Axial Compressive Capacity of Square Rectangular Steel Tube Confined Ultra-High Strength Concrete Short Columns

Literature Overview

This study by Wei Jiangang, Zhang Hanwen, Luo Xia, and Yang Yan (2022), published in Progress in Steel Building Structures (Vol. 24, No. 7, pp. 75–83), investigates the axial compressive behavior of square and rectangular steel tube confined ultra-high strength concrete (UHSC) short columns. The research was funded by the National Natural Science Foundation of China (No. 51878172) and Fujian Provincial Industry-Academia Cooperation Project (No. 2019H6020). The paper is classified under TU398 (steel-concrete composite structures).

Core Technical Content

Ultra-high strength concrete (UHSC), with compressive strengths typically exceeding 100 MPa, offers significant advantages in reducing structural member sizes and improving load-bearing capacity. When confined by square or rectangular steel tubes, the concrete is subjected to lateral restraint, which enhances its ductility and compressive capacity. However, the interaction between the steel tube confinement and the UHSC is complex, and existing design formulas may not adequately predict the behavior of such members when material strengths exceed the ranges covered by conventional research.

The authors used a validated finite element model to conduct parametric analyses on the ultimate axial compressive capacity of square and rectangular steel tube confined UHSC short columns. The parameters studied included concrete strength, steel ratio, and the aspect ratio of the rectangular section. The results were used to evaluate the accuracy of existing capacity calculation methods and to propose a modified formula.

Parametric Analysis Variables

Parameter	Range Studied	Effect on Capacity
Concrete compressive strength	100–150 MPa (UHSC)	Higher strength reduces confinement effectiveness
Steel ratio (A_s/A_c)	5%–20%	Higher ratio increases confinement and capacity
Aspect ratio (b/h)	1.0 (square) to 2.0 (rectangular)	Higher ratio reduces confinement effectiveness
Corner radius ratio	0.1–0.5 (of short side)	Moderate effect on confinement

Key Findings and Conclusions

Confinement effectiveness decreases with increasing concrete strength: At the same confining pressure coefficient, the confinement effect on UHSC is less pronounced than on conventional strength concrete. This is because UHSC has a lower Poisson's ratio and exhibits more brittle behavior, resulting in less lateral expansion and therefore less confining pressure developed by the steel tube.
Steel ratio and corner radius have limited influence on UHSC confinement: For members filled with ultra-high strength concrete, the confinement effectiveness is relatively insensitive to changes in steel ratio and corner radius ratio, in contrast to conventional strength concrete where these parameters have a more significant effect.
Rectangular section aspect ratio reduces confinement: As the ratio of the long side to the short side increases, the confinement effectiveness decreases. This is because the flat sides of the rectangular tube provide less effective confinement than the curved surfaces of a circular tube, and the corners of a highly rectangular section may experience localized buckling.
Existing calculation methods are inadequate for UHSC: When material strengths exceed the ranges covered by existing formulas (typically developed for conventional strength concrete), the predicted capacities deviate significantly from the actual behavior. The proposed modified formula provides improved predictions for UHSC-confined members.

Modified Capacity Formula

The authors proposed a modification to the existing capacity calculation method to account for the reduced confinement effectiveness at high concrete strengths. The modification adjusts the confinement pressure coefficient to reflect the lower lateral expansion of UHSC, thereby providing more accurate predictions of the ultimate axial load.

Method	Applicable Concrete Strength	Accuracy for UHSC
Existing formula (e.g., Mander, Park)	20–80 MPa	Poor for >100 MPa
Proposed modified formula	100–150 MPa	Good agreement with FE results
Finite element analysis	All ranges	Benchmark (validated model)

Engineering Practice Implications

For engineers designing UHSC-confined columns, the conventional design formulas should not be used without modification, as they may overestimate the confinement benefit and lead to unsafe designs.
The modified formula proposed in this study should be considered for preliminary design, but finite element analysis is recommended for critical members.
Square sections are preferred over highly rectangular sections for UHSC-confined columns because they provide more uniform confinement.
Increasing the steel ratio beyond a certain threshold may not significantly improve the capacity of UHSC-confined columns, and economic optimization should be considered.
The brittle nature of UHSC means that ductility requirements must be carefully evaluated, particularly for seismic applications.

Study Insights

This research addresses an important gap in the design of steel-concrete composite members using ultra-high strength concrete. The finding that confinement effectiveness decreases with increasing concrete strength is counterintuitive but physically well-explained by the reduced Poisson's ratio and lateral expansion of UHSC. The proposed modified formula provides a practical tool for engineers, but further experimental validation is needed, particularly for members subjected to cyclic loading. The parametric study also highlights the importance of considering the geometric effects of rectangular sections, which are often overlooked in design practice.