Experimental Bending Capacity of Steel Tube Lightweight Aggregate Concrete Space Truss Beams

Literature Overview

This paper by Fu Zhongqiu et al. from Hohai University, published in the Journal of Chang'an University (Natural Science Edition) in 2016, presents a systematic pure bending test on steel tube lightweight aggregate concrete (STLAC) space truss beams. The study aims to establish a design method for the bending capacity of such composite members, which are increasingly used in bridge engineering due to their lightweight characteristics and favorable structural performance. The research was supported by the National Natural Science Foundation of China (Grant No. 51208176) and the Central University Basic Research Fund (Grant No. 2015B17414).

Core Technical Content

The experimental program focused on pure bending loading of STLAC space truss beam specimens, with instrumentation measuring deflection and surface strain on the steel tube. The authors applied the plane section assumption while neglecting the influence of web members to derive a bending capacity calculation method for the mid-span cross-section.

Key Test Findings

Parameter	Finding
Web member force	Relatively small during bending
Chord deformation	Upper and lower chords deform consistently
Strain distribution	Chord longitudinal strain follows plane section assumption
Deflection threshold	At L/50 deflection, no local buckling observed on chord or web surfaces
Failure mode	Ductile failure
Capacity deviation	Test values within 5% of calculated values (serviceability limit state basis)

Engineering Relevance for Steel Pipe Fabrication

From the perspective of steel pipe fabrication and quality control, several observations merit attention:

The steel tube surface remained smooth without buckling up to L/50 deflection, indicating that the tube wall thickness and material grade were adequately specified to prevent local instability during service and ultimate loading.
The ductile failure mode implies that the steel tube material possessed sufficient elongation and strain-hardening capacity, which is critical for seismic and overload scenarios in bridge applications.
The plane section assumption validity suggests that the bond between the steel tube and the lightweight aggregate concrete core was adequate, meaning no significant slippage or debonding occurred during loading.

Technical Points and Process Implications

For pipe fabricators supplying steel tubes for such composite truss beams, the following quality considerations arise:

Wall thickness uniformity: Since local buckling was not observed, the tube wall thickness must be controlled within tight tolerances. Deviations in wall thickness due to rolling or forming defects could compromise the buckling resistance.
Surface quality: The absence of surface imperfections that initiated buckling suggests that internal and external surface defects (such as seam welds, scale, or dents) must be controlled per applicable standards (e.g., GB/T 8163, ASTM A500).
Material grade selection: The steel tube must exhibit sufficient yield strength and ductility to participate effectively in the composite action while allowing the concrete core to contribute to bending resistance.

Study Insights and Reflections

This study provides valuable empirical support for the design of lightweight composite bridge members. The finding that web members carry relatively small forces during pure bending simplifies the design approach, allowing engineers to focus on chord member capacity. However, in practical engineering, the loading conditions are rarely pure bending; combined bending and shear must be considered, and the web members may experience significant shear forces. The 5% deviation between test and calculated values is acceptable for design purposes but highlights the need for continued refinement of composite member design formulas, particularly for non-standard cross-sections and connection details.

For steel pipe manufacturers, the study reinforces the importance of producing tubes with consistent mechanical properties and dimensional accuracy, as these directly influence the composite member's structural performance and ultimately the bridge's service life.