Bearing Capacity Calculation of Steel Tube Confined Reactive Powder Concrete Short Columns

Literature Overview

This paper by Wang Qiuwei and colleagues, published in the Chinese Journal of Applied Mechanics (Volume 37, Issue 4, 2020, pp. 1573-1581), investigates the axial compressive behavior of circular steel tube confined reactive powder concrete (RPC) short columns. Twenty specimens with an outer diameter of 133 mm were tested under two loading configurations: full-section loading and core-concrete-only loading. The study explores the influence of loading method, steel tube thickness-to-diameter ratio, and RPC strength on axial compressive performance, and establishes bearing capacity calculation methods based on the double-shear unified strength theory.

Core Technical Points

The research addresses a critical engineering challenge: RPC achieves ultra-high compressive strength (typically 120-200 MPa or higher) but exhibits severe brittleness, limiting its practical application in structural members. Steel tube confinement provides the necessary lateral restraint to enhance ductility while maintaining high strength. The key findings are summarized below:

Parameter	Test Range	Key Observation
Steel tube thickness	4.5, 6, 8, 10 mm	8 and 10 mm yield stable or rising load-displacement curves
Confinement coefficient	>0.9	Bulging failure mode; below 0.9, shear failure dominates
Load-displacement behavior (4.5-6 mm)	-	Pronounced descending branch observed
Load-displacement behavior (8-10 mm)	-	Stable plateau or continued ascending branch
Bearing capacity enhancement (full-section)	-	12% above simple superposition of steel and concrete
Bearing capacity enhancement (core only)	-	24% above simple superposition

Technical Interpretation from a Welding and Steel Pipe Perspective

From the standpoint of steel pipe manufacturing and welding quality control, several aspects of this study merit careful attention:

Steel Tube Geometry and Confinement Efficiency

The confinement coefficient (defined as the ratio of steel tube cross-sectional area to concrete core area) is the dominant parameter governing failure mode. When this coefficient exceeds 0.9, the steel tube exerts sufficient lateral confining pressure to prevent concrete spalling, resulting in a bulging failure where the steel tube itself undergoes plastic deformation. Below this threshold, the concrete core fails in shear before the steel tube can develop full confining action. This has direct implications for the selection of steel tube wall thickness in composite column design.

Steel Tube Thickness Effects on Post-Yield Behavior

The observation that 4.5 mm and 6 mm wall thickness specimens exhibit a pronounced descending branch, while 8 mm and 10 mm specimens maintain stable or ascending behavior, is particularly relevant to weld quality. Thinner-walled tubes are more susceptible to local buckling at the weld zone, especially if the longitudinal seam weld (if any) or the end connection weld introduces geometric discontinuities. In practice, ERW or HFW welded tubes with thinner walls may exhibit premature local deformation at the seam due to residual stress concentrations, which would exacerbate the descending branch behavior.

Bearing Capacity Calculation Method

The proposed calculation method integrates the double-shear unified strength theory with a mechanistic model of the composite column. The key innovation is distinguishing between two loading scenarios:

Full-section loading: The steel tube and concrete core are loaded simultaneously through end plates, with friction at the interface contributing to load transfer.
Core-only loading: Only the concrete core receives axial load, and the steel tube is engaged through interface friction and dilatancy effects.

The 24% enhancement in the core-only case (versus 12% for full-section) indicates that the steel tube's confining action is more effective when it is not pre-loaded, allowing it to develop larger lateral deformation and thus higher confining pressure.

Process and Standards Analysis

Aspect	Relevance to Steel Pipe Engineering
Steel tube material grade	Typically Q235B or Q345B per GB/T 3091 or GB/T 8163
Weld quality requirement	For HFW tubes, the seam must withstand lateral expansion without cracking
Dimensional tolerance	Wall thickness uniformity directly affects confinement uniformity
End preparation	Bevel quality at column ends is critical for composite action
Concrete placement	Pumping pressure must not deform thin-walled tubes prior to concrete hardening

The paper's findings suggest that for optimal confinement efficiency, steel tube wall thickness should not be less than 8 mm for a 133 mm diameter tube (thickness-to-diameter ratio of approximately 6%). This ratio aligns with the minimum wall thickness requirements in ASME B31.3 for pressure vessels and with the practical limits for HFW welding, where thinner walls are more prone to seam defects.

Engineering Practice Implications

In engineering practice, composite columns using RPC and steel tubes present specific challenges:

Welding of steel tubes to end plates: The high strength of RPC means that the steel tube-to-end-plate connection must be designed to prevent premature weld failure. Full-penetration GTAW or SAW welds with post-weld heat treatment may be required.
Tube fabrication: Seamless tubes are preferred for uniformity, but HFW tubes can be acceptable if the seam quality meets API 5L or GB/T 9711 requirements. The longitudinal seam should be oriented perpendicular to the maximum bending moment plane if the column is subjected to eccentric loading.
Concrete pumping: RPC has low slump (typically 0-30 mm), requiring high-pressure pumping. The steel tube must be designed to resist pumping-induced ovalization, especially for thin-walled tubes.
Quality assurance: Hydrostatic testing per ASTM A530 or GB/T 241 should be performed on steel tubes before concrete placement. The composite column should undergo non-destructive evaluation (UT or MT) of welds prior to concrete pouring.

Key Questions and Reflections

The paper raises an important question: how does the presence of a longitudinal seam weld in HFW tubes affect the confinement behavior? The specimens appear to use seamless tubes, but in practice, HFW tubes are often used for cost reasons. The residual stress at the HFW seam could create a preferential buckling location, potentially reducing the effective confinement efficiency. This warrants further investigation.

Additionally, the study does not address fatigue behavior under cyclic loading. In seismic applications, the steel tube may undergo repeated plastic deformation, and the weld zone (if any) could become a fatigue crack initiation site. The combination of RPC's brittleness and potential weld fatigue could be detrimental.

Study Insights and Conclusions

This research provides a solid theoretical and experimental foundation for the design of steel tube confined RPC columns. The proposed bearing capacity calculation method, with errors below 10%, is suitable for practical engineering applications. From a steel pipe and welding quality perspective, the study reinforces the importance of adequate wall thickness (at least 6% of diameter) and highlights the need for careful consideration of steel tube manufacturing method and weld quality in composite column design. The distinction between full-section and core-only loading provides valuable insight into the load transfer mechanism, which should inform connection design and construction sequencing. Engineers working with ultra-high strength concrete composite members should pay particular attention to the interface behavior and ensure that steel tube dimensions and material properties are selected to achieve the desired confinement coefficient above 0.9.