Zhuojin Pipe Fitting Co., LtdAnti-Collapse Performance of Prefabricated Concrete-Filled Steel Tube Frame Nodes
Research Significance and Background
Progressive collapse is one of the most feared failure modes in building structures, where the loss of a single load-bearing element can trigger a chain reaction leading to the collapse of an entire structural system. Prefabricated concrete-filled steel tube (CFST) frame nodes, which are increasingly used in modern construction for their speed of assembly and high load capacity, must be designed to provide adequate resistance against progressive collapse. This study presents experimental investigation of the anti-collapse performance of prefabricated CFST frame nodes under various loading scenarios, providing critical data for performance-based design.
Test Configuration and Loading Protocol
The experimental program includes full-scale and reduced-scale node specimens subjected to quasi-static and dynamic loading to simulate both gradual and sudden loss of support conditions.
| Specimen Type | Scale | Steel Tube | Concrete | Loading Condition | Purpose |
|---|---|---|---|---|---|
| FS-1 | Full-scale | Q345, 200×10 | C50 | Single column removal | Progressive collapse initiation |
| FS-2 | Full-scale | Q345, 250×12 | C60 | Two-column removal | Multi-element failure |
| RS-1 | 1:2 scale | Q345, 100×5 | C50 | Quasi-static pull | Connection strength verification |
| RS-2 | 1:2 scale | Q390, 125×6 | C60 | Impact loading | Dynamic response assessment |
The loading protocol follows the Progressive Collapse Procedures (PCP) recommended by the US GSA and FEMA, involving three stages: (1) normal service loading, (2) sudden removal of the critical column (simulated by rapid load release), and (3) continued loading until the structure reaches a new equilibrium or fails completely.
Connection Types and Performance Comparison
Prefabricated CFST frame nodes employ various connection types, each with distinct anti-collapse characteristics.
| Connection Type | Assembly Method | Ductility Index | Load Redistribution Capacity | Ease of Field Assembly |
|---|---|---|---|---|
| Bolted flange with shear keys | High-strength bolts | 2.5–3.5 | Medium | Excellent |
| Welded end plate | Site welding | 3.0–4.0 | High | Moderate |
| Friction-fit sleeve with grout | Precast sleeve, post-grouting | 2.0–3.0 | Medium-High | Good |
| Mechanically locked joint | Interlocking geometry | 3.5–5.0 | High | Good |
| Hybrid bolted-welded | Combined approach | 4.0–5.5 | Very High | Moderate |
The hybrid bolted-welded connection demonstrates the best anti-collapse performance due to its ability to undergo significant deformation while maintaining load-carrying capacity. The bolted component provides initial stiffness and serviceability, while the welded component ensures ductile failure and energy dissipation under extreme loading.
Progressive Collapse Resistance Mechanisms
When a column is removed, the adjacent beams must redistribute the load through three mechanisms: (1) catenary action, where the beams develop tensile forces and act as cables; (2) flexural redistribution, where plastic hinges form and moments are transferred to adjacent spans; and (3) membrane action, where the slab develops in-plane tensile forces. For CFST frames, the concrete-filled steel tubes provide additional benefits through their high bending capacity and ductility, allowing the catenary action to develop fully before failure.
The displacement capacity of the node (measured as the vertical deflection of the removed column position at the point of structural failure) is a key performance indicator. Test results show that properly designed CFST frame nodes can achieve displacement capacities of 30–50% of the span length, well exceeding the typical 20% requirement specified in progressive collapse design guidelines.
Welding Quality and Its Impact on Anti-Collapse Performance
The welding quality at prefabricated CFST node connections directly influences the progressive collapse resistance. Poor weld quality can lead to premature brittle fracture, which eliminates the ductile failure mode essential for progressive collapse resistance. The following quality control measures are critical:
- All field welds should be inspected by qualified Level II NDT personnel using MT for surface defects and UT for volumetric defects.
- Weld repair procedures must follow the original welding procedure specification (WPS) with documented qualification records.
- Impact testing of weld metal coupons at -20 °C should verify a minimum absorbed energy of 27 J for Charpy V-notch specimens.
- Dimensional tolerance of weld penetration must be within ±0.5 mm of the designed throat thickness to ensure uniform stress distribution.
Key Findings and Design Recommendations
The experimental results demonstrate that prefabricated CFST frame nodes can achieve excellent anti-collapse performance when properly designed. The key design parameters are: (1) the beam-to-column connection should be designed for a plastic moment capacity at least 1.2 times the design moment from gravity loads; (2) the CFST column should have a D/t ratio below 40 to ensure adequate ductility; and (3) the connection should provide sufficient rotation capacity (at least 0.04 radians) to develop catenary action.
For prefabricated construction, the friction-fit sleeve with post-grouting connection offers a good balance between assembly convenience and progressive collapse resistance. However, the grouting quality must be rigorously controlled, with 100% radiographic inspection of critical joints to verify complete sleeve filling. The grout strength should exceed the concrete fill strength by at least 20 MPa to ensure that the connection, not the concrete core, governs the failure mode.