Fire Resistance of Hollow Steel Tube Concrete Composite Long Columns Under Axial Compression and Three-Side Fire Exposure

Literature Overview

This paper by Zhang Bo, Xu Guangpeng, Ren Qingxin, and Qin Xiaoxiao, published in the Journal of Shenyang Jianzhu University (Natural Science Edition) (2023, Vol. 39, No. 4, pp. 615-623), investigates the fire resistance performance of hollow steel tube concrete (HSTC) composite long columns subjected to axial compression and three-side fire exposure. Supported by the National Natural Science Foundation of China (Grant 51808351) and the Shenyang Science and Technology Program (Grant 21-108-9-34), the study employs finite element analysis using ABAQUS to establish coupled thermal-mechanical models and evaluate the influence of geometric and loading parameters on fire resistance limits. The authors are affiliated with Shenyang Jianzhu University, Liaoning Qingchuang Gaoke Building Industrialization Consulting Co., Ltd., and Foshan University of Science and Technology.

Thermal-Mechanical Modeling Approach

The finite element model incorporates a coupled temperature-stress analysis that captures the following physical phenomena:

1. Heat transfer through the hollow steel tube wall, concrete infill, and air cavity within the hollow section.
2. Temperature-dependent material properties for steel, concrete, and reinforcing steel.
3. Thermal expansion and thermal stress development in all structural components.
4. Internal force redistribution as material properties degrade with increasing temperature.
5. Geometric nonlinearity arising from column deflection under axial load and thermal gradients.

The three-side fire exposure condition represents a realistic fire scenario for columns located at building edges or in open-plan layouts, where fire can impinge on three faces while the fourth face remains relatively unexposed. This asymmetric heating creates complex thermal gradients through the cross-section, leading to eccentric thermal expansion and additional bending moments superimposed on the axial load.

Key Results and Parameter Influence

The study systematically varies four parameters to assess their influence on the fire resistance limit:

The most striking finding is the dramatic increase in fire resistance with increasing section side length. A tripling of the side length from 300 mm to 900 mm results in a 221.81% increase in fire resistance limit. This outcome reflects the improved thermal mass and the increased distance between the fire-exposed surface and the neutral axis of the cross-section, which reduces the thermal gradient-induced bending moment. The hollow ratio effect is particularly relevant because the air cavity within the hollow section acts as a thermal insulator, reducing heat transfer to the inner steel tube wall and the concrete infill. However, excessive hollow ratios compromise the structural capacity of the cross-section, creating a trade-off between thermal insulation and mechanical strength.

The slenderness ratio effect is significant because long columns are susceptible to buckling under combined thermal and mechanical loading. As the slenderness ratio increases, the critical buckling load decreases, and the column becomes more sensitive to the additional bending moments induced by asymmetric heating. The load ratio effect is similarly pronounced because higher axial loads reduce the margin between the applied load and the residual load capacity of the heated column.

Internal Force Redistribution and Failure Mechanism

A key observation from the finite element analysis is the occurrence of internal force redistribution during fire exposure. As the steel tube and concrete degrade at different rates, the load initially carried by the stronger material transfers to the weaker material, following the compatibility of deformations. This redistribution follows a progressive pattern:

1. At early fire stages, the steel tube carries the majority of the axial load due to its higher stiffness relative to concrete.
2. As temperatures increase, the steel tube modulus degrades more rapidly than concrete, causing progressive load transfer to the concrete infill.
3. At advanced fire stages, both materials have significantly degraded, and the hollow section geometry becomes critical in maintaining structural stability.
4. Failure occurs when the combined residual capacity of the heated cross-section can no longer sustain the applied axial load and thermal bending moment.

The three-side fire condition creates an asymmetric temperature distribution that induces a bending moment perpendicular to the unheated face. This thermal moment adds to the mechanical moment from any initial eccentricity, accelerating the failure process. The hollow section, while providing thermal insulation benefits, also reduces the section modulus relative to a solid section, making the column more susceptible to thermal bending.

Comparison with Hollow Reinforced Concrete Columns

The paper concludes that the fire resistance of hollow steel tube concrete composite columns is superior to that of hollow reinforced concrete columns. This advantage stems from the steel tube's ability to confine the concrete infill, preventing spalling and maintaining cross-sectional integrity even at elevated temperatures. The steel tube also provides continuous structural continuity around the perimeter of the column, which is not achieved by discrete reinforcing bars in conventional reinforced concrete columns. From a steel pipe manufacturing perspective, the seamless steel tubes used in these composite columns must maintain their mechanical properties at elevated temperatures, which requires careful selection of steel grades with favorable high-temperature strength retention.

Engineering Practice and Simplified Calculation Formula

The paper proposes a simplified calculation formula for the fire resistance limit of hollow steel tube concrete composite long columns. The formula incorporates the effects of section geometry, hollow ratio, load ratio, and slenderness ratio in a closed-form expression that can be applied during preliminary design stages without the need for full finite element analysis. The good agreement between the formula predictions and finite element results validates its applicability for engineering practice.

For quality control purposes, the seamless steel tubes used in these composite columns should be inspected for dimensional accuracy, wall thickness uniformity, and surface integrity using ultrasonic testing or eddy current testing. The welding of end plates or connection plates to the steel tubes should follow qualified welding procedures with appropriate heat input control to minimize the extent of the heat-affected zone. The concrete infill should be placed and compacted with sufficient care to ensure full filling of the hollow section and adequate bond development between the concrete and the steel tube inner wall.

Study Insights and Reflections

This research provides valuable insights into the fire behavior of a relatively novel structural system that combines the advantages of steel tube confinement with the thermal insulation benefits of a hollow section. The dramatic sensitivity of fire resistance to section size, slenderness ratio, and load ratio underscores the importance of careful geometric and loading optimization in the design of fire-resistant composite columns. The simplified formula offers a practical tool for engineers to rapidly assess design alternatives and identify critical design parameters that warrant further investigation.

The internal force redistribution phenomenon highlights the complexity of fire resistance design for composite structures. The assumption of proportional load sharing between steel and concrete, common in ambient temperature design, breaks down under fire conditions as material properties degrade at different rates. Engineers must account for this progressive redistribution in their design calculations and ensure that the connection details between the steel tube and concrete infill can accommodate the differential thermal expansion without causing bond failure or local buckling. The hollow section geometry introduces an additional design consideration: the air cavity provides thermal insulation but also reduces the effective cross-sectional area and moment of inertia, requiring a careful balance between thermal and structural performance.