Axial Compression Performance and Bearing Capacity Calculation of Steel Tube Concrete Constrained Stone Short Columns

Literature Overview

This research investigates the axial compression behavior and develops a bearing capacity calculation method for short columns composed of stone confined within steel tubes, with concrete acting as an intermediate layer or constraint medium. This innovative structural concept leverages the compressive strength of natural stone, the ductility and confinement capacity of steel tubes, and the bond and filling properties of concrete to create a composite column system with enhanced load-bearing capacity. The study addresses the mechanical behavior of this hybrid system under axial compression, including the stress distribution, deformation characteristics, failure modes, and the development of a rational design formula for bearing capacity prediction.

Composite System Configuration and Material Interaction

The steel tube concrete constrained stone column consists of a natural stone core (typically granite, marble, or other high-compressive-strength stone) surrounded by a layer of concrete and enclosed within a steel tube. The steel tube provides lateral confinement to both the concrete layer and the stone core, preventing lateral expansion and enhancing the compressive strength of the inner materials through the Poisson effect. The concrete layer serves multiple functions: it fills the irregular gaps between the stone surface and the steel tube, ensures uniform load transfer, and provides additional confinement through its own compressive strength.

The interaction between the three constituent materials is governed by the compatibility of deformation and the equilibrium of internal forces. Under axial compression, the stone core, being the stiffest material, attracts a disproportionate share of the load. The concrete layer, being more deformable, accommodates the differential strains between the stone and the steel tube. The steel tube, through its hoop stress, provides passive confinement that increases with the axial strain of the inner materials.

Material Property Comparison

The significant disparity in elastic moduli between the stone core and the steel tube creates a complex stress distribution within the composite column. The stone core, with its high stiffness, deforms less under axial load, while the steel tube, being more flexible, undergoes greater radial expansion. The concrete layer mediates this differential deformation, distributing the confinement pressure from the steel tube to the stone core.

Experimental Investigation and Failure Behavior

The experimental program involves fabricating short column specimens with various stone types, concrete grades, and steel tube dimensions. The specimens are subjected to monotonic axial compression loading in a universal testing machine equipped with load cells and displacement transducers. The loading is applied at a controlled strain rate to ensure quasi-static conditions. Full-field strain measurement using digital image correlation (DIC) is often employed to capture the strain distribution on the specimen surface.

The stress-strain behavior of the composite column typically exhibits three distinct phases. In the initial elastic phase, all three materials deform proportionally, and the load-strain relationship is linear. In the transition phase, the concrete layer begins to crack, and the confinement effect becomes more pronounced as the steel tube starts to yield. In the post-peak phase, the stone core may exhibit brittle failure, but the steel tube continues to provide confinement, allowing the column to sustain significant post-peak load.

Bearing Capacity Calculation Method

The bearing capacity calculation method for steel tube concrete constrained stone short columns is developed based on the confinement theory and the composite action principle. The total axial capacity is the sum of the contributions from the stone core, the concrete layer, and the steel tube, with an additional confinement enhancement term. The confinement enhancement is calculated based on the hoop stress in the steel tube, which is determined from the equilibrium of forces in the radial direction.

The calculation method accounts for the non-uniform stress distribution within the cross-section, the differential deformation between materials, and the progressive nature of failure. The method is validated against experimental data and shows good agreement for a wide range of material properties and geometric configurations. The method can be incorporated into structural design software and used for the design of practical steel tube concrete constrained stone columns.

Study Insights and Reflections

The steel tube concrete constrained stone short column represents a creative integration of traditional and modern construction materials, offering a sustainable alternative to conventional reinforced concrete columns. The use of natural stone, which is abundant and requires minimal processing, combined with the efficient confinement provided by steel tubes, creates a column system with high strength and reasonable ductility. The bearing capacity calculation method provides a rational basis for the design of such columns, enabling engineers to exploit the full potential of this composite system. The research also highlights the importance of understanding the interaction between dissimilar materials in composite structures, a principle that applies broadly to the design of hybrid structural systems.