Bending Capacity of Grouted Square Steel Tube Truss-Concrete Composite Beams

Overview of the Study

This paper by Wang Kun et al. (2020), published in the Journal of Yangzhou University (Natural Science Edition), investigates the bending behavior of composite beams formed by combining square steel tube (SST) trusses with concrete decks, where the chord tubes are filled with grout or concrete. The research is supported by multiple funding sources including the National Natural Science Foundation (51878589), Jiangsu Province Talent Program (2017-JZ-038), and Yangzhou Science and Technology Bureau (YZ2018068). The study combines experimental testing with Abaqus finite element analysis to develop a bending capacity calculation formula.

Core Technical Findings

Experimental and Analytical Results

The study demonstrates that the grouted SST truss-concrete composite beam exhibits high bending capacity and good ductility. The key findings regarding parameter sensitivity are summarized below:

This parameter sensitivity pattern is physically intuitive: in a simply supported beam under gravity loading, the bottom chord acts in tension and its cross-sectional area directly controls the tensile capacity. The top chord, being in compression, benefits from the concrete deck and grout infill, so its steel contribution is less critical.

Finite Element Validation

The Abaqus numerical model accurately reproduces the experimental load-deflection curves and chord strain distributions. The agreement between experimental and numerical results validates the modeling approach and provides confidence in the proposed analytical formula.

Bending Capacity Formula

The authors develop a normal section bending capacity calculation formula that accounts for the composite action between the steel truss and the concrete deck. The formula is validated against experimental results and shows good agreement.

Fabrication and Welding Considerations

The construction of SST truss-concrete composite beams involves several critical fabrication and welding operations:

• Chord tube preparation: The square steel tubes are typically manufactured by ERW or HFW processes. The surface quality, dimensional accuracy, and flatness of the tubes directly affect the quality of chord-to-web connections. Tubes with excessive out-of-squareness or surface defects should be rejected or corrected before welding.
• Truss node welding: The connection of chord tubes to web members at the truss nodes is a critical welding operation. These are typically fillet welds or groove welds on gusset plates. The weld size and quality must be designed to transfer the full design forces without premature failure. For seismic applications, full-penetration welds with qualified welding procedures are recommended.
• Grout/concrete infill: The grouting of chord tubes is essential for developing the full composite action. The grout material should have adequate flowability to fill the tube completely, sufficient compressive strength to contribute to the flexural capacity, and compatibility with the steel tube to prevent corrosion. The infill process should be monitored to ensure no voids or honeycombing.
• Shear connectors: The composite action between the steel truss and the concrete deck depends on the effectiveness of shear connectors (typically welded studs or bolted connectors). The welding of shear studs must be inspected for lack of fusion and root cracks, which are common defects in stud welding.

Quality Control and Inspection

The following quality control measures are recommended for this type of composite beam:

Study Insights and Practical Recommendations

The grouted SST truss-concrete composite beam is an efficient structural system for medium-span applications where high bending capacity and good ductility are required. The parameter sensitivity analysis provides valuable guidance for economical design: increasing the bottom chord cross-sectional area is the most effective way to enhance bending capacity, while increases in top chord area or reductions in node spacing offer diminishing returns. Engineers should ensure that the grout infill is complete and that weld quality at truss nodes and shear connectors is rigorously controlled. The proposed bending capacity formula provides a practical design tool, but should be supplemented with finite element analysis for unusual loading conditions or non-standard geometries. Future research should investigate the long-term behavior under sustained loading, the effects of corrosion on the grout-steel interface, and the seismic performance of this composite beam system.