrasonic inspection for small-diameter thick-wall pipes. Topic 3 provides a scientifically grounded reassessment of the medium-frequency hot expansion process. Topic 4 develops a practical restoring force model for seismic design of composite columns. Topic 5 identifies and addresses a critical gap in the numerical modelling of CFDST columns with high-strength concrete. Together, these studies highlight the importance of integrating manufacturing considerations with structural performance, the value of specialized testing and modelling approaches, and the need for continuous refinement of analytical methods to match evolving material and design practices.