Axial Compression Performance of Hollow Sandwich Steel Tube Concrete Composite Long Columns

Literature Overview

The paper by Li Minglun, Ren Qingxin, Wang Qinghe, and Ding Jinan (2023), published in the Journal of Shenyang Jianzhu University (Natural Science Edition), investigates the axial compression performance of square hollow sandwich steel tube concrete (HSCFST) composite long columns. The research is supported by multiple funding sources including the National Natural Science Foundation of China (Grant No. 51808351), the Liaoning Province Xingliao Talent Plan (XLYC1902027), the Liaoning Provincial Key R&D Program (2020JH2/10300110), the Ministry of Housing and Urban-Rural Development Science and Technology Program (2019-K-054), the Liaoning Provincial Doctoral Research Start-up Fund (2019-BS-193), and the Shenyang City Young and Middle-aged Science and Technology Innovation Talent Support Program (RC200144, RC200143).

The study establishes and validates a finite element model, defines six mechanical characteristic points on the load-deformation curve, analyzes stress distributions at different characteristic points, and investigates the influence of material strength, reinforcement ratio, hollow rate, section aspect ratio, steel tube thickness-to-diameter ratio, and slenderness ratio on the axial compression performance. The authors propose a calculation method for axial compression bearing capacity based on the superposition and reduction methods, with an average ratio of 1.013 and a standard deviation of 0.038 between calculated and FE results.

Core Technical Points and Interpretation

Structural Configuration of Hollow Sandwich CFST Columns

The hollow sandwich steel tube concrete composite column is an innovative structural system that combines the advantages of conventional CFST columns with enhanced structural efficiency. The configuration consists of:

An outer square steel tube that provides lateral confinement and compressive resistance
An inner hollow core that reduces self-weight while maintaining structural integrity
A concrete layer sandwiched between the outer steel tube and the inner hollow space
Internal reinforcement within the concrete layer

This configuration offers several advantages over conventional CFST columns:

Feature	Conventional CFST	Hollow Sandwich CFST
Self-weight	Higher	Reduced by 15-30%
Material usage	More concrete	Less concrete, optimized steel
Confinement effect	Single-layer	Dual-layer (outer tube + internal reinforcement)
Stiffness	Higher	Slightly reduced but acceptable
Constructability	Standard	Requires inner formwork
Cost	Higher material cost	Potentially lower overall cost

Six Mechanical Characteristic Points

The definition of six characteristic points on the load-deformation curve is a methodologically sound approach that allows for detailed comparison between FE results and analytical predictions. These points typically correspond to:

Initial linear elastic stage (proportional limit)
Steel tube yielding
Concrete cracking
Concrete crushing initiation
Peak load
Residual load stage

By analyzing stress distributions at each of these characteristic points, the authors provide a comprehensive understanding of the progressive failure mechanism of the hollow sandwich CFST long column. The stress distribution analysis reveals that the outer steel tube bears the majority of the load in the early stages, while the concrete layer progressively takes on more load as the steel tube yields. The hollow core reduces the concrete volume but does not significantly compromise the confinement effectiveness because the outer steel tube continues to provide lateral restraint to the concrete layer.

Parametric Study Results

The parametric study identifies the most influential parameters on axial compression performance:

Parameter	Influence Level	Description
Outer steel tube thickness-to-diameter ratio	High	Directly affects confinement effectiveness
External concrete strength	High	Primary load-bearing component
Slenderness ratio	High	Governs buckling behavior of long columns
Material strength	Medium	Affects overall load capacity
Reinforcement ratio	Medium	Provides additional confinement
Hollow rate	Medium	Trade-off between weight reduction and capacity
Section aspect ratio	Low-Medium	Affects bending behavior under eccentric loading

The finding that the outer steel tube thickness-to-diameter ratio has a high influence on performance is consistent with the fundamental principle that confinement effectiveness in CFST columns is directly proportional to the hoop stress capacity of the steel tube. A thicker tube can sustain higher hoop stresses before yielding, thereby providing more effective lateral confinement to the concrete core.

Axial Compression Bearing Capacity Calculation

The proposed calculation method based on the superposition and reduction approach is a practical contribution to the design of hollow sandwich CFST columns. The superposition method calculates the individual contributions of the steel tube and the concrete layer, while the reduction method accounts for the buckling effects due to the slenderness of the column.

The accuracy of the proposed method, with an average ratio of 1.013 and a standard deviation of 0.038, indicates excellent agreement between the analytical predictions and FE results. This level of accuracy is sufficient for practical design purposes, though engineers should note that the method has been validated primarily against FE results and should be further validated against experimental data before widespread adoption.

Engineering Practice Implications

Design Considerations for Hollow Sandwich CFST Columns

For engineers considering the use of hollow sandwich CFST columns in practice, the following considerations are important:

The hollow rate should be optimized to balance weight reduction against capacity loss, with typical values in the range of 20-40% of the total cross-sectional area.
The outer steel tube thickness-to-diameter ratio should be selected to ensure adequate confinement effectiveness, with minimum values typically in the range of 1/30 to 1/50.
The slenderness ratio must be carefully controlled, as long columns are more sensitive to buckling effects, and the hollow core configuration may reduce the effective moment of inertia.
The internal reinforcement should be designed to provide additional confinement and to resist potential shear forces that may develop during seismic loading.

Constructability and Quality Control

The hollow sandwich CFST column requires inner formwork to create the hollow core, which adds complexity to the construction process. Engineers should consider:

The method of inner formwork installation and removal
The quality of concrete placement within the sandwich layer, particularly at the interface with the inner formwork
The inspection methods for verifying the thickness and uniformity of the concrete layer
The potential for voids or honeycombing in the concrete layer, which could compromise confinement effectiveness

Study Insights and Conclusions

This paper presents a well-structured investigation of an innovative composite column system that offers meaningful advantages in terms of material efficiency and structural performance. The parametric study provides valuable guidance for designers, and the proposed analytical method offers a practical tool for capacity calculation.

The key insight from this study is that the hollow sandwich configuration does not sacrifice structural performance to a significant degree while providing substantial weight reduction. This makes it an attractive option for tall buildings and long-span structures where self-weight is a critical design consideration. However, further experimental validation and code development are needed before this system can be widely adopted in practice.