Temperature Field Analysis of Steel Tubes Under Solar Radiation

Literature Overview

This research published in "Spatial Structures" (2011, Vol. 17, Issue 2) investigates the temperature field distribution within steel tubes subjected to solar radiation, employing finite element thermal analysis based on the ASHRAE clear sky model. The study was conducted by researchers at the College of Engineering, Tianjin University, and was supported by the National Natural Science Foundation of China (50778122) and the Tianjin Municipal Commission of Urban Construction (2008-24). The work addresses a critical but often overlooked aspect of steel structural design: thermal effects from solar radiation on exposed steel tube members in spatial structures.

Thermal Analysis Methodology

The study employs a dual analytical approach: transient finite element analysis for capturing the diurnal temperature variation process, and steady-state thermal analysis for establishing a simplified calculation model. The ASHRAE clear sky model provides the solar radiation boundary conditions, accounting for atmospheric conditions, solar altitude, and azimuth angles throughout the day.

Key Thermal Behavior Findings

The diurnal temperature variation analysis reveals that the temperature field within a steel tube is not uniform even under steady solar radiation conditions. The temperature distribution exhibits significant gradients across the tube cross-section, with the sun-facing surface reaching substantially higher temperatures than the shaded surface. The temperature difference between the hottest and coldest points on the tube cross-section can be significant enough to induce thermal stresses that affect structural performance.

The parametric analysis demonstrates that the solar radiation absorption coefficient has the most pronounced effect on the peak temperature reached by the steel tube surface. Dark-colored or weathered steel surfaces absorb more radiation and reach higher temperatures compared to light-colored or coated surfaces. The ground radiation reflection coefficient contributes to the temperature of the lower surface of horizontal tubes, creating a secondary heating effect that is often neglected in conventional design.

Impact of Geometric and Orientation Parameters

The steel tube diameter and wall thickness influence the thermal gradient magnitude and the time lag between surface temperature and core temperature. Larger diameter tubes exhibit more pronounced internal temperature gradients due to the longer thermal diffusion path. The spatial orientation of the tube—its inclination angle relative to the horizontal and its azimuth angle relative to true south—determines the incident radiation angle and thus the absorbed radiation intensity.

For vertical tubes, the temperature distribution is primarily governed by the azimuth angle, with the maximum temperature occurring on the surface facing the sun. For inclined or horizontal tubes, the inclination angle determines the projected area exposed to direct solar radiation, while the azimuth angle determines the time of day at which peak heating occurs.

Simplified Calculation Model

Based on the transient and steady-state thermal analyses, the researchers developed a simplified calculation model for predicting the temperature field of steel tubes under solar radiation. This simplified model reduces computational complexity while maintaining acceptable accuracy for engineering design purposes. The model was validated through numerical examples that compared the simplified predictions with the full finite element results.

The simplified model is particularly useful for preliminary design stages where rapid assessment of thermal effects is needed. It can be integrated into structural analysis software to evaluate thermal stresses in spatial steel structures during the design phase, rather than requiring full transient thermal analysis for every design iteration.

Engineering Practice and Design Implications

From a steel pipe manufacturing and structural design perspective, this research has several important implications. First, the thermal expansion of steel tube members under solar radiation can cause significant displacements in spatial structures, particularly long-span trusses and tension structures. Second, the non-uniform temperature distribution across the tube cross-section induces bending moments and stresses that are superimposed on the structural loads, potentially affecting the fatigue life of welded connections.

The coating system applied to steel tubes in outdoor applications directly affects the solar radiation absorption coefficient. Standard hot-dip galvanized coatings have moderate absorption coefficients, while painted coatings can range from low (white, light gray) to high (dark colors). The selection of coating color should be considered as a thermal management strategy, not merely an aesthetic or corrosion protection decision.

Welding and Fabrication Considerations

The thermal cycling experienced by steel tubes under solar radiation has implications for the long-term integrity of welded joints. Repeated daily thermal cycling can contribute to fatigue cracking at weld toes, particularly in fillet welds connecting steel tubes to gusset plates or other structural elements. The thermal gradient across the tube cross-section means that the weld metal on the sun-facing side experiences higher temperatures than the weld metal on the shaded side, creating differential thermal expansion that induces additional cyclic stresses.

For the steel pipe manufacturer, the residual stresses from the welding process interact with the operational thermal stresses from solar radiation. If the residual stress distribution from welding is not properly managed, the superposition with thermal stresses could accelerate fatigue crack initiation and propagation.

Key Reflections and Study Insights

The research highlights a design aspect that has historically received insufficient attention in spatial steel structure engineering. While structural engineers routinely consider thermal effects from temperature changes, the localized and non-uniform thermal effects from direct solar radiation on individual members are often overlooked. This oversight can lead to unexpected structural behavior, including excessive displacements, unexpected stress concentrations, and premature fatigue failure.

The development of a simplified calculation model is a significant practical contribution, as it makes thermal analysis accessible to practicing engineers who may not have the resources or expertise to perform full transient finite element thermal analysis. The model should be further validated against field measurements under various climatic conditions to establish its reliability across different geographical regions.

Summary

This study provides a comprehensive framework for understanding and predicting the temperature field of steel tubes under solar radiation, combining transient and steady-state thermal analysis with parametric investigation of key influencing factors. The development of a simplified calculation model offers practical value for engineering design, enabling engineers to account for solar thermal effects without resorting to computationally intensive full-scale analyses. The findings emphasize the importance of considering surface properties, geometric parameters, and spatial orientation in the thermal design of outdoor steel structures, with particular attention to the implications for welded connection integrity and long-term structural performance under thermal cycling.