Axial Compression Bearing Capacity of Square Steel Tube Confined Ultra-High-Strength Concrete Short Columns

Literature Overview

This study by Shi Bochao (Journal of Highway and Transportation Research and Development, 2024, Vol. 41, No. 9, pp. 180-189), supported by the Fujian Provincial Science and Technology Department Industry-University-Research Cooperation Project (Grant 2022H6009), investigates the axial compression behavior and ultimate bearing capacity of square steel tube confined ultra-high-strength concrete (UHSC) short columns. The research combines experimental testing with finite element analysis, examining the influence of concrete compressive strength, steel yield strength, and steel ratio on structural performance, and proposes an improved calculation method for ultimate bearing capacity.

Research Significance and Technical Context

The application of ultra-high-strength concrete (UHSC) in steel tube confined columns represents a frontier in composite structural engineering. UHSC, typically defined as concrete with compressive strength exceeding 100 MPa, offers significant advantages in terms of strength, durability, and dimensional efficiency. However, the confinement behavior of UHSC differs fundamentally from that of ordinary concrete due to its lower compressive-to-tensile strength ratio and reduced plastic deformation capacity. This study addresses the critical question of how square steel tubes confine UHSC and how this differs from the well-established behavior of steel tube confined ordinary concrete.

Test Parameters

Parameter	Values Examined	Description
Concrete compressive strength	Multiple grades (UHSC range)	Core material strength
Steel yield strength	Multiple grades	Confinement material strength
Steel ratio	Variable	Cross-sectional steel area ratio
Column aspect ratio	Short column	Axial compression dominant

Key Findings and Technical Analysis

Confinement Coefficient and Failure Mode

The study establishes that the confinement coefficient (defined as the ratio of steel tube confinement force to concrete strength) is the primary factor governing the failure mode of square steel tube confined UHSC short columns:

When the confinement coefficient is less than 0.83, the failure mode is characterized as shear failure, where diagonal cracking and shear deformation dominate the failure process
When the confinement coefficient is greater than or equal to 0.83, the failure mode transitions to crushing failure, where the confined concrete is crushed under the combined action of axial compression and lateral confinement

This threshold value of 0.83 represents a critical design parameter that should be explicitly considered in the design of UHSC confined columns to ensure predictable failure behavior and adequate ductility.

Confinement Effectiveness and Load-Displacement Behavior

The load-displacement and load-strain curves reveal important characteristics of the confinement behavior:

Pre-ultimate stage: The lateral confinement capacity of the steel tube is not fully utilized before the ultimate load is reached. The confinement effect primarily manifests after the ultimate load, during the post-peak softening phase.
Post-ultimate behavior: Increasing the steel ratio effectively improves the post-failure compressive ductility of the column, enabling continued load-carrying capacity after the initial concrete crushing.
Low steel ratio behavior: Specimens with the minimum steel ratio exhibit sudden load drops after peak load, indicating brittle failure characteristics that are undesirable in structural applications.

Material Interaction Differences

A critical finding of this study is that the interaction between square steel tubes and UHSC differs significantly from the interaction between steel tubes and ordinary concrete:

UHSC exhibits a lower compressive-to-tensile strength ratio compared to ordinary concrete, resulting in less plastic development before approaching the ultimate load
The reduced plastic deformation capacity of UHSC decreases the mutual interaction between the steel tube and the core concrete
Consequently, the ultimate load improvement achieved by square steel tube confinement of UHSC is smaller than that achieved when confining ordinary concrete

This finding has significant implications for the design of UHSC confined columns, as the confinement benefit is less pronounced than might be expected based on experience with ordinary concrete applications.

Bearing Capacity Calculation Method

The study proposes an improved calculation method for the ultimate axial compression bearing capacity of square steel tube confined UHSC short columns. The method introduces a confinement reduction coefficient to account for the reduced confinement effectiveness observed in UHSC applications.

Comparison of Calculation Methods

Calculation Method	Applicable Range	Accuracy for UHSC Columns
Mander model	Confinement coefficient < 0.3	Significant overestimation for higher coefficients
Chinese standard method	General confinement range	Better accuracy, smaller errors
Proposed method (with reduction coefficient)	Full confinement coefficient range	Within 10% deviation from test results

The study demonstrates that both the Mander model and the Chinese standard calculation method fail to accurately predict the bearing capacity of UHSC confined columns when the confinement coefficient exceeds 0.3. The Mander model, in particular, significantly overestimates the bearing capacity in this range. The Chinese standard method provides better predictions with smaller errors, but still requires modification for UHSC applications.

The proposed method introduces a confinement reduction coefficient that accounts for the reduced plastic interaction between the steel tube and UHSC core. With this modification, the calculation deviation can be controlled within 10% of the experimental and finite element results, providing an acceptable level of accuracy for engineering design purposes.

Steel Pipe Engineering Perspective

The application of square steel tubes in UHSC confined columns imposes specific requirements on steel pipe selection and fabrication:

Material grade selection: The steel yield strength must be matched to the concrete compressive strength to achieve the target confinement coefficient. For UHSC with compressive strength exceeding 100 MPa, higher grade steels such as Q390 or Q420 may be required to achieve adequate confinement.
Wall thickness optimization: The wall thickness must be sufficient to provide the required confinement force without causing local buckling. The confinement coefficient threshold of 0.83 provides a design target for wall thickness selection.
Fabrication quality: Square steel tubes for UHSC applications require high dimensional accuracy and weld quality, as the high concrete strength amplifies the effects of any geometric imperfections or weld defects.
Surface preparation: The interior surface of the steel tube should be prepared to ensure adequate bonding with the UHSC, which may require mechanical roughening or chemical treatment due to the low permeability of UHSC.

Design Recommendations

Based on the findings of this study, the following design recommendations are proposed for square steel tube confined UHSC short columns:

Design the confinement coefficient to exceed 0.83 to ensure crushing failure mode rather than shear failure
Use the proposed calculation method with the confinement reduction coefficient for accurate bearing capacity prediction
Select steel ratio values that ensure adequate post-ultimate ductility, avoiding the brittle failure behavior observed at minimum steel ratios
Match the steel yield strength to the concrete compressive strength to achieve optimal confinement effectiveness
Consider the reduced confinement benefit of UHSC compared to ordinary concrete when evaluating the structural efficiency of the composite system

Study Insights and Implications

This research makes a significant contribution to the understanding of steel tube confined UHSC behavior and provides practical tools for the design of such structural elements. The identification of the confinement coefficient threshold of 0.83 as the transition between shear and crushing failure modes provides a clear design criterion that can be directly applied in engineering practice. The demonstration that conventional calculation methods (Mander model and Chinese standard) are inadequate for UHSC applications, particularly at higher confinement coefficients, highlights the need for specialized design approaches for ultra-high-strength concrete composite systems. The proposed calculation method with the confinement reduction coefficient offers a practical solution that achieves acceptable accuracy within 10% deviation. For steel pipe manufacturers, this research validates the application of square steel tubes in UHSC confined columns and provides specific material and dimensional requirements that can inform product development for this specialized application. The finding that the confinement benefit of UHSC is less than that of ordinary concrete should be carefully considered when evaluating the economic and structural advantages of using UHSC in composite column applications.