Bending Capacity of Square Steel Tube Truss-Concrete Composite Beam with Grouted Chords

Literature Overview

Published in the Journal of Yangzhou University (Natural Science Edition) (2020, Vol. 23, No. 4), this paper by Wang Kun, Zha Zhiyuan, Hu Pengfei, and Zhong Zhenpeng investigates the flexural behavior of a composite beam system where both the top and bottom chords of a square steel tube truss are grouted with concrete. The research is supported by multiple funding sources including the National Natural Science Foundation of China (51878589), the Jiangsu Provincial Six Major Talent Peaks Program (2017-JZ-038), and the Jiangsu Provincial Double Innovation Program. This work is directly relevant to steel pipe manufacturing and composite structural engineering.

Structural Configuration and Design Rationale

The composite beam system studied features:

• A square steel tube truss as the primary load-bearing framework
• Both top and bottom chords fully grouted with concrete
• A reinforced concrete deck or topping slab forming the upper flange
• Diagonal web members connecting the chords in a truss configuration

The grouting of the square steel tube chords serves multiple purposes:

1. Enhancing the bending capacity of the chords by utilizing the full cross-section
2. Improving local buckling resistance of the thin-walled square tubes
3. Providing composite action between the steel tubes and the concrete deck
4. Reducing overall structural weight compared to fully solid sections

Design Parameters Investigated

Test Program and Numerical Analysis

Static loading tests were conducted on composite beam specimens under vertical concentrated loads. The tests measured:

• Load-midspan deflection curves
• Strain distribution in top and bottom chord steel tubes
• Strain distribution in the concrete deck
• Crack patterns and failure modes

Finite element models were developed using Abaqus software to complement the experimental investigation. The FEA models captured:

• Nonlinear material behavior of steel and concrete
• Interface bonding between steel tubes and grouted concrete
• Contact and separation at chord-deck interfaces
• Progressive damage and failure mechanisms

Load-Deflection Behavior

The load-midspan deflection curves exhibit a typical three-stage behavior:

1. Elastic stage: Linear relationship between load and deflection, with composite action fully engaged
2. Yielding stage: Gradual increase in deflection rate as steel chord materials yield
3. Post-peak stage: Significant deflection increase with limited load-carrying capacity, indicating ductile failure

The numerical simulation results show good agreement with experimental data, validating the FEA modeling approach.

Bending Capacity Calculation Formula

Based on the experimental and numerical results, the authors develop a normal section bending capacity calculation formula for the grouted square steel tube truss-concrete composite beam. The formula accounts for:

• The flexural resistance of the bottom chord (steel tube + grouted concrete)
• The flexural resistance of the top chord (steel tube + grouted concrete)
• The contribution of the concrete deck slab
• The interaction effects between steel and concrete components

Key Design Insights

Engineering Practice Implications

From a steel pipe manufacturing and structural engineering perspective, this research has several important implications:

• Square tube fabrication: The grouting of square steel tubes requires precise fabrication tolerances to ensure uniform wall thickness and straightness. Deviations in wall thickness can lead to uneven concrete distribution and reduced composite action.
• Concrete placement: Grouting concrete into square steel tube chords requires careful placement procedures to avoid voids. The square cross-section provides better access for concrete placement compared to circular sections, but the corners may still trap air pockets. Vibratory compaction or self-compacting concrete should be used to ensure full consolidation.
• Welding of truss nodes: The connections between chord tubes and diagonal web members must be designed for the combined effects of axial forces, bending moments, and shear forces. Welded connections require careful design to accommodate the complex stress states at truss nodes.
• Quality control: Non-destructive testing (NDT) of welds at truss nodes and inspection of concrete grout quality within the chords are essential quality control measures. Ultrasonic testing (UT) can verify weld integrity, while radiographic testing (RT) or impact echo methods can detect voids in the grout.

Critical Reflection

The paper's finding that bottom chord area has a significant effect on bending capacity while top chord area has minimal effect is consistent with the flexural behavior of composite beams where the bottom chord is in tension and the top chord is in compression. However, this conclusion should be applied with caution in design practice, as the top chord area affects stability and buckling resistance, which are critical for slender composite beams. The paper does not address lateral-torsional buckling, which may govern the design of long-span composite beams. Additionally, the study focuses on static loading behavior, and the fatigue performance of the welded truss nodes under cyclic loading—particularly relevant for bridge applications—requires further investigation. The interface bonding between the grouted concrete and the steel tube walls, which is critical for composite action, could benefit from more detailed investigation using push-out tests or strain gauge measurements at the interface.

Summary

This paper provides a well-documented investigation into the flexural behavior of grouted square steel tube truss-concrete composite beams, offering a validated bending capacity calculation formula suitable for design application. The key engineering insights are that bottom chord cross-sectional area is the primary design variable for bending capacity, grout quality is critical for achieving full composite action, and finite element analysis with proper modeling of material nonlinearity and interface behavior provides reliable prediction of structural response. For steel pipe manufacturers and structural engineers, this work underscores the importance of precise fabrication tolerances, high-quality concrete grouting, and rigorous welding quality control in achieving the designed performance of grouted composite beam systems.