Experimental Investigation of Rectangular Steel Tube Concrete Long-Span Cantilever Truss Structures

Literature Overview

Yang Chun, Cai Jian, Chen Guodong, Luo Qizhi, and Zuo Zhiliang from South China University of Technology and Foshan University of Science and Technology present comparative experimental research on rectangular steel tube concrete (RSTC) long-span cantilever truss structures. Published in the Journal of South China University of Technology (Natural Science) in 2008, Volume 36, Issue 3, this study investigates the structural performance of RSTC truss systems through direct comparison with conventional rectangular steel tube (RST) truss models. The research addresses the practical question of whether concrete infill in rectangular steel tube truss members provides meaningful structural benefits for long-span applications.

Experimental Configuration and Test Matrix

The study employed a comparative test approach with two large-scale model specimens: one RSTC truss and one RST truss, both featuring long-span configurations with cantilever extensions. The specimens were subjected to loading tests to evaluate failure modes, deformation characteristics, and load-bearing capacity. The comparative design isolates the effect of concrete infill on truss structural performance, providing clear evidence for or against the use of composite construction in truss applications.

Core Findings: Effect of Concrete Infill

The experimental results clearly demonstrate the structural benefits of concrete infill in rectangular steel tube truss members:

The stiffness improvement is the most pronounced benefit of concrete infill. The concrete core provides additional bending stiffness to the rectangular tube members by increasing the effective second moment of area and by confining the steel walls against local buckling. This stiffness enhancement translates directly into reduced deformations under service loads, improving the structural performance of long-span truss systems where deflection limits are often critical design constraints.

Local Buckling Prevention Mechanism

One of the most important findings is that concrete infill effectively prevents local buckling of the steel tube walls. In conventional steel tube truss members, particularly those with relatively thin walls, local buckling can occur before the material reaches its full strength capacity. The concrete core provides lateral support to the steel walls, raising the local buckling resistance and allowing the member to achieve a more fully plastic failure mode governed by material strength rather than geometric instability.

This finding has significant implications for the economic efficiency of RSTC truss structures. By preventing premature local buckling, concrete infill allows engineers to use thinner-walled steel tubes while maintaining adequate structural performance, potentially reducing material costs. The concrete also provides additional compressive resistance in compression members, further enhancing the structural efficiency of the composite system.

Connection Performance and Node Behavior

A critical finding with direct design implications concerns the behavior of welded connections in both RST and RSTC trusses. The study identifies that web members and chord members develop significant end moments at the nodes due to the direct welded connection configuration. These node moments are substantial enough to affect the connection's load-bearing capacity and must be considered in the structural design.

The presence of these moments arises from the semi-rigid nature of welded truss connections, where the weld restraint creates moment continuity between members. In conventional truss analysis, members are often idealized as pin-jointed, but the actual welded connections introduce moment transfer that can significantly alter the stress distribution. This finding is particularly relevant for RSTC trusses because the increased stiffness of concrete-filled members amplifies the moment transfer at connections.

Engineering Practice Implications

The experimental findings provide several actionable recommendations for the design of RSTC long-span truss structures:

1. Concrete infill should be specified for truss members where local buckling is a concern, particularly for compression members with high slenderness ratios.
2. Connection design must account for moment transfer at welded joints, using either semi-rigid connection models or moment-resisting connection details.
3. The increased stiffness of RSTC members should be reflected in the structural analysis model, avoiding the pin-jointed idealization that underestimates connection moments.
4. Weld quality verification should be enhanced for RSTC truss connections, as the welds transfer both axial forces and bending moments.
5. Long-term performance should be monitored, as the concrete infill may experience shrinkage and creep that could affect the steel-concrete bond and overall structural behavior.

FMEA Analysis of Structural Risks

Applying the Failure Mode and Effects Analysis framework to the RSTC truss structure:

The highest risk priority corresponds to cantilever tip deflection, highlighting the importance of stiffness design in cantilever truss configurations.

Key Questions and Reflections

The study raises important questions about the long-term durability of RSTC truss members. The concrete infill provides excellent structural performance during the elastic and early plastic stages, but its long-term behavior under cyclic loading (such as wind or seismic actions) is less well understood. Concrete shrinkage, creep, and potential cracking could progressively degrade the composite action between the steel tube and the concrete core, particularly in environments with temperature and humidity variations.

Another consideration is the constructability of RSTC truss members. Pouring concrete into rectangular steel tubes in the field, particularly for long-span trusses erected at height, presents practical challenges including concrete placement, vibration, and curing. The experimental study does not address these construction-phase issues, which can be equally important as the structural performance in determining the overall project success. Engineers should carefully evaluate the constructability of RSTC truss systems before specifying them for practical applications.

Summary and Concluding Remarks

This comparative experimental study provides compelling evidence that concrete infill significantly enhances the structural performance of rectangular steel tube truss members for long-span applications. The improvements in stiffness, deformation control, local buckling resistance, and ultimate load capacity make RSTC trusses a viable and often superior alternative to conventional steel tube trusses. However, the critical finding regarding connection moments demands that engineers abandon the simplified pin-jointed idealization and adopt semi-rigid connection models that accurately capture the moment transfer at welded joints. The practical success of RSTC truss structures depends on careful attention to both structural design and constructability, ensuring that the theoretical benefits demonstrated in laboratory testing are realized in field applications. The research contributes valuable experimental data to the design of composite truss structures and should inform future code provisions for RSTC structural systems.