Axial Compression Performance of Square Steel Tube Concrete Columns with Internal Circular Tubes at High Temperature

Literature Overview

This paper by Wang Weihua, Zhang Wei, Bai Yang, and Tan Qinghua, published in the Engineering Mechanics Journal in 2018 (Vol. 35, No. 3, pp. 141-150), investigates the axial compression mechanical properties of square steel tube concrete (STC) columns with internal circular tubes at high temperatures. The study develops a nonlinear finite element model incorporating modified high-temperature constitutive relationships for the concrete core, validates the model against experimental data, and conducts parametric analysis on fire resistance performance.

Core Technical Content

The proposed composite column combines a square outer steel tube with an internal circular steel tube, both enclosing a concrete core. This hybrid configuration leverages the high axial load capacity of the square outer tube, the efficient confinement of the circular inner tube, and the composite action of the concrete core. The primary objective is to evaluate whether this composite configuration can meet fire resistance requirements without external fire protection, which would reduce construction cost and complexity.

High-Temperature Material Behavior

The constitutive model for the concrete core was modified to account for high-temperature effects. At elevated temperatures, concrete undergoes significant degradation in strength and stiffness. The thermal expansion mismatch between steel and concrete also introduces thermal stresses that can cause cracking and spalling. The modified constitutive model incorporates temperature-dependent compressive strength, elastic modulus, and Poisson's ratio for the concrete.

The steel tubes also experience property degradation at high temperatures. The yield strength and elastic modulus of structural steel decrease significantly above approximately 400°C, with rapid degradation above 600°C. The thermal expansion of steel causes the tubes to expand, which can either enhance or reduce the confinement effect on the concrete depending on the temperature and the relative expansion rates.

Finite Element Model and Validation

A nonlinear finite element model was established in a finite element software package, incorporating material nonlinearity, geometric nonlinearity, and contact nonlinearity at the interfaces between the outer square tube, inner circular tube, and concrete core. The model was validated against experimental test data, confirming its accuracy in predicting the load-displacement response, failure modes, and internal force distribution of the composite column.

Parametric Analysis Results

The parametric analysis investigated the influence of several key parameters on the fire resistance performance:

The non-monotonic relationship between the inner tube diameter ratio and fire resistance is particularly interesting. At small ratios, the inner tube provides limited confinement benefit. As the ratio increases, the confinement effect improves, enhancing fire resistance. However, beyond an optimal ratio, the increased inner tube diameter reduces the concrete volume and may alter the heat transfer characteristics, leading to decreased fire resistance. The optimal ratio represents a balance between confinement benefit and thermal performance.

Engineering Practice Implications

The finding that the composite column can achieve fire resistance requirements without external fire protection is practically significant. In conventional design, steel tube concrete columns require fire protection coatings or encasement to meet fire resistance criteria, which adds cost and construction complexity. If the composite configuration can meet fire resistance requirements through its inherent thermal and structural properties, this simplifies the design and reduces the overall cost.

From a fabrication perspective, the assembly of the internal circular tube within the square outer tube requires precise alignment and secure attachment. The inner tube must be positioned concentrically within the outer tube to ensure uniform concrete filling and consistent confinement. Welding or mechanical fastening of the inner tube to the outer tube is necessary to prevent relative movement during concrete placement and service loading. The welding details must be designed to accommodate thermal expansion during fire exposure without compromising structural integrity.

The concrete core properties are critical for the composite action. The concrete mix design should be optimized for both room-temperature structural performance and high-temperature durability. High-performance concrete or fiber-reinforced concrete may be beneficial for enhancing the high-temperature performance of the core.

Study Insights and Reflections

This research contributes valuable insight into the fire resistance design of composite steel tube concrete columns. The finding that fire resistance can be more than doubled compared to conventional STC columns is a substantial improvement that could have significant implications for design practice. The parametric identification of the optimal inner tube diameter ratio provides actionable guidance for engineers designing these columns.

However, I note that the study focuses on axial compression loading, which is the primary load case for columns. In practice, columns are often subjected to combined axial and bending loads, particularly in seismic regions. The interaction between axial compression and bending at high temperatures may differ from the pure compression case, and further investigation is warranted. Additionally, the study does not address the behavior of the column under cyclic loading at high temperatures, which could be relevant for structures subjected to both fire and seismic events.

From a welding quality perspective, the connections between the inner and outer tubes must be designed and fabricated with attention to thermal cycling. During a fire, the steel tubes expand, and the welds connecting the inner and outer tubes experience thermal stresses. If these welds are brittle or poorly designed, they may crack under thermal stress, compromising the composite action and reducing the fire resistance. Weld procedures should be qualified for the specific steel grades used, and weld metal properties should be verified for adequate ductility at elevated temperatures.

The practical significance of this research is enhanced by the potential for simplified fire protection design. If composite columns can meet fire resistance requirements without external protection, this reduces the need for fire-resistant coatings, intumescent paints, or concrete encasement, all of which add cost and construction time. This is particularly relevant for large-scale infrastructure projects where the cumulative cost of fire protection can be substantial.