Experimental Research on Closely-Jointed Pre-Stressed Concrete Steel Pipe Truss Composite Slabs

Literature Overview

The paper by Yu Jinghai, He Mengjie, Zhao Yuyang, Zhang Bo, and Tang Yuxuan (2020), published in Industrial Construction (Vol. 50, No. 6, pp. 58–66), presents experimental research on closely-jointed pre-stressed concrete steel pipe truss composite slabs. The research was supported by the Tianjin Innovation and Entrepreneurship Team Special Support Project (TJTZJH-GCCCXCYTD-1-3). The authors are affiliated with Tianjin University, Tianjin University Architectural Design and Research Institute, Tianjin Jinzheng Real Estate Development Co., Ltd., and Shandong Wansida Building Technology Co., Ltd. This work addresses an important topic in precast construction: the structural performance of composite slabs formed by joining multiple precast units with closely-spaced joints.

Experimental Program and Specimen Configuration

The experimental program involved eight closely-jointed pre-stressed concrete steel pipe truss composite slabs subjected to static loading under uniformly distributed load. The loading direction was perpendicular to the pre-stressing direction, which represents a critical structural condition for these composite slabs.

The steel pipe truss embedded in the precast bottom plate serves as the shear connector between the precast unit and the cast-in-place topping layer. The closely-spaced joints between adjacent precast units are designed to provide continuity and load transfer between units.

Key Experimental Findings

The experimental results reveal several important structural behaviors:

1. Overall structural performance: The closely-jointed pre-stressed concrete steel pipe truss composite slabs exhibit good overall structural performance under bending loads perpendicular to the pre-stressing direction.
2. Cracking behavior:

• For two-unit composite slabs: Cracks appear only at the mid-span joint, indicating that the joint is the critical location for crack initiation.
• For three-unit and four-unit composite slabs: Cracks are distributed more uniformly across the slab, suggesting that the joint reinforcement effectively distributes the stresses.

1. Joint reinforcement effectiveness: The joint construction reinforcement shares the stress with the transverse reinforcement. Although the joint reinforcement does not significantly increase the cracking load, it causes the cracks to be finer and more uniformly distributed, which is beneficial for durability and serviceability.
2. Ultimate load and composite layer thickness: The ultimate load increases with the thickness of the cast-in-place composite layer, as expected from the increased section modulus and reinforcement.
3. Plane-section assumption: The plane-section assumption is valid at locations away from the joints, confirming the applicability of conventional beam theory for analysis of these composite slabs.

Parametric Analysis and Stiffness Model

Based on the experimental results, a parametric analysis was conducted to identify the primary factors influencing the flexural stiffness in the direction perpendicular to the pre-stressing direction. The key parameters identified are:

The authors proposed a stiffness influence coefficient calculation formula based on nonlinear surface fitting of the parametric analysis results. This formula provides a practical tool for predicting the flexural stiffness of closely-jointed composite slabs under various geometric configurations.

Engineering Practice Implications

From the perspective of steel pipe fabrication and composite slab construction, several practical considerations emerge:

• Steel pipe truss quality: The steel pipe truss elements serve as the primary shear connectors between the precast bottom plate and the cast-in-place topping layer. Their fabrication quality — including pipe diameter accuracy, truss height consistency, and weld quality — directly affects the composite action and overall structural performance.
• Joint detail design: The closely-spaced joints between precast units are critical for load transfer and crack control. The joint reinforcement must be properly detailed and installed to ensure effective stress distribution.
• Steel pipe specifications: The steel pipes used in the truss elements are typically small-diameter pipes (10–20 mm outer diameter) with thin walls (1.5–3 mm). These dimensions require precise fabrication to ensure consistent performance.
• Welding quality: The welding of the steel pipe truss to the precast bottom plate reinforcement must be inspected for full fusion and adequate weld size. Incomplete welds or weak welds would compromise the shear transfer capacity.

Key Questions and Reflections

Several questions arise from this research:

• The experimental program involves only 8 specimens. While this provides valuable insight, the statistical basis for the proposed stiffness formula may be limited. Further experimental validation with a larger specimen set would strengthen the conclusions.
• The research focuses on static loading. In practical applications, composite slabs may be subjected to dynamic loads, impact loads, and cyclic loading. The performance under these conditions would need separate investigation.
• The long-term durability of the closely-spaced joints is a concern. Cracking at the joints, even if fine and distributed, could provide pathways for moisture and chloride ingress, potentially leading to corrosion of the embedded steel reinforcement and steel pipe truss elements.
• The steel pipe truss elements are typically made from low-carbon structural steel. The corrosion protection strategy for these elements — whether through galvanizing, epoxy coating, or increased concrete cover — is critical for long-term performance.

Study Insights and Conclusion

This paper provides valuable experimental evidence for the structural performance of closely-jointed pre-stressed concrete steel pipe truss composite slabs. The findings confirm that the composite slab system works effectively when properly designed and constructed, with the closely-spaced joints providing adequate continuity and load transfer between precast units. For steel pipe manufacturers and precast concrete producers, the work highlights the importance of maintaining precise fabrication tolerances for the steel pipe truss elements and ensuring high-quality welding connections. The proposed stiffness influence coefficient formula offers a practical design tool that can be incorporated into structural analysis software. The research contributes to the growing body of knowledge on precast composite slab systems, which are increasingly used in industrial and commercial construction for their speed of erection, quality control advantages, and material efficiency.