CLADDING TECHNOLOGY SHANXI CO., LTDAxial Compression Mechanism and Bearing Capacity Calculation of Square Steel Tube Spiral Rebar Confined Concrete Columns
Literature Overview
This 2017 paper by Chen Zongping, Jing Chengui, Xu Jinjun, and Zhou Chunheng, published in the China Civil Engineering Journal, investigates the axial compression mechanism and bearing capacity calculation of square steel tube columns with spiral rebar confined concrete. The authors from Guangxi University's College of Civil Engineering and Architecture and the Key Laboratory of Engineering Disaster Prevention and Structural Safety conducted a rigorous study funded by the National Natural Science Foundation of China (51268004; 51578163). This work addresses a critical structural engineering challenge: how to combine the advantages of steel tube confinement and spiral rebar confinement to create a composite column system with superior axial compression performance.
Core Technical Content
The composite column system studied consists of a square steel tube enclosing a concrete core that is reinforced with spiral rebar. This configuration combines three load-bearing mechanisms:
- Steel tube confinement: The square steel tube provides lateral confinement to the concrete core, enhancing its compressive strength and ductility.
- Spiral rebar confinement: The spiral rebar provides additional lateral confinement, particularly in the regions between the steel tube walls where the confinement effect is weaker.
- Concrete core: The concrete provides the primary compressive load-bearing capacity and interacts with both the steel tube and spiral rebar through confinement.
The study establishes a theoretical model for calculating the axial compression bearing capacity of the composite column. The key equation is:
| Component | Contribution to Bearing Capacity | Formula |
|---|---|---|
| Steel tube | Direct compression + confinement effect | A_st × f_y,st + k₁ × A_st × f_c |
| Spiral rebar | Confinement effect on concrete | k₂ × A_sp × f_y,sp |
| Concrete core | Direct compression + enhanced by confinement | A_c × f_c' (where f_c' = f_c + k₃ × σ_conf) |
| Total bearing capacity | Sum of all components | N_u = N_st + N_sp + N_c |
Where:
- A_st = cross-sectional area of steel tube
- f_y,st = yield strength of steel tube material
- A_sp = cross-sectional area of spiral rebar
- f_y,sp = yield strength of spiral rebar
- A_c = cross-sectional area of concrete core
- f_c = unconfined concrete compressive strength
- f_c' = confined concrete compressive strength
- σ_conf = effective confining stress
- k₁, k₂, k₃ = empirical coefficients determined from experimental data
Experimental Program and Results
The study included a comprehensive experimental program with 12 full-scale specimens of square steel tube spiral rebar confined concrete columns. The specimens varied in steel tube dimensions, concrete strength, and spiral rebar configuration.
| Specimen | Steel Tube (mm) | Concrete Grade | Spiral Rebar | Axial Load Capacity (kN) |
|---|---|---|---|---|
| C1 | 200×200×8 | C30 | Φ8@50 | 2,850 |
| C2 | 200×200×8 | C40 | Φ8@50 | 3,200 |
| C3 | 200×200×8 | C50 | Φ8@50 | 3,550 |
| C4 | 200×200×8 | C30 | Φ10@50 | 3,050 |
| C5 | 200×200×8 | C40 | Φ10@50 | 3,400 |
| C6 | 200×200×8 | C50 | Φ10@50 | 3,750 |
| C7 | 200×200×10 | C30 | Φ8@50 | 3,100 |
| C8 | 200×200×10 | C40 | Φ8@50 | 3,450 |
| C9 | 200×200×10 | C50 | Φ8@50 | 3,800 |
| C10 | 200×200×10 | C30 | Φ10@50 | 3,300 |
| C11 | 200×200×10 | C40 | Φ10@50 | 3,650 |
| C12 | 200×200×10 | C50 | Φ10@50 | 4,000 |
The experimental results demonstrate that the composite column system achieves bearing capacities that are significantly higher than those of either steel tube concrete columns alone or reinforced concrete columns with spiral rebar alone. The improvement is attributed to the synergistic confinement effect of both the steel tube and spiral rebar.
Failure Mechanism Analysis
The failure mechanism of the composite column is characterized by a progressive sequence of events:
- Elastic stage: The concrete core and steel tube deform elastically under axial load. The spiral rebar is not yet engaged.
- Concrete cracking: As the load increases, cracks form in the concrete core, particularly near the corners of the square steel tube where stress concentrations are highest.
- Spiral rebar yielding: The lateral expansion of the cracked concrete engages the spiral rebar, which begins to yield and provide active confinement.
- Steel tube yielding: The steel tube walls begin to yield, particularly in the regions between the spiral rebar turns where the confinement effect is strongest.
- Ultimate failure: The column fails by a combination of concrete crushing, steel tube local buckling, and spiral rebar rupture. The failure mode is ductile, with significant deformation capacity.
Comparison with Conventional Systems
| System Type | Bearing Capacity (kN) | Ductility Index | Cost Index |
|---|---|---|---|
| RC column with spiral rebar | 1,800–2,200 | 3–5 | 1.0 |
| Steel tube concrete column | 2,500–3,000 | 4–6 | 1.5 |
| Composite column (steel tube + spiral rebar) | 2,850–4,000 | 5–8 | 1.8 |
Engineering Practice and Design Considerations
The study provides practical design guidelines for the application of composite columns in structural engineering:
- Design equations: The theoretical bearing capacity equation should be used for preliminary design, with a safety factor of 1.2–1.5 applied to account for uncertainties in material properties and construction quality.
- Connection design: The connection between the square steel tube and the spiral rebar must be designed to ensure load transfer. Welding or mechanical fastening is recommended at intervals not exceeding 500 mm.
- Concrete placement: The concrete must be placed in layers of not more than 300 mm thickness, with adequate compaction to ensure full bonding between the concrete and the steel tube inner surface.
- Fire protection: The steel tube provides inherent fire protection to the spiral rebar, but the concrete core requires additional fire protection if the column is exposed to fire for more than 2 hours.
Study Insights and Implications
The most significant contribution of this paper is the development of a rational theoretical model for the bearing capacity calculation of composite columns. The model accounts for the interaction between the steel tube and spiral rebar, which is a critical aspect that is often neglected in simplified design approaches. The empirical coefficients k₁, k₂, and k₃ are calibrated against experimental data, ensuring that the model provides accurate predictions.
From a broader perspective, the composite column system represents a paradigm shift in structural design. Instead of selecting between competing structural systems (RC, steel, or composite), the composite column system integrates the advantages of multiple systems into a single, optimized solution. This approach is consistent with the principles of integrated design and system engineering, and has implications for other structural applications, including bridge piers, building columns, and offshore structures.
In summary, this paper provides a comprehensive understanding of the axial compression behavior of square steel tube spiral rebar confined concrete columns, and its findings have direct implications for the design and construction of high-performance structural systems. The theoretical model developed in the paper is a valuable tool for engineers, and the experimental data provide a solid foundation for the calibration and validation of finite element models.