Experimental and Simulation Study on Flexural Capacity of Rectangular CFST Beams

Literature Overview

This paper by Liu Xiang and Wu Yongbo (2010), published in Industrial Construction (Vol. 40, No. 7, pp. 91–94), presents a combined experimental and numerical investigation into the flexural behavior of rectangular steel tube concrete (CFST) beam specimens. Six specimens with width-to-thickness ratios exceeding 55 were subjected to pure bending tests, followed by full-process finite element simulation using SAP2000. The research was supported by the Inner Mongolia Autonomous Region Natural Science Foundation (Grant 200711020701) and conducted at the School of Architecture and Civil Engineering, Inner Mongolia University of Science and Technology, in collaboration with Baosteel Group.

Core Technical Findings

The principal conclusions drawn from this study are of considerable practical significance for structural engineers working with composite members in industrial and civil construction.

Flexural Capacity and Ductility Characteristics

The test results demonstrate that rectangular CFST flexural members possess high flexural capacity and satisfactory ductility. A particularly noteworthy observation is that even when the mid-span deflection reaches L/20, the applied external load can still increase. This behavior indicates that the composite action between the steel tube and the concrete core continues to provide reserve strength well beyond conventional serviceability limits, which is highly relevant for seismic design and overload scenarios.

Influence of Geometric Parameters

The study explicitly identifies that the width-to-thickness ratio exerts a more pronounced influence on flexural capacity than the height-to-width ratio. This finding aligns with the fundamental mechanics of thin-walled sections, where local buckling of the flanges (governed by width-to-thickness ratio) becomes the critical failure mode before global flexural yielding is fully mobilized.

Simulation Verification

The SAP2000 finite element model captured the full loading process with good agreement against experimental results. This validation provides confidence that numerical tools can be reliably employed for parametric studies and preliminary design optimization of CFST beams, reducing the need for extensive physical testing.

Engineering Practice Implications

Connection to Steel Pipe and Welding Practice

From a steel pipe fabrication standpoint, the emphasis on width-to-thickness ratio is directly relevant to the selection of rectangular hollow section (RHS) pipe specifications. Engineers must ensure that the selected pipe grade and wall thickness comply with local buckling limits specified in relevant standards such as GB 50017-2017 (Standard for Design of Steel Structures) and EN 1993-1-1. For high-strength steels (Q345, Q390, Q460), the allowable width-to-thickness ratio limits are more restrictive, and this must be considered when specifying seamless or welded rectangular tubes for CFST applications.

Welding Considerations for Rectangular CFST Beams

The fabrication of rectangular CFST beams involves critical welding operations at the tube ends and at connection interfaces. Key welding concerns include:

• End plate welding: Full-penetration groove welds (GTAW + SMAW or SAW) are typically required at beam-to-column or beam-to-beam connections. The weld HAZ in high-strength steel grades (e.g., Q345B, Q390) requires careful preheating and interpass temperature control to avoid cold cracking.
• Residual stress management: The combination of welding residual stresses and the confined concrete state can lead to complex stress redistribution. Pre-welding stress relief treatment (intermediate stress relief at 550–650°C) may be warranted for thick-walled sections.
• Concrete pouring interface: The weld quality at the top opening of the rectangular tube (if used for concrete placement) must be verified by MT or PT to prevent leakage and ensure proper concrete-steel bond.

Standards Reference

Key Questions and Reflections

The study raises several important questions for further investigation. First, the specimens tested had width-to-thickness ratios greater than 55, which places them in the locally yielding or elastically buckling range depending on the steel grade. The transition behavior between local buckling and global flexural failure is not fully explored. Second, the effect of concrete strength grade (C30, C40, C50, C60) on the flexural ductility is not systematically varied, which limits the generalizability of the findings. Third, the SAP2000 model, while validated, relies on simplified constitutive models for the concrete and steel interaction; more advanced shell-element approaches (e.g., in ABAQUS or ANSYS) with explicit concrete-steel bond modeling could provide deeper insight into the failure mechanisms.

Study Insights

The most valuable contribution of this paper is the quantitative demonstration that CFST beams maintain increasing load capacity even at large deflections, which supports their use in ductile structural systems where post-yield energy absorption is critical. For engineers involved in pipe fabrication and welding, the key takeaway is that the geometric proportions of rectangular tubes must be carefully selected to balance flexural capacity against local buckling susceptibility. The width-to-thickness ratio should be treated as the primary design variable, with careful attention to weld quality at tube ends and connection interfaces to ensure that the composite member achieves its full design potential.