Zhuojin Pipe Fitting Co., LtdNonlinear Finite Element Analysis of Temperature Field in Square-Section CFST Members Under Fire
Literature Overview
Xu Lei and Han Linhai from the Metal Structure Research Laboratory at Harbin University of Architecture published this study in the Journal of Harbin University of Architecture in 1999 (Volume 32, Issue 5, pages 34-38). Funded by the National Natural Science Foundation of China, the research addresses the nonlinear finite element analysis of temperature distribution in square-section concrete-filled steel tube (CFST) members subjected to fire loading. The work considers different protective layer thicknesses, including both concrete cover and fire-resistant coatings, as well as different steel tube dimensions.
Core Technical Content and Methodology
The study establishes a reasonable surface temperature boundary condition for CFST members exposed to fire and then employs nonlinear finite element methods to compute the internal temperature field. The nonlinear nature of the analysis is essential because thermal conductivity, specific heat capacity, and emissivity of both steel and concrete are temperature-dependent properties that vary significantly as temperatures rise from ambient to over 1000 degrees Celsius.
The key finding is that the finite element results correlate well with experimental data, validating the computational approach. The authors systematically investigate how protective layer thickness and steel tube geometry influence the temperature gradients within the CFST member, particularly at the critical interface between the steel tube and the concrete core.
Thermal Analysis Parameters and Results
| Parameter | Effect on Temperature Field | Engineering Significance |
|---|---|---|
| Concrete cover thickness | Increases thermal resistance, delays core temperature rise | Critical for fire resistance rating |
| Fire-resistant coating thickness | Provides additional insulation layer | Allows thinner steel tubes |
| Steel tube wall thickness | Thicker walls delay heat transfer to concrete | Affects both structural and fire performance |
| Steel tube outer dimension | Larger sections have lower surface-to-volume ratio | Reduces temperature gradient magnitude |
| Steel tube corner radius | Affects stress concentration and thermal gradient | Important for square section analysis |
Steel Pipe Manufacturing and Welding Implications
From a steel pipe engineering perspective, this research highlights the critical role that steel tube geometry and material properties play in fire performance. The wall thickness of the steel tube directly influences the rate of heat transfer from the fire-exposed surface to the concrete core. Thinner-walled tubes, which are often preferred for economic reasons, transfer heat more rapidly to the concrete, potentially leading to earlier spalling and loss of structural capacity.
The welding joints in square-section steel tubes present particular challenges in fire conditions. The longitudinal weld seam, whether produced by ERW, HFW, or SAW processes, may exhibit different thermal expansion characteristics compared to the base metal due to microstructural differences in the heat-affected zone. During fire exposure, differential thermal expansion between the weld zone and base metal can create additional stresses that may accelerate crack initiation.
For square-section CFST tubes fabricated by bending and welding flat steel plates, the corner welds are particularly vulnerable. The geometric discontinuity at the corner creates both a stress concentration and a thermal concentration under fire loading. The finite element results suggest that the temperature gradient at corner weld regions is steeper than at mid-side regions, which has implications for weld metal selection and post-weld treatment in fire-exposed applications.
Key Reflections and Study Insights
The correlation between finite element predictions and experimental results provides confidence in using computational methods for fire design of CFST members. However, the accuracy of these predictions depends critically on the material property databases used, which must account for the actual microstructure and composition of the steel pipe material. Different steel grades, and even different heat treatment conditions within the same grade, will exhibit different thermal behavior under fire loading.
For steel pipe manufacturers, this research underscores the importance of providing comprehensive material property data to structural engineers. Beyond standard mechanical properties, thermal conductivity, specific heat, and thermal expansion coefficient as functions of temperature are essential inputs for fire design calculations. The quality of the steel pipe, including its homogeneity and freedom from inclusions or segregation, directly affects the reliability of fire performance predictions.
Summary
This study provides a validated computational framework for predicting temperature distributions in square-section CFST members under fire conditions, with direct implications for steel pipe design specifications in fire-exposed applications. Steel pipe manufacturers should recognize that fire performance is as important as ambient-temperature mechanical performance, and that material property data for elevated temperatures must be made available to designers. The welding quality of steel tube seams, particularly at geometric discontinuities such as corners in square sections, requires special attention because weld zones may behave differently from base metal under thermal cycling conditions.