Comparative Analysis of Rectangular and Circular Steel Tube Concrete Trusses in Bridge Engineering

Literature Overview

The paper by Lin Hongyu, Dong Lining, and Gao Yingshe from Xi'an University of Science and Technology, published in the journal "Concrete" (2008, Vol. 8, pp. 24-27), presents a nonlinear finite element study comparing the load-bearing performance of rectangular steel tube concrete (STC) trusses against their circular counterparts. Funded by the National Western Transportation Construction Science and Technology Program and the Ministry of Transport Applied Basic Research Project, this work addresses a practically significant question in bridge engineering: whether the manufacturing and erection convenience of rectangular sections can compensate for their relatively lower load capacity compared to circular sections.

Core Technical Points

The authors employed nonlinear finite element analysis to model both rectangular and circular STC truss systems under realistic loading conditions. The key finding is that rectangular STC trusses exhibit lower ultimate load capacity than circular STC trusses, yet demonstrate comparable or even superior plasticity and ductility characteristics. This observation is consistent with the well-known geometric efficiency of circular cross-sections in confining concrete, where the hoop stress distribution is more uniform in circular tubes compared to rectangular ones.

From a materials and manufacturing perspective, this distinction carries significant implications. Circular steel tubes—whether seamless, ERW, or HFW welded—are produced to high dimensional accuracy and exhibit excellent hydrostatic pressure resistance and uniform wall thickness. Rectangular tubes, typically formed by roll-forming or welding of flat strips, introduce additional complexity at the corners and weld seams, which become critical stress concentration zones.

Comparison of Structural Performance

Welding and Fabrication Considerations

From my experience in pipe fabrication and welding quality control, the rectangular tube presents specific challenges that must be carefully managed. The corner regions of rectangular tubes typically involve either continuous weld seams (in welded rectangular hollow sections) or cold-bent corners (in formed sections). In welded rectangular tubes conforming to standards such as EN 10219 or GB/T 6728, the longitudinal welds at the corners are susceptible to incomplete fusion, lack of penetration, and residual stress accumulation. These defects can significantly reduce the effective load-bearing capacity, particularly under cyclic or impact loading as encountered in bridge applications.

The circular tube, by contrast, benefits from the inherent symmetry of the cross-section. Whether manufactured as seamless pipe (ASTM A519, EN 10210) or as welded pipe (ERW or HFW), the weld seam geometry is simpler and more amenable to consistent non-destructive testing protocols such as eddy current testing or ultrasonic examination.

Integration with Engineering Practice

In practical bridge engineering, the choice between rectangular and circular STC trusses involves a multi-criteria decision process. For large-span truss bridges where transportation logistics, erection sequence, and connection detailing dominate the cost structure, rectangular sections offer tangible advantages. The flat faces of rectangular tubes facilitate bolted connection plates and gusset fabrication, reducing field welding requirements and improving constructability.

However, engineers must ensure that the weld quality of rectangular tube connections meets stringent criteria. According to ASME B31.3 and relevant GB/T standards, fillet welds at tube-to-gusset intersections in rectangular trusses should be inspected using magnetic particle testing (MT) or penetrant testing (PT), with acceptance criteria aligned to AWS D1.1 or ISO 5817 Grade B or better. The thicker plate sections typical of rectangular tubes also demand preheating considerations for high-strength steels (Q345, Q390, or higher) to prevent cold cracking in the heat-affected zone.

Study Insights and Implications

This research reinforces a fundamental principle in structural engineering: the optimal cross-sectional geometry is not solely determined by material efficiency but must also account for fabrication feasibility, connection detailing, and constructability. The rectangular STC truss, despite its theoretical load capacity disadvantage, achieves overall engineering superiority through practical advantages in production and assembly. For engineers involved in pipe and fitting supply for such structures, this means that rectangular hollow sections require particular attention to corner weld integrity, dimensional tolerances, and surface quality to ensure that the theoretical benefits predicted by finite element models are realized in actual construction.

The study also highlights the importance of nonlinear analysis in capturing the true behavior of STC members, where concrete confinement effects, steel tube yielding, and composite action interact in complex ways. Future work should investigate the long-term performance of rectangular STC trusses under fatigue and corrosion conditions, particularly in marine or aggressive environments where the corner welds may become initiation sites for localized corrosion.