Measured Analysis of Temperature Field in Steel-Concrete Members Under Solar Radiation

Literature Overview and Research Context

The paper by Chen Baochun and Liu Zhenyu from the College of Civil Engineering at Fuzhou University, published in Highway Traffic Science and Technology in 2008 (Vol. 25, No. 12, pp. 117-122), presents an experimental investigation into the temperature field distribution within steel-concrete composite members under solar radiation. Funded by the Fujian Provincial Major Science and Technology Project (Grant 2003F007), this research addresses a practical and often overlooked aspect of structural engineering: the thermal effects of solar radiation on steel-concrete composite bridge members and their implications for structural performance and durability.

From a steel pipe engineering perspective, this research is highly relevant because solar radiation induces non-uniform temperature distributions in steel tubes, which in turn cause differential thermal expansion, thermal stresses, and potential fatigue damage. The temperature field within a steel-concrete composite member is influenced by the solar radiation direction, the orientation of the member, the section geometry, and the thermal properties of the steel tube and concrete core. Understanding these effects is essential for the design of durable and reliable steel-concrete composite bridge structures.

Experimental Methodology and Temperature Measurement

The authors conducted temperature field measurements on steel-concrete composite members under natural solar radiation conditions. The experimental setup included multiple temperature sensors embedded at various locations within the cross-section to capture the spatial distribution of temperature. The measurements were taken under different solar radiation conditions, including different times of day and different orientations of the member.

The key experimental findings reveal that the temperature field within the steel-concrete composite member is highly non-uniform under solar radiation. The temperature at the center point of the cross-section is more influenced by the east-west direction (the solar radiation direction) than by other directions. This finding is intuitive because the solar radiation primarily heats the surface of the member facing the sun, and the thermal wave propagates inward from the heated surface.

Temperature Field Distribution Patterns

The experimental data revealed several important patterns in the temperature field distribution. First, the temperature variation at the center point of the cross-section is more influenced by the east-west direction (the solar radiation direction) than by other directions. This is because the solar radiation primarily heats the surface of the member facing the sun, and the thermal wave propagates inward from the heated surface. The center point, being equidistant from all surfaces, experiences the integrated effect of thermal waves from all directions, but the east-west component dominates due to the direct solar radiation.

Second, except for the center point, the temperature variation along the radial direction shows a progressive lag as one moves toward the center of the cross-section. This thermal lag is a consequence of the finite thermal conductivity of the steel tube and the concrete core. The thermal wave from the heated surface takes time to propagate through the material, and the phase lag increases with distance from the heated surface. This finding is consistent with the theory of thermal wave propagation in solids and has important implications for the thermal stress distribution within the member.

Third, under the same solar radiation conditions, the temperature variation amplitude of a horizontally placed member is greater than that of a vertically placed member. This is because the horizontal member has a larger exposed surface area facing the sun throughout the day, while the vertical member has a more limited exposed surface area that changes with the solar elevation angle. The larger temperature variation amplitude in the horizontal member implies higher thermal stresses and a greater risk of thermal fatigue.

Fourth, the sun-facing side of the cross-section experiences larger temperature variations than the shaded side. This is obvious but important because it means that the thermal stress distribution is asymmetric, with the sun-facing side experiencing higher tensile or compressive stresses depending on the temperature gradient. The maximum temperature difference across the cross-section is larger for horizontal members than for vertical members, which further confirms the greater thermal stress risk for horizontal members.

Thermal Stress and Fatigue Implications for Steel Pipes

From a steel pipe engineering perspective, the non-uniform temperature field induced by solar radiation creates significant thermal stresses in the steel tube. The thermal stress is proportional to the temperature difference across the cross-section and the coefficient of thermal expansion of the steel. For typical structural steel with a coefficient of thermal expansion of approximately 12 × 10⁻⁶ /°C, a temperature difference of 30°C across the cross-section can generate thermal stresses of approximately 36 MPa, which is a significant fraction of the yield strength of common structural steels.

The cyclic nature of solar radiation—daily heating and cooling—induces cyclic thermal stresses that can lead to thermal fatigue damage in the steel tube. The fatigue life of the steel tube depends on the stress amplitude, the stress ratio, and the number of cycles. In bridge applications, the daily thermal cycling can accumulate over the service life of the structure, potentially leading to fatigue cracks in the steel tube, particularly at locations of stress concentration such as welds, geometric discontinuities, and connections.

The thermal stresses from solar radiation interact with the structural stresses from traffic loads, wind loads, and other external loads. The combined stress state can accelerate fatigue damage and reduce the service life of the steel-concrete composite member. This interaction is particularly important for steel tubes with existing defects or residual stresses from fabrication and welding, as these can act as fatigue crack initiation sites.

Practical Engineering Considerations

For steel pipe fabrication and welding engineers, the findings from this research have several practical implications. First, the orientation of steel-concrete composite members in bridge structures should be considered in the design to minimize thermal stress effects. Horizontal members should be designed with additional consideration for thermal fatigue, and the weld details should be optimized to reduce stress concentrations. Second, the thermal protective measures, such as reflective coatings or thermal insulation, can be applied to reduce the temperature variation amplitude and thereby reduce the thermal stresses.

The welding joints in the steel tube are particularly susceptible to thermal fatigue because the heat-affected zone (HAZ) has different thermal expansion properties and lower fatigue resistance compared to the base metal. The residual stresses from welding can also interact with the thermal stresses to create a more severe stress state. Quality control of welds, including non-destructive testing and residual stress measurement, is essential to ensure the long-term durability of steel-concrete composite members under solar radiation.

Key Questions and Reflections

Several important questions arise from this research. First, the study focuses on the temperature field distribution, but the resulting thermal stresses and their interaction with structural loads are not explicitly analyzed. A comprehensive study should include the calculation of thermal stresses and the assessment of fatigue damage under combined thermal and structural loading. Second, the long-term effects of solar radiation on the steel-concrete bond interface are not addressed. The differential thermal expansion between the steel tube and the concrete core can cause cracking at the bond interface, which can compromise the composite action and reduce the load-bearing capacity of the member.

From a standards perspective, current bridge design standards often do not explicitly account for the thermal effects of solar radiation on steel-concrete composite members. The thermal stresses from solar radiation can be significant and should be included in the structural analysis. The development of design guidelines that incorporate the thermal effects of solar radiation is an important area for future research and standardization.

Study Insights and Implications

The research by Chen and Liu provides valuable experimental data on the temperature field distribution in steel-concrete composite members under solar radiation. The findings on the non-uniformity of the temperature field, the thermal lag effect, and the influence of member orientation are important for the design of durable and reliable steel-concrete composite bridge structures. The cubic polynomial representation of the radial temperature difference provides a convenient mathematical tool for thermal stress calculation.

For steel pipe engineers, the research highlights the importance of considering thermal effects in the design and fabrication of steel-concrete composite members. The cyclic thermal stresses from solar radiation can lead to fatigue damage in the steel tube, particularly at welds and stress concentrations. Quality control of steel tube fabrication and welding, including residual stress measurement and non-destructive testing, is essential to ensure the long-term durability of these members. The research also underscores the need for integrated design approaches that consider thermal, structural, and material aspects as a unified system.

In summary, this research makes a valuable contribution to the understanding of thermal effects in steel-concrete composite members under solar radiation and provides practical guidance for engineers designing durable bridge structures with steel pipe components.