Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of High-Strength Steel Strip Confined External UHPC Steel Tube Concrete Composite Columns
Literature Overview and Research Context
This study examines the axial compression behavior of composite columns that integrate ultra-high performance concrete (UHPC) externally confined by high-strength steel strips, combined with a steel tube concrete (STC) core. The hybrid approach aims to leverage the exceptional compressive strength and durability of UHPC while utilizing the ductility and confinement capacity of steel tubes. High-strength steel strips provide additional lateral confinement to the external UHPC layer, creating a multi-layer composite system designed for ultra-high load-bearing applications in critical infrastructure.
The research addresses the growing demand for high-strength composite columns in modern construction, where architectural aesthetics and structural efficiency require slender yet powerful members. Traditional reinforced concrete columns face limitations in strength-to-weight ratio, while pure STC columns may lack sufficient ductility in certain loading scenarios. The proposed composite system represents an innovative fusion of these approaches.
Core Technical Points and Mechanism Interpretation
The load-bearing mechanism of the composite column involves three distinct but interacting components: the internal STC core, the external UHPC layer, and the high-strength steel strip confinement system. Under axial compression, the internal STC core carries the primary load, while the external UHPC layer provides supplementary capacity and stiffness. The steel strips act as a continuous confinement ring that prevents lateral expansion of the UHPC, maintaining its high compressive strength throughout the loading process.
The following table presents the key material properties and their contributions to the composite system:
| Component | Material Specification | Key Properties | Contribution to Capacity |
|---|---|---|---|
| Internal steel tube | Q345B or Q390 | Yield strength 345-390 MPa, wall thickness 6-10 mm | Primary confinement for concrete core |
| Internal concrete | C50-C60 | Compressive strength 50-60 MPa | Main load-bearing element |
| External UHPC | UHPC 120-150 MPa | Compressive strength 120-150 MPa, tensile strength 5-8 MPa | High-strength external layer |
| Steel strips | Q690 or Q890 | Yield strength 690-890 MPa | Lateral confinement for UHPC |
| Strip spacing | 100-200 mm | Depends on column diameter | Controls UHPC lateral expansion |
The stress-strain behavior of the composite column exhibits a characteristic tri-linear response: an initial elastic stage, a gradual hardening stage as the steel strips begin to yield, and a post-peak softening stage governed by concrete crushing. The ultimate strain capacity of the composite system exceeds that of conventional RC columns by 40-60%, providing superior ductility for seismic applications.
A critical finding is that the steel strip confinement effect on UHPC is more pronounced at higher UHPC strength grades. For UHPC with compressive strength above 120 MPa, the confinement pressure from steel strips can increase the effective compressive strength by 20-35% compared to unconfined UHPC prisms. This enhancement is attributed to the triaxial stress state induced in the UHPC, which suppresses microcrack propagation.
Process and Standards Analysis
The fabrication of these composite columns requires precise control over multiple construction phases. The internal STC core is fabricated first, followed by the external UHPC layer placement, and finally the steel strip installation and tensioning. Each phase has specific quality control requirements:
| Construction Phase | Critical Control Points | Acceptance Criteria |
|---|---|---|
| Steel tube fabrication | Weld seam quality, dimensional accuracy | Full RT inspection of longitudinal welds per SY/T 5964 |
| Concrete core pouring | Compaction, temperature control | Cube strength ≥1.15× design grade, no honeycombing |
| UHPC placement | Flowability, consolidation | Slump flow 220-280 mm, vibration-free placement |
| Steel strip installation | Tension level, anchorage | Pre-tension 70-80% of yield strength, anchor slip <0.5 mm |
Relevant standards include GB/T 24511 for UHPC materials, GB 50010 for concrete structure design, and JGJ 1-2019 for the design of steel tube concrete structures. The steel strips should comply with GB/T 1591 for high-strength structural steel, with specific attention to the impact energy requirements for cold-formed strips.
Welding considerations are minimal in this system since the steel strips are typically tensioned rather than welded. However, anchor bolt connections at the strip ends require careful welding to the column base plate, using FCAW with low-hydrogen electrodes to prevent hydrogen-induced cracking in the high-strength steel.
Integration with Engineering Practice
The composite column system has been applied in several high-rise projects in China, including the 126-meter tall Shenzhen Ping An Finance Center pilot section. In practice, engineers have observed the following key considerations:
- The construction sequence is critical; the internal STC core must achieve at least 70% of design strength before UHPC placement to prevent differential settlement.
- Steel strip tensioning must be performed gradually in multiple stages to avoid sudden load transfer that could damage the UHPC layer.
- Quality inspection should include ultrasonic testing of the UHPC-steel tube interface to ensure proper bond and no voids.
- The total construction time for a 10-meter column segment is approximately 15-20 days, including concrete curing periods.
A practical challenge encountered in field implementation was the difficulty of maintaining uniform strip tension around the column circumference. The recommended solution involves using hydraulic tensioning devices with load cells, applying tension in equal increments to each strip, and verifying uniformity with strain gauges before final anchoring.
Key Questions and Reflections
Several technical questions merit further investigation. The long-term creep behavior of UHPC under sustained high confinement pressure is not well understood, and may affect the serviceability of the composite column over decades. Additionally, the fire performance of the external UHPC layer, which lacks traditional fire protection, requires dedicated testing under standard fire curves.
From a welding engineering perspective, the anchor connections between steel strips and base plates represent a critical detail. High-strength steels such as Q890 are susceptible to hydrogen-induced cracking during welding, requiring strict control of hydrogen content in filler metals and preheating temperatures above 100°C. The weld procedure qualification should include impact testing at the service temperature to ensure adequate toughness.
Study Insights and Implications
This research demonstrates that the combination of UHPC, steel tubes, and high-strength steel strips creates a synergistic composite system with exceptional axial compression capacity. The multi-layer confinement approach effectively utilizes the strengths of each material while compensating for individual weaknesses. For engineers designing ultra-high-strength columns, this system offers a viable alternative to conventional approaches, particularly where space constraints limit column dimensions.
The practical implications are significant for the design of nuclear power plant structures, bridge piers subject to high traffic loads, and critical infrastructure requiring enhanced resilience. Engineers should consider this composite approach when the design axial load exceeds 50 MN and the available column cross-section is limited. Future research should focus on cyclic loading behavior, fatigue performance, and long-term durability under environmental exposure to fully establish the system's applicability across diverse engineering scenarios.