Zhuojin Pipe Fitting Co., LtdBending Performance of Prestressed Concrete Steel Tube Truss Composite Soffit During Construction Stage
Literature Overview
This study by Yu Jinghai and colleagues from Tianjin University, published in Structural Engineer journal in 2020, investigates the bending performance of prestressed concrete steel tube truss composite soffit panels during the construction stage. Three test specimens with different truss heights and spans were designed and tested under simply supported and mid-span support conditions. Funded by the Tianjin Innovation and Entrepreneurship Team Special Support Project, this research addresses a critical practical issue: ensuring the structural integrity of composite floor systems during the construction phase before the cast-in-place concrete achieves full strength.
Test Configuration and Parameters
The specimens incorporated steel tube trusses as the structural core of the composite soffit panel, with prestressed concrete bottom plates. The primary variables were truss height, span length, and support conditions.
| Parameter | Variation | Observed Effect |
|---|---|---|
| Truss height | Different levels | Higher truss increases bearing capacity and stiffness |
| Span length | Different lengths | Affects moment distribution and deflection |
| Support condition | Simply supported vs. mid-span support | Changes cracking pattern and failure sequence |
| Concrete cracking | Left/right mid-span first | Then middle support top surface |
| Section behaviour | Non-planar deformation | Plane section assumption not satisfied |
Core Technical Findings
The research reveals several important structural behaviours. First, increasing the steel tube truss height directly improves both the bearing capacity and stiffness of the composite soffit panel. This is because a taller truss provides a larger internal lever arm between the top and bottom chords, enhancing the section modulus for bending resistance.
Second, the steel tube truss and the prestressed concrete bottom plate exhibit good composite action, meaning the interface between the two components effectively transfers shear forces. However, the section deformation during bending does not conform to the plane section assumption, indicating non-uniform strain distribution across the section height. This deviation from Euler-Bernoulli beam theory has implications for analytical design methods.
Third, the presence of temporary mid-span support fundamentally changes the cracking sequence. With temporary support, the left and right mid-span regions crack first in the bottom concrete, while the middle support top surface cracks later. This crack propagation pattern reflects the complex moment redistribution in the continuous beam behaviour created by the temporary support.
Steel Tube Truss Fabrication and Welding Quality
The steel tube truss is a critical component whose fabrication quality directly affects the composite panel performance. The truss typically consists of:
- Top chord: Steel tube elements connected by welded joints
- Bottom chord: Steel tube elements forming the compression zone
- Web members: Diagonal and vertical steel tube members providing shear resistance
Welding quality of the truss joints is paramount. Common fabrication methods include:
| Joint Type | Welding Process | Quality Concern |
|---|---|---|
| Chord-web connection | GMAW or SAW | Root penetration, undercut |
| Chord-chord splice | Butt weld or fillet weld | Full penetration verification |
| End plate connections | Fillet weld | Throat thickness adequacy |
| Shear connector attachment | Stud welding or fillet weld | Pull-out resistance |
The steel tubes used in the truss are typically small-diameter ERW or seamless tubes. The wall thickness of these tubes affects the local buckling resistance of the truss members under compression. For the bottom chord in compression, tube ovalization and local buckling must be checked against the applicable design codes.
Stiffness Calculation and Construction Stage Support Recommendations
The authors derived a stiffness calculation formula for the composite soffit panel, which accounts for the composite action between the steel tube truss and the prestressed concrete bottom plate. This formula is essential for construction stage design, where the panel must support its own weight and construction loads before the cast-in-place concrete achieves design strength.
The research provides practical recommendations for temporary support arrangement during construction. The key insight is that the support location and spacing must be designed to prevent premature cracking in critical regions. The finding that cracking initiates at the left and right mid-spans first suggests that these regions are most vulnerable and may require closer support spacing or additional shoring.
Study Insights and Engineering Implications
This research provides valuable guidance for the design and construction of steel tube truss composite floor systems. The non-planar deformation behaviour observed experimentally suggests that simplified analytical methods based on plane section assumptions may underestimate deflections and overestimate stiffness. Engineers should adopt more refined analytical approaches or rely on validated finite element models for critical applications. The construction stage support recommendations are particularly practical and should be incorporated into construction method statements. The composite action between the truss and concrete plate, while good, should be verified through interface testing in production settings to ensure proper bond and shear transfer at the construction site.