Shear Capacity of Steel Tube Lightweight Aggregate Concrete Members

Research Motivation and Scope

This study by Ji Bohai and colleagues from Hohai University investigates the shear behavior of steel tube-confined lightweight aggregate concrete (LWAC) members through an extensive experimental program involving 29 specimens. Lightweight aggregate concrete offers significant weight reduction advantages for structural applications, but its lower density and typically reduced strength compared to normal-weight concrete raise concerns about shear capacity. The steel tube confinement is expected to compensate for these material deficiencies, but the interaction mechanisms require systematic experimental investigation.

Experimental Program and Test Parameters

Specimen Configuration

The 29 specimens were tested under combined axial compression and shear loading, with the following parameters varied systematically:

Failure Mode Classification

The experimental results clearly demonstrate that the failure mode is governed by the shear span ratio:

• λ ≤ 0.85: Shear-type failure (brittle diagonal shear)
• λ > 0.85 with axial load: Flexural-type failure (more ductile)

This threshold value of λ = 0.85 is consistent with the behavior of normal-weight CFST members, suggesting that the lightweight aggregate does not fundamentally alter the failure mode transition mechanism.

Key Findings and Technical Analysis

Influence of Shear Span Ratio

The shear span ratio is the dominant geometric parameter affecting shear capacity. As λ increases, the shear capacity decreases monotonically, which is consistent with the general behavior of reinforced concrete and CFST members. The reduction in capacity is attributed to the transition from shear-dominated to flexure-dominated failure mechanisms. For engineering design, this means that the shear span ratio must be carefully controlled in the layout of steel tube lightweight aggregate concrete columns and beams to avoid brittle shear failure.

Influence of Axial Compression Ratio

The axial compression ratio has a positive effect on both shear capacity and ductility. This is attributed to the confining effect of axial compression on the concrete core, which delays diagonal cracking and enhances the shear friction resistance across the crack surfaces. The steel tube's hoop confinement further amplifies this effect by maintaining the integrity of the concrete core under combined loading.

Influence of Lightweight Aggregate Concrete Strength

A notable finding is that the strength of the lightweight aggregate concrete has a relatively minor influence on the shear capacity. This is attributed to the fact that the steel tube confinement effectively compensates for the lower concrete strength, making the shear capacity more dependent on the steel tube's contribution than on the concrete's compressive strength. This finding has important implications for material selection, as it suggests that lower-strength lightweight concrete can be used without significant penalty to shear capacity, provided the steel tube confinement is adequate.

Influence of Steel Tube Volume Fraction

Higher steel tube volume fractions (achieved through thicker walls or smaller concrete-to-steel ratios) consistently improve shear capacity. This is expected, as the steel tube provides direct shear resistance through its shear capacity and indirect resistance through enhanced concrete confinement.

Shear Capacity Formula

Through regression analysis of the experimental data, the authors derive a shear capacity formula that incorporates the influence of all identified parameters. The formula is validated against the experimental data and found to be conservative (on the safe side), which is appropriate for design applications. The formula structure is consistent with the additive approach commonly used in CFST design codes, where the total shear capacity is the sum of contributions from the concrete core, the steel tube, and the interaction effect between them.

Engineering Practice Considerations

Steel Tube Selection for Lightweight Aggregate Applications

From a steel pipe manufacturing and supply perspective, this research highlights several practical considerations:

• Wall thickness optimization: Since the steel tube volume fraction is a key parameter, engineers should optimize the wall thickness to achieve the required confinement without excessive material usage. For lightweight aggregate concrete with lower compressive strength, thinner steel tubes may suffice compared to normal-weight concrete applications.
• Surface treatment: The bond between the steel tube and lightweight aggregate concrete may differ from that with normal-weight concrete due to the different surface texture and porosity of lightweight aggregates. Internal surface roughening or mechanical keying may be necessary to ensure adequate bond strength.
• Quality control emphasis: Given that the concrete strength has limited influence on shear capacity, the quality control emphasis should be placed on the steel tube properties—particularly the yield strength and wall thickness uniformity—which are the primary determinants of shear capacity.

Study Insights and Reflections

This research contributes valuable experimental data for the design of steel tube lightweight aggregate concrete structures, a topic that has received relatively limited attention compared to normal-weight CFST. The finding that concrete strength has limited influence on shear capacity is particularly significant, as it opens the door to using lower-cost, lower-strength lightweight concrete without compromising shear performance. For steel pipe engineers, this reinforces the principle that in CFST design, the steel tube is not merely a passive container but an active structural component whose properties govern the overall member performance. The experimental methodology—systematically varying multiple parameters across a comprehensive matrix—is exemplary and provides a solid basis for the derived design formula.