Flexural Test Study on Steel Tube Concrete Truss Beams

Literature Overview

This paper by Huang Wenjin and Chen Baochun from Fuzhou University, published in the Journal of Architecture and Civil Engineering (2006, Vol. 23, No. 1, pp. 29-33), presents an experimental investigation into the flexural behavior of steel tube concrete (STC) truss beams. The study designs and fabricates STC truss beam specimens, subjects them to four-point symmetric loading, and analyzes deformation patterns, strain distribution, failure modes, and load-bearing capacity. The research provides valuable insights into the structural behavior of this composite truss system, particularly regarding joint performance and load redistribution mechanisms.

Core Technical Findings

Overall Structural Behavior

The STC truss beam exhibits several distinctive behavioral characteristics compared to solid-web beams:

Deformation and Strain Distribution

The experimental results reveal important patterns in deformation and strain:

1. Deflection concentration: The majority of beam deflection occurs in the edge segments rather than the mid-span, which is counterintuitive for a simply supported beam under symmetric loading
2. Joint strain complexity: The joints experience multi-axial stress states with significant interaction between axial, shear, and bending stresses
3. Chord member behavior: The concrete-filled chord members exhibit enhanced radial stiffness due to concrete confinement, which constrains joint deformation

Load-Bearing Capacity Analysis

The joint load-bearing capacity is identified as the governing factor for overall structural capacity:

• Joint capacity controls the ultimate load of the truss beam
• Chord member concrete fill increases radial stiffness and constrains joint deformation
• The concrete fill prevents chord member plastic instability, thereby enhancing joint capacity
• Under four-point symmetric loading, the actual axial force in web members is less than the pin-jointed truss analytical value
• Chord members carry significant bending moments in addition to axial forces

Failure Mode Characteristics

The failure mode is characterized by:

• Progressive yielding of chord members near joint locations
• Shear deformation in edge segment web members prior to joint failure
• Joint failure as the ultimate limit state
• Ductile behavior with significant deformation prior to failure

Process Analysis and Engineering Implications

Specimen Design and Fabrication

The fabrication of STC truss beam specimens requires careful attention to several process aspects:

1. Steel tube preparation: Chord and web members must be precisely cut and prepared for connection
2. Concrete placement: Concrete must be placed into tubes before assembly or through designated openings, with proper compaction
3. Joint fabrication: Welded joints require careful procedure qualification and execution
4. Assembly sequence: The assembly sequence affects residual stresses and final geometry

Welding Considerations

The truss beam joints involve critical weld connections:

Weld quality directly affects joint capacity, which in turn governs overall beam capacity. Weld procedure qualification per applicable codes (ASME B31.3, EN 10216, or GB/T standards) is essential.

Design Implications

The experimental findings have significant implications for STC truss beam design:

1. Joint design governs: Joint capacity must be designed to exceed member capacity to ensure member-controlled failure
2. Concrete fill is beneficial: The concrete fill in chord members provides radial stiffness and prevents local buckling
3. Semi-rigid behavior: The joints are not perfectly rigid, leading to moment redistribution that must be accounted for in design
4. Web member forces: Actual web member forces differ from ideal pin-jointed truss analysis, requiring frame analysis for accurate design

Comparison with Alternative Truss Types

Key Reflections and Study Insights

The most significant finding from this research is that joint capacity governs the overall load-bearing capacity of STC truss beams. This is a critical insight for design, as it means that joint design must receive as much attention as member design. The traditional approach of designing members first and then detailing joints may not be adequate for STC truss systems.

The observation that web member axial forces are less than pin-jointed truss analytical values, while chord members carry significant bending moments, highlights the importance of semi-rigid joint analysis. The actual structural behavior is intermediate between ideal truss and frame behavior, and design methods must account for this semi-rigid characteristic.

The concentration of deformation in edge segments rather than mid-span is an unexpected but important finding. This suggests that the truss action is most effective in the mid-span region, where the geometry provides optimal load paths, while edge segments rely more on flexural action. This has implications for serviceability design, as deflection limits must be checked at the locations of maximum deformation.

The role of concrete fill in enhancing chord member radial stiffness and preventing plastic instability is a key advantage of STC truss beams over all-steel trusses. This confinement effect is well-established for individual CFT members but its application to truss systems is a valuable contribution.

Reference Value and Outlook

This research provides essential experimental data for the design and analysis of STC truss beams. The identification of joint capacity as the governing design factor is particularly important, as it directs design attention to the most critical aspects of the structure. Future research should investigate seismic performance through cyclic loading tests, develop design equations based on parametric studies, and explore the application of STC truss beams in bridge structures and long-span buildings. The findings also have implications for construction methodology, as the fabrication and assembly of STC truss beams requires specialized techniques and quality control procedures.