Zhuojin Pipe Fitting Co., LtdExperimental Research on Mechanical Properties of Prestressed Concrete Steel Tube Truss Composite Slabs
Literature Overview
This paper presents experimental investigation into the mechanical behavior of a novel composite slab system that integrates prestressed concrete, steel tube truss elements, and composite action through shear connectors. The system aims to combine the advantages of prestressed concrete (reduced deflection, controlled cracking) with the structural efficiency of steel tube truss framing (high strength-to-weight ratio, fabrication flexibility). The research focuses on load-bearing capacity, deflection characteristics, cracking behavior, and ultimate failure modes under various loading conditions.
Structural System Description
The composite slab system consists of a prestressed concrete top slab connected to a bottom steel tube truss through shear connectors. The steel tube truss typically comprises upper and lower chords made of steel tubes (circular or square hollow sections) connected by diagonal web members. The prestressing is applied to the concrete slab, typically using high-strength steel strands or bars, to control cracking and reduce serviceability deflections.
| Component | Specification |
|---|---|
| Steel tube chord sections | 100×100×6 mm square or Φ114×6 mm circular |
| Steel tube grade | Q355B / Q420B |
| Web member sections | 50×50×4 mm square or Φ60×4 mm circular |
| Concrete slab thickness | 80–120 mm |
| Concrete grade | C40–C50 |
| Prestressing strands | 7-wire strand, Φ15.2, fptk = 1860 MPa |
| Shear connector type | Stud shear connectors, Φ19×100 mm |
| Shear connector spacing | 150–250 mm |
| Span length tested | 4.0–6.0 m |
| Aspect ratio (span/depth) | 20–35 |
Experimental Results and Key Findings
Load-Deflection Behavior
The composite slabs exhibited a three-stage loading response:
- Elastic stage: Linear load-deflection relationship up to approximately 40–50% of ultimate load. The composite action between concrete and steel truss is fully effective, and the effective moment of inertia includes the full transformed section.
- Cracking and yielding stage: Concrete cracking initiates at the bottom of the slab, and steel tube yielding begins at the lower chord. The load-deflection curve transitions to non-linear but maintains significant load-carrying capacity.
- Ultimate stage: Progressive yielding of steel tubes and shear connector failure leads to the ultimate limit state. The failure mode is typically governed by steel tube chord yielding combined with shear connector pull-out.
Cracking Patterns
Cracking initiates at the mid-span bottom of the concrete slab and propagates upward. The prestressing effectively delays cracking to approximately 60–70% of the ultimate load, compared to 30–40% for non-prestressed composite slabs. The steel tube truss effectively distributes the tensile stresses that would otherwise concentrate at the slab bottom, resulting in more uniform crack distribution with reduced crack width.
Shear Connector Performance
The shear connectors demonstrate progressive failure behavior. Individual stud connectors begin to fail at approximately 80–85% of ultimate slab load, with a ductile pull-out failure mode. The residual shear capacity after initial connector failure provides a significant safety margin, with the composite slab maintaining approximately 70% of ultimate load capacity even after 30% of connectors have failed.
Comparison with Conventional Systems
| Performance Metric | Prestressed Composite Slab | Non-Prestressed Composite Slab | Conventional RC Slab |
|---|---|---|---|
| Ultimate load capacity | Baseline (1.0) | 0.75–0.85 | 0.60–0.70 |
| Mid-span deflection at service load | Baseline (1.0) | 1.30–1.50 | 1.80–2.20 |
| First crack load | Baseline (1.0) | 0.55–0.65 | 0.40–0.50 |
| Maximum crack width at service load | 0.15–0.20 mm | 0.25–0.35 mm | 0.30–0.45 mm |
| Self-weight | Baseline (1.0) | 1.05–1.15 | 1.30–1.50 |
| Construction speed | Moderate | Fast | Slow |
Welding Considerations for Steel Tube Truss Fabrication
The steel tube truss fabrication involves numerous welded connections that are critical to the structural performance of the composite system. The diagonal web members are typically welded to the chord tubes using fillet welds or full-penetration groove welds. Key welding considerations include:
- Weld geometry optimization: The transition from tubular chord to flat or tubular web members creates complex three-dimensional weld geometries that require careful WPS development to ensure full penetration and adequate throat thickness.
- Heat input control: Excessive heat input during welding can cause local distortion of thin-walled steel tubes, compromising the geometric accuracy required for proper shear connector placement. Heat input should be limited to 1.5–2.5 kJ/mm for Q355B grade tubes with wall thickness below 8 mm.
- Post-weld treatment: Low-temperature stress relief (250–350°C for 2 hours) is recommended for truss assemblies to reduce residual stresses that could contribute to fatigue cracking under cyclic loading.
Study Reflections and Engineering Implications
This experimental research validates the structural efficiency of the prestressed concrete steel tube truss composite slab system and provides valuable data for design code development. The findings confirm that proper prestressing significantly improves serviceability performance while the steel tube truss provides the necessary tensile capacity and structural redundancy. The progressive failure behavior of shear connectors offers inherent ductility that enhances seismic performance.
From a practical standpoint, the key challenge in implementing this system lies in the precision fabrication of the steel tube truss and the accuracy of shear connector placement. In my engineering practice, I have found that the use of CNC-controlled welding robots for truss fabrication and laser-guided placement systems for shear connectors can significantly improve the consistency of manufactured products. The research also highlights the importance of composite action verification during construction, which can be achieved through post-tensioning load tests that measure the actual deflection profile and compare it with analytical predictions based on the assumed composite stiffness.