Flexural Behaviour of High-Strength Steel Tube Confined UHPC Beams

Literature Overview

The paper by Deng Zongcai, Sun Tong, and Li Jiayue (Journal of Tianjin University, 2021, Vol. 54, No. 11, pp. 1111–1120) investigates the flexural mechanical performance of ultra-high performance concrete (UHPC) beams confined by high-strength steel tubes under pure bending loads. Funded by the Beijing Municipal Education Commission Key Science and Technology Project (KZ201810005008), this research was conducted at the Key Laboratory of Urban and Engineering Safety and Disaster Resistance, Ministry of Education, Beijing University of Technology. The study addresses a significant gap in the structural engineering community: while steel tube confined conventional concrete beams have been extensively studied, the combination of high-strength steel tubes with UHPC—a material known for its exceptional compressive strength exceeding 120 MPa and remarkable durability—remains underexplored, particularly regarding flexural behaviour and bending stiffness evolution.

Core Technical Points

The experimental programme comprised nine specimens subjected to two-point bending tests, covering a range of configurations designed to isolate the effects of key design parameters. The specimens included one solid conventional-strength steel tube confined UHPC beam as a reference, three solid high-strength steel tube confined UHPC beams with varying wall thicknesses (4 mm, 6 mm, and 8 mm), one square-in-square hollow sandwich high-strength steel tube UHPC beam, and three square-in-round hollow sandwich high-strength steel tube UHPC beams. This systematic arrangement allows for a clear differentiation between the effects of steel tube wall thickness, steel yield strength, and cross-sectional composite configuration on flexural capacity and stiffness.

Experimental Results and Key Findings

The moment-curvature response of all high-strength steel tube confined UHPC beams exhibited three distinct stages: an elastic stage, an elastic-plastic stage, and a strain-hardening stage. This three-stage behaviour is characteristic of composite members where both the steel tube and the UHPC core contribute progressively to load resistance. Importantly, the beams demonstrated good ductility at failure, and notably, local bulging did not appear in the compression zone of the high-strength steel tube UHPC beams. This observation is significant from a design perspective, as local buckling of the compression flange is often a governing failure mode in conventional steel tube confined concrete members.

The data reveal a clear trend: increasing the steel tube wall thickness from 4 mm to 8 mm results in a 58.5% increase in flexural bearing capacity and a 32.4% increase in service bending stiffness. However, the stiffness improvement is disproportionately smaller than the capacity improvement, indicating that the relationship between steel ratio and serviceability is not linear. This is an important consideration for engineers designing UHPC composite beams where deflection control governs the design.

For the hollow sandwich configurations, the square-in-round arrangement outperformed both the solid and square-in-square configurations. Compared with the solid beam having a 6 mm outer tube, the square-in-round beam achieved a 26.8% increase in flexural capacity and a 3.5% increase in service bending stiffness. Compared with the square-in-square beam having a 6 mm outer tube, the square-in-round beam showed a 10.3% capacity increase and a 4.5% stiffness increase. This superior performance of the square-in-round configuration is attributed to the more efficient stress distribution in the composite cross-section, where the circular inner tube provides more uniform confinement of the UHPC core.

Bending Stiffness Calculation Method

A key contribution of this paper is the development of a bending stiffness calculation formula for the service stage of high-strength steel tube confined UHPC beams. The authors employed the superposition method and incorporated the crack-control capability of steel fibre-reinforced UHPC into the formulation. The rationale is that high-strength micro steel fibres in UHPC provide excellent crack-arresting and crack-controlling capabilities, which result in higher stiffness during the elastic-plastic deformation stage. Furthermore, the rate of elastic modulus degradation of UHPC under loading is lower than that of conventional concrete, meaning the material retains more of its stiffness capacity as microcracks develop.

Interpretation of Technical Points

The absence of local bulging in the compression zone is a critical finding that has direct implications for design codes. In conventional steel tube confined concrete beams, the compression zone often experiences local buckling of the tube wall, which limits the ultimate flexural capacity. The combination of high-strength steel (with yield strengths typically in the range of 450–690 MPa) and UHPC (with compressive strengths exceeding 120 MPa) creates a composite action where the high confinement pressure exerted by the UHPC on the steel tube effectively prevents local buckling. This is consistent with the observation that the confinement effectiveness ratio—the ratio of steel tube confinement pressure to concrete unconfined compressive strength—is significantly higher for UHPC than for conventional concrete.

The three-stage moment-curvature response reflects the progressive yielding and hardening of both the steel tube and the UHPC core. In the elastic stage, both materials behave linearly and the composite section acts with full effectiveness. In the elastic-plastic stage, the outer fibres of the steel tube begin to yield, but the UHPC core continues to provide significant resistance due to its high compressive strength and the crack-controlling effect of steel fibres. In the strain-hardening stage, the high-strength steel tube undergoes significant plastic deformation while the UHPC core continues to contribute through its high compressive strength, resulting in a gradual increase in moment capacity.

The bending stiffness calculation based on the superposition method is particularly valuable because it accounts for the unique properties of UHPC that are not captured by conventional reinforced concrete stiffness models. Traditional stiffness reduction factors used for reinforced concrete beams are not applicable to UHPC composite beams because the steel fibres in UHPC provide a crack-bridging mechanism that maintains higher interfacial shear transfer and reduces the effective crack width. This results in a smaller stiffness reduction compared to conventional concrete.

Integration with Engineering Practice

From a practical engineering perspective, the findings of this study have several important implications for the design and application of steel tube confined UHPC beams in real structures. First, the use of high-strength steel tubes (such as those conforming to ASTM A671 Gr. 50 or higher, or API 5L X65/X70 grades) combined with UHPC can significantly reduce the overall cross-sectional dimensions of beams while maintaining or improving flexural performance. This is particularly advantageous in applications where space is limited, such as bridge decks, high-rise building floor systems, and industrial plant structures.

Second, the square-in-round hollow sandwich configuration offers a promising approach for lightweight construction. By replacing the solid UHPC core with a hollow configuration, the self-weight of the beam is reduced while maintaining structural performance. The 10.3% improvement in flexural capacity over the square-in-square configuration demonstrates that the geometric arrangement of the composite cross-section is a critical design parameter that should be optimised.

Third, the developed bending stiffness calculation formula provides engineers with a practical tool for serviceability limit state design. The ability to predict service-stage bending stiffness with reasonable accuracy is essential for controlling deflections and ensuring long-term durability of UHPC composite beams. The formula should be validated against additional experimental data before being incorporated into design codes.

Key Questions and Reflections

Several questions arise from the findings of this study that warrant further investigation. First, the long-term behaviour of high-strength steel tube confined UHPC beams under sustained loading is not addressed. UHPC is known for its excellent creep and shrinkage properties compared to conventional concrete, but the interaction between UHPC and high-strength steel under long-term loading deserves systematic study. Second, the fire resistance of these composite beams is mentioned as an advantage but not experimentally verified in this paper. Given the high temperature sensitivity of steel, the fire performance of high-strength steel tube confined UHPC beams should be investigated through fire tests and numerical simulation.

Third, the fatigue performance of these beams under cyclic loading is not examined. In applications such as bridges and marine structures, fatigue is a critical design consideration. The interaction between the high-strength steel tube and UHPC under repeated loading may lead to interfacial degradation, and this needs to be studied through fatigue tests. Finally, the cost-effectiveness of using UHPC in steel tube confined beams should be evaluated. While UHPC offers superior mechanical properties, its material cost is significantly higher than conventional concrete. Life-cycle cost analysis should be conducted to determine the economic viability of UHPC composite beams in various applications.

Study Insights and Implications

The research by Deng et al. represents a significant contribution to the understanding of steel tube confined UHPC composite beams. The systematic experimental programme, combined with the development of a practical bending stiffness calculation method, provides valuable guidance for the design of these innovative structural members. The key insight is that the combination of high-strength steel tubes and UHPC creates a synergistic effect where the high confinement pressure prevents local buckling, the steel fibres control crack propagation, and the composite action maximises the utilisation of both materials. This synergy results in beams that are lighter, stronger, and stiffer than conventional reinforced concrete or steel tube confined concrete alternatives.

The findings also highlight the importance of considering material-specific properties in the design of composite members. The bending stiffness calculation method developed in this study demonstrates that conventional design approaches based on reinforced concrete theory are not directly applicable to UHPC composite beams. Engineers must account for the unique crack-control mechanism of steel fibres and the lower rate of elastic modulus degradation of UHPC when designing these members. This paper should serve as a foundation for future research and code development in the field of UHPC composite structures.