Zhuojin Pipe Fitting Co., LtdCombined Compression-Bending-Shear Behavior of Square CFST Members with Internal Steel Sections
Literature Overview
This study by Wang Wenda and colleagues from Lanzhou University of Technology investigates the combined compression-bending-shear behavior of square concrete-filled steel tube (CFST) members with internal steel sections. Published in the China Civil Engineering Journal (Vol. 54, No. 1, 2021, pp. 76–87), the research was supported by the National Natural Science Foundation of China (Grants 51768038 and 51468037) and the Gansu Provincial Collaborative Innovation Team Program (2018C-08). The study addresses a structurally important but relatively underexplored configuration in which steel sections are embedded within the concrete core of square CFST columns, creating a hybrid composite member that combines the confinement benefits of CFST with the additional strength and ductility of internal steel reinforcement.
Experimental Program and Key Findings
Ten test specimens were subjected to combined compression-bending-shear loading, with the experimental variables encompassing shear span ratio, axial compression ratio, and the cross-sectional configuration of the internal steel section. Finite element (FE) simulations were conducted to validate the test results and to perform parametric analyses that extend beyond the experimental scope.
Specimen Configuration and Test Variables
| Parameter | Variation Range | Number of Specimens |
|---|---|---|
| Shear span ratio (λ) | Multiple values (including λ < 0.8) | 10 total |
| Axial compression ratio (n) | Multiple values (including n ≤ 0.4) | 10 total |
| Internal steel section type | Different cross-sectional forms | 10 total |
| Loading axis | Strong axis and weak axis | 10 total |
Key Experimental Results
The study reports several significant findings that have direct implications for structural design:
- Ductile behavior: All specimens exhibited good ductile deformation characteristics, indicating that the internal steel sections contribute positively to the overall ductility of the composite member.
- Steel tube deformation: The steel tube on the compression side developed pronounced outward bulging (drumming) deformation, confirming the interaction between the steel tube and the internal components.
- Composite action: The internal steel section and the surrounding concrete worked together as a composite unit throughout the loading process.
- Axis-dependent behavior: Specimens loaded about the weak axis exhibited horizontal load-bearing capacity approximately 13% lower on average compared to those loaded about the strong axis.
- Stiffener effect: The introduction of stiffening ribs increased the horizontal load-bearing capacity by an average of 21%.
- Load redistribution: Throughout the loading process, the axial load underwent redistribution among the steel tube, internal steel section, and concrete, with concrete carrying the largest share of both horizontal and axial loads.
- Uneven interaction: The interaction forces between the internal steel section and concrete were non-uniformly distributed, with stronger interaction observed at the flanges of the steel section in the compression zone.
- Shear span effect: When the axial compression ratio was held constant, specimens with shear span ratios below 0.8 showed a dramatic increase in horizontal load-bearing capacity as the shear span ratio decreased.
- Axial force effect: When the axial compression ratio did not exceed 0.4, the presence of axial force improved the horizontal load-bearing capacity of the specimens.
Finite Element Analysis and Parametric Study
The FE models were validated against experimental results, showing good agreement between simulated and measured responses. The validated models were then used to extend the parametric analysis to a wider range of structural parameters, providing design-oriented insights that supplement the experimental findings.
Mechanism of Action
The combined compression-bending-shear loading state creates a complex stress environment within the composite member. The axial compression provides a confining effect that enhances the shear capacity of the concrete core, while the internal steel section acts as a shear-resisting element that bridges across potential shear cracks. The steel tube provides lateral confinement that prevents the concrete from spalling under combined loading, and the internal steel section distributes the shear stresses more uniformly through the cross-section.
The non-uniform distribution of interaction forces between the internal steel section and concrete is a critical observation. In the compression zone, the flanges of the internal steel section experience higher contact pressures with the surrounding concrete because the flanges are closer to the outer surface of the steel tube where the concrete is more heavily confined. This uneven interaction has implications for the design of connection details between the internal steel section and the steel tube, as well as for the prediction of local failure modes.
Engineering Practice Integration
From a structural engineering practice standpoint, this hybrid CFST member with internal steel sections offers several advantages for applications where high shear and bending resistance are required in a compact cross-section. Potential applications include:
- Bridge pier columns: Where combined gravity and seismic loads demand high shear and bending capacity within limited architectural dimensions.
- High-rise building columns: Where the column must resist both axial gravity loads and lateral seismic or wind forces.
- Offshore platform legs: Where the compact cross-section and high load-bearing capacity are advantageous in space-constrained environments.
Design Recommendations Based on Study Findings
| Design Parameter | Recommendation | Basis |
|---|---|---|
| Loading axis orientation | Prefer strong-axis loading where possible | 13% capacity advantage |
| Stiffener provision | Incorporate stiffening ribs at critical sections | 21% capacity improvement |
| Shear span ratio | Design for λ ≥ 0.8 to avoid shear-critical behavior | Dramatic capacity increase below λ = 0.8 |
| Axial compression ratio | Maintain n ≤ 0.4 for beneficial axial force effect | Axial force enhances shear capacity |
| Internal steel section | Use sections with prominent flanges | Stronger flange-concrete interaction |
Welding and Fabrication Considerations
Although this paper focuses on structural performance rather than fabrication, the hybrid member configuration presents significant welding challenges that must be addressed in practice. The connection between the internal steel section and the steel tube typically involves fillet welds or bolted connections, and the concrete pouring process must ensure adequate bond between the steel components and the concrete core.
The internal steel section must be accurately positioned within the steel tube before concrete placement, requiring temporary bracing and alignment fixtures. The welding of stiffening ribs to the internal steel section must be performed with attention to heat input control to avoid excessive distortion of the steel tube, particularly in large-diameter members. For thick-walled steel tubes, the welding sequence should be planned to minimize residual stresses that could compromise the member's structural performance under combined loading.
Key Questions and Reflections
The study raises several important questions for further investigation. First, the long-term behavior of the hybrid member under sustained loading, including creep and shrinkage effects of the concrete core, is not addressed. The redistribution of axial loads among the steel tube, internal steel section, and concrete over time could alter the stress distribution and potentially lead to premature failure.
Second, the fatigue performance of the internal steel section under cyclic loading is not examined. In seismic applications, the member may experience numerous loading cycles, and the weld connections between the internal steel section and the steel tube could be vulnerable to fatigue cracking.
Third, the study does not address the corrosion protection of the internal steel section, which is embedded within the concrete core but is also in direct contact with the steel tube. Differential corrosion rates between the steel tube and internal steel section could lead to degradation of the composite action over time.
Study Insights and Implications
This research provides valuable experimental and numerical data for the design of hybrid CFST members with internal steel sections. The key insight for practicing engineers is that the composite action between the steel tube, internal steel section, and concrete is robust under combined loading, and the member exhibits favorable ductile behavior. The 21% capacity improvement from stiffening ribs and the 13% axis-dependent capacity difference are practical design parameters that can be directly incorporated into structural design calculations.
The finding that axial compression enhances shear capacity when the axial compression ratio is below 0.4 is particularly useful for seismic design, where the member is expected to experience combined axial and lateral loading. This beneficial interaction means that the member's shear capacity is not compromised by the gravity loads it must carry, which simplifies the design process and potentially allows for more economical member sizing.