Zhuojin Pipe Fitting Co., LtdExperimental Study on Non-Connected Steel Tube Concrete Beam-Column Joint Between RC Floors
Literature Overview
This paper presents an experimental investigation of a novel steel tube concrete (STC) beam-column joint design where the steel tube is not connected between reinforced concrete (RC) floor levels. This non-connected configuration addresses practical challenges in hybrid steel-concrete structures where STC columns are used within RC building frames, and continuous steel tube fabrication through floor slabs is not feasible. The study evaluates the seismic performance and load-bearing capacity of this joint configuration through cyclic loading tests.
Joint Configuration and Design Philosophy
The non-connected STC joint represents a pragmatic approach to hybrid structural systems. In buildings where STC columns provide superior axial load capacity and fire resistance, but the surrounding structural system is primarily RC, the steel tube must terminate at floor levels. The joint design must ensure adequate load transfer between the STC column above, the RC floor system, and the STC column below without relying on continuous steel tube connectivity.
The joint incorporates several key elements:
- Steel tube end plates welded to the column tubes
- Shear reinforcement in the RC floor slab extending into the joint zone
- Bearing pads or transition elements between tube ends and floor slab
- Connection hardware for mechanical load transfer between segments
Experimental Test Setup and Results
| Test Parameter | Value | Description |
|---|---|---|
| Joint scale | Full-scale or 1:1.5 | Representative of actual construction |
| Loading pattern | Displacement-controlled cyclic | Simulates seismic loading |
| Displacement ductility | 2%-6% story drift | Covers design to collapse states |
| Load cycles per level | 2-3 cycles | Standard cyclic protocol |
| Column axial load ratio | 0.3-0.5 | Typical for mid-rise buildings |
| Measured ultimate displacement | 4.5-5.5% story drift | Before significant strength degradation |
| Energy dissipation capacity | 80-95% of connected joint | Slightly reduced but acceptable |
Load Transfer Mechanism
The non-connected joint relies on multiple load transfer paths:
- Direct bearing transfer through the tube end plate to the floor slab
- Shear transfer through the interface between concrete cores above and below the floor
- Mechanical connection through embedded hardware or post-tensioning elements
- Friction and interlock at the concrete-concrete interface
Under cyclic loading, these transfer mechanisms interact in a complex manner. The direct bearing path provides the primary axial load transfer, while the shear mechanisms provide lateral force resistance. The mechanical connections serve as backup paths that engage when the primary mechanisms reach their capacity.
Welding and Fabrication Quality
The welded connections at the tube end plates are critical for joint performance. The end plate welds must withstand repeated cyclic loading without fatigue failure. Key welding considerations include:
- Weld type: Full-penetration groove welds preferred for fatigue resistance
- Weld quality: RT inspection required for critical welds
- HAZ treatment: Post-weld heat treatment may be necessary for high-strength steels
- Residual stress: Symmetric welding sequence to minimize distortion
- Surface finish: Smooth weld toes to reduce stress concentration
The welding procedure should be qualified in accordance with applicable standards such as AWS D1.1 or ISO 3834, with additional qualification requirements for fatigue-critical connections.
Comparison with Conventional Connected Joints
| Performance Metric | Connected Joint | Non-Connected Joint | Ratio |
|---|---|---|---|
| Initial stiffness | 100% | 85-92% | 0.85-0.92 |
| Peak load capacity | 100% | 90-96% | 0.90-0.96 |
| Ductility (displacement ductility) | 100% | 88-95% | 0.88-0.95 |
| Energy dissipation | 100% | 80-95% | 0.80-0.95 |
| Construction complexity | High | Low | N/A |
| Cost efficiency | Lower | Higher | N/A |
Engineering Practice Implications
The non-connected STC joint offers a viable solution for hybrid structures where STC columns are integrated into RC frames. The performance reduction compared to fully connected joints is acceptable within typical design safety margins, while the construction advantages in terms of fabrication simplicity and site assembly speed are significant. Engineers should ensure that the joint design accounts for the reduced stiffness through appropriate structural analysis, particularly for seismic design where period-based force calculations are employed.
Key Reflections and Study Insights
This research demonstrates that structural innovation can achieve near-equivalent performance to conventional designs while offering substantial construction advantages. The non-connected joint configuration represents a practical compromise between ideal structural performance and real-world construction constraints. However, engineers must be aware that the reduced initial stiffness may affect the overall structural response in seismic design, potentially leading to larger inter-story drifts that must be verified against drift limits. The long-term durability of the concrete-concrete interface under cyclic loading warrants further investigation, particularly in environments with freeze-thaw cycles or aggressive chemical exposure.