Zhuojin Pipe Fitting Co., LtdExperimental Study on Steel Bracket External Strengthening Ring-Steel Tube Coal Gangue Concrete Column and Steel Beam Joint
Literature Overview
The paper by Li Guochang, Liu Lin, and Xing Zhonghua was published in the Journal of Shenyang Jianzhu University (Natural Science Edition) in 2008 (Vol. 24, No. 3, pp. 407-410). Funded by the Ministry of Construction Science and Technology Program (03-20027) and the Shenyang Science and Technology Fund (1053125-1-28), this research investigates the seismic performance of a novel joint connecting a steel tube coal gangue concrete (STCGC) column to a steel beam using a steel bracket with an external strengthening ring. The study includes low-cycle reversed loading tests and theoretical analysis to evaluate the hysteresis behavior, ductility, and energy dissipation capacity of the joint.
Core Technical Content
Background and Motivation
Steel tube concrete (STC) columns have gained widespread acceptance in modern structural engineering due to their high load-bearing capacity, compact cross-section, and favorable seismic performance. However, the connection of steel beams to STC columns presents a significant engineering challenge, particularly in seismic design where the joint must exhibit adequate ductility and energy dissipation capacity. The use of coal gangue concrete—a recycled concrete made with coal gangue as a partial aggregate replacement—offers environmental and economic benefits but raises questions about the structural performance of the composite column and its connections under seismic loading.
Joint Configuration
The studied joint consists of the following key components:
| Component | Function | Material Specification |
|---|---|---|
| Steel tube column | Primary vertical load-bearing member | Carbon structural steel (e.g., Q235/Q345) |
| Coal gangue concrete core | Compressive load sharing; enhances column stiffness | Concrete with coal gangue aggregate replacement |
| Steel bracket (corbel) | Transfers beam shear and moment to column | Structural steel plate, welded to column |
| External strengthening ring | Reinforces column at joint zone; prevents local buckling | Steel plate ring, welded to column exterior |
| Steel beam | Horizontal load-bearing member | Structural steel (I-section or H-section) |
The external strengthening ring is a critical innovation in this joint design. It is welded to the exterior surface of the steel tube column at the joint location, providing additional local confinement and preventing the steel tube from local buckling under the concentrated forces transmitted by the beam connection.
Test Results and Key Performance Indicators
The low-cycle reversed loading test revealed the following performance characteristics:
- Ductility coefficient: 5.3, indicating excellent deformation capacity under cyclic loading.
- Energy dissipation ratio: 0.891, demonstrating superior energy absorption capability.
- Failure mode: The joint exhibited a ductile failure pattern with plastic deformation concentrated in the steel bracket and beam end, while the column and strengthening ring maintained structural integrity throughout the test.
- Hysteresis behavior: The joint displayed well-defined, full hysteresis loops with minimal strength degradation over multiple loading cycles, indicating stable energy dissipation performance.
Welding Analysis and Quality Control
Welding Configuration in the Joint
The joint involves several critical welded connections that must be carefully designed and executed:
- Strengthening ring to steel tube column: The external strengthening ring is attached to the column by circumferential welds. These welds must achieve full penetration to ensure that the ring effectively transfers loads to the column. The weld type is typically a fillet weld or a groove weld, depending on the design requirements and accessibility.
- Steel bracket to steel tube column: The corbel (bracket) is welded to the column surface, creating a T-joint configuration. This connection is subjected to combined shear and bending stresses and requires high-quality welds to prevent premature failure.
- Steel beam to steel bracket: The beam is connected to the bracket, typically through a bolted or welded connection. In the studied configuration, the beam is welded to the bracket, requiring full-penetration welds at the critical load transfer locations.
- Coal gangue concrete placement: While not a welding process, the concrete placement within the steel tube must be carefully controlled to ensure full compaction and avoid voids, particularly near the joint zone where the bracket and strengthening ring are located.
Welding Process Selection and Parameters
| Welded Joint | Recommended Process | Key Parameters | Quality Standard |
|---|---|---|---|
| Strengthening ring to column | SMAW or SAW | Current: 150-250A; Speed: 5-10 cm/min | GB/T 19866 Level II |
| Bracket to column | SMAW or FCAW | Full penetration groove weld | GB/T 19866 Level I |
| Beam to bracket | SMAW or SAW | Full penetration; stress-relieved | GB/T 19866 Level I |
| Column longitudinal weld (if applicable) | SAW (submerged arc) | Multi-pass; interpass temperature control | GB/T 19866 Level I |
Residual Stress and Distortion Control
The welding of the strengthening ring and bracket introduces significant residual stresses in the joint zone. These stresses can have both beneficial and detrimental effects:
- Beneficial effect: Compressive residual stresses in the strengthening ring can enhance the local buckling resistance of the steel tube column.
- Detrimental effect: Tensile residual stresses at the weld toes can initiate fatigue cracks under cyclic loading, potentially reducing the fatigue life of the joint.
To manage residual stresses, the following measures are recommended:
- Weld sequence optimization: Welding should proceed in a symmetric pattern to minimize distortion and residual stress concentration.
- Post-weld stress relief: Local or global stress relief treatment (e.g., thermal treatment at 550-650°C) should be applied to critical joints.
- Weld toe grinding: Mechanical grinding of weld toes can reduce stress concentration and improve fatigue performance.
- Peening: Shot peening or hammer peening of weld toes can introduce beneficial compressive residual stresses that enhance fatigue resistance.
Defect Analysis and FMEA for the Joint
| Failure Mode | Potential Cause | Detection Method | Mitigation |
|---|---|---|---|
| Strengthening ring weld cracking | High residual stress; poor weld quality | MT / UT | Stress relief; welder qualification |
| Local buckling of column tube | Insufficient ring confinement; high axial load | Visual inspection post-test | Increase ring thickness; reduce axial load ratio |
| Beam end fracture | Over-strengthened joint; brittle failure | Strain measurement during test | Ensure ductile connection design |
| Concrete crushing near joint | Poor compaction; high stress concentration | UT / visual inspection | Improved concrete placement; joint detailing |
| Fatigue crack initiation at weld toe | Tensile residual stress; cyclic loading | MT / acoustic emission | Peening; weld toe grinding |
Seismic Performance Analysis
Hysteresis Behavior Interpretation
The hysteresis loops obtained from the low-cycle reversed loading test provide valuable information about the joint's seismic performance:
- Loop shape: The full and stable hysteresis loops indicate that the joint maintains its load-bearing capacity throughout the loading cycles. The absence of pinching in the loops suggests that there is no significant slippage or separation at the connection interfaces.
- Strength degradation: The minimal reduction in peak load over successive cycles indicates that the joint does not suffer from progressive damage accumulation, which is essential for seismic resilience.
- Stiffness degradation: While some stiffness reduction is expected under cyclic loading, the joint maintains adequate stiffness to control lateral displacements, as evidenced by the consistent loop width throughout the test.
Ductility and Energy Dissipation
The ductility coefficient of 5.3 indicates that the joint can undergo significant inelastic deformation without catastrophic failure. This is well above the typical minimum requirement of 3.0 for seismic joints in Chinese seismic design codes (GB 50011). The energy dissipation ratio of 0.891 is also excellent, indicating that the joint effectively absorbs and dissipates seismic energy through plastic deformation of the steel bracket and beam end.
The primary mechanism of energy dissipation in this joint is the plastic deformation of the steel bracket (corbel) and the beam end region. The strengthening ring ensures that the column does not become the weak link in the system, thereby promoting a ductile failure mode where the energy is dissipated in the intended locations.
Study Insights and Engineering Implications
This study demonstrates that the steel bracket with external strengthening ring is an effective and practical solution for connecting steel beams to steel tube coal gangue concrete columns in seismic regions. The joint achieves excellent ductility and energy dissipation performance while maintaining structural integrity under severe cyclic loading.
From a steel pipe and welding engineering perspective, the success of this joint depends critically on the quality of the welded connections between the strengthening ring, bracket, and column. The strengthening ring, in particular, must be welded with full penetration and high integrity to ensure that it provides the intended local confinement and prevents premature local buckling of the column tube. Any welding defect in the ring-column connection could compromise the entire joint's seismic performance.
The use of coal gangue concrete as the infill material introduces additional considerations for the joint design. Coal gangue concrete typically has lower compressive strength and elastic modulus compared to conventional concrete, which means that the steel tube and strengthening ring must be designed to compensate for the reduced contribution of the concrete core. The welding quality of the strengthening ring becomes even more critical in this context, as the ring must provide sufficient confinement to prevent local buckling of the steel tube under the combined action of axial load and cyclic bending.
The study also highlights the importance of experimental validation in the development of new structural joint systems. The low-cycle reversed loading test provides irreplaceable information about the actual behavior of the joint under seismic loading, including the failure mode, damage progression, and residual strength capacity. This information is essential for calibrating analytical models and developing design guidelines. Future research should extend to full-scale substructure tests and numerical modeling to provide a comprehensive understanding of the joint's behavior under various loading conditions and to develop practical design procedures for engineering applications.