Comparative Shear Performance of Concrete-Filled High-Strength Square Steel Tubes with Different Concrete Strengths

Literature Overview

This study conducts a comparative analysis of the shear performance of concrete-filled square steel tubes (CFST) constructed with high-strength steel tubes and filled with concrete of varying compressive strengths. The research is particularly relevant to modern high-rise and long-span structural applications where material efficiency and structural compactness are critical design objectives. The work provides empirical and analytical data to guide the selection of concrete strength grades for shear-critical CFST members.

Core Technical Content

The investigation examines how the compressive strength of the infill concrete (ranging from ordinary strength to ultra-high-strength grades) influences the shear capacity, shear ductility, and failure mode of CFST members made with high-strength steel tubes. The study considers the interaction between the steel tube confinement effect and the concrete's compressive and tensile behavior under shear loading.

Specimen Configuration and Test Parameters

Parameter	Specification
Steel tube grade	Q345, Q390, Q420, Q460
Concrete strength grades	C30, C40, C50, C60, C80
Square tube dimensions	100×100, 150×150, 200×200 mm
Wall thickness	4–8 mm
Shear span-to-depth ratio (a/h)	1.0, 2.0, 3.0
Loading method	Pure shear via four-point bending or direct shear

Shear Capacity Analysis

The shear capacity of CFST members is governed by the combined contribution of the steel tube web, the concrete core in compression, and the shear friction along the interface between the steel and concrete. The study reveals several important trends:

Non-linear contribution of concrete strength: While increasing the concrete compressive strength from C30 to C60 significantly improves the shear capacity (by approximately 15–20%), further increases to C80 yield diminishing returns (only 3–5% additional improvement). This is because the shear capacity is not solely dependent on concrete compressive strength but also on the tensile strength and fracture energy of the concrete, which do not scale proportionally with compressive strength.
Steel tube confinement effect: The high-strength steel tube provides lateral confinement to the concrete core, which enhances the concrete's effective compressive strength under shear. The confinement pressure is approximately proportional to the steel tube yield strength and inversely proportional to the tube width.
Shear span effect: At low shear span ratios (a/h ≤ 1.5), the shear capacity is dominated by the steel tube web yielding and concrete strut action. At higher shear span ratios (a/h ≥ 2.5), the concrete contribution becomes more significant, and the steel tube primarily provides shear reinforcement through web shear resistance.

Shear Capacity Comparison

Concrete Grade	Shear Capacity (kN)	Improvement over C30	Failure Mode
C30	Baseline	—	Steel tube web yielding
C40	+8–12%	Moderate	Combined steel-concrete failure
C50	+15–18%	Significant	Concrete strut crushing
C60	+18–22%	Significant	Concrete strut crushing
C80	+20–25%	Marginal over C60	Concrete strut crushing

Ductility and Failure Mode Analysis

A critical finding of this study is that higher concrete strength does not necessarily improve shear ductility. In fact, the opposite trend is observed:

Low-strength concrete (C30–C40): The failure is characterized by steel tube web yielding followed by gradual concrete crushing. The hysteretic loops are full and stable, indicating good ductility. The displacement ductility factor ranges from 3.5 to 5.0.
Medium-strength concrete (C50–C60): The failure mode transitions to concrete strut crushing with partial steel tube web yielding. The ductility is moderate, with displacement ductility factors of 2.5 to 3.5.
High-strength concrete (C80): The failure is brittle, dominated by sudden concrete strut crushing with limited steel tube yielding. The displacement ductility factor drops to 1.5–2.0, which is unacceptable for seismic applications.

This observation has significant implications for the design of CFST members in seismic zones. While high-strength concrete may appear attractive for capacity reasons, the associated reduction in ductility can compromise the overall seismic performance of the structure.

Engineering Practice Recommendations

Based on the study findings, the following recommendations are proposed for practical engineering applications:

For non-seismic applications: High-strength concrete (C60–C80) can be used to maximize shear capacity in space-constrained situations, provided that the design accounts for the reduced ductility.
For seismic applications: Concrete grades of C40–C50 are recommended as they provide an optimal balance between shear capacity and ductility. The steel tube should be designed to yield before the concrete crushes, ensuring a ductile failure mode.
For shear-critical members: The shear span ratio should be kept below 2.0 to ensure that the steel tube web contributes significantly to the shear resistance and to prevent brittle concrete strut failure.
Interface treatment: The bond strength between the steel tube and concrete should be enhanced through surface roughening or mechanical interlock provisions, as the interface shear friction is a significant contributor to the overall shear capacity.

FMEA Analysis of Shear Failure

Failure Mode	Severity	Occurrence	Detection	Risk Priority	Mitigation
Steel tube web yielding	Medium	Low	High	Low	Adequate wall thickness; steel tube yielding is ductile
Concrete strut crushing	High	Medium	Low	High	Limit concrete strength; ensure steel tube confinement
Interface debonding	Medium	Medium	Low	Medium	Surface treatment; mechanical interlock
Shear buckling of steel tube	High	Low	Medium	Medium	Limit slenderness ratio; add stiffeners

Study Insights and Reflections

This study challenges the common assumption that higher material strength always leads to better structural performance. The finding that ultra-high-strength concrete (C80) provides only marginal shear capacity improvement over C60 while significantly reducing ductility is particularly noteworthy. From a life-cycle cost perspective, the use of C80 concrete may not be justified when C50 or C60 can achieve comparable capacity with superior ductility at lower material cost.

Another important insight is the role of the steel tube confinement effect. The study demonstrates that the confinement pressure provided by the steel tube is more effective at moderate concrete strengths (C40–C60) than at very high strengths (C80). This is because the confinement pressure is limited by the steel tube's yield strength, and at very high concrete strengths, the concrete's inherent strength exceeds the confinement enhancement, rendering the confinement effect less significant.

The study also highlights the importance of the shear span ratio in determining the failure mode. Engineers should carefully evaluate the shear span ratio during design and avoid configurations where a/h exceeds 2.5, as these are prone to brittle shear failure regardless of the concrete strength used.

Summary

This comparative analysis provides clear guidance on the selection of concrete strength grades for concrete-filled high-strength square steel tubes subjected to shear loading. The study demonstrates that C40–C50 concrete offers the optimal balance between shear capacity and ductility for most practical applications, while ultra-high-strength concrete (C80) should be used with caution due to its brittle failure characteristics. The findings underscore the importance of ductility in structural design, particularly for seismic applications, and suggest that material strength should be selected based on overall structural performance rather than capacity alone. Engineers should integrate these findings into their design workflows, paying particular attention to the interaction between steel tube confinement, concrete strength, and shear span ratio.