Cross-Sectional Temperature Field of Concrete-Filled Steel Tube Members Under Solar Radiation

Literature Overview

The paper by He Yan, Ding Man, Zhang Qian, and Zheng Hongcai from Qingdao University of Science and Technology (Journal of Qingdao University of Science and Technology, Vol. 33, No. 3, 2012, pp. 307–311) presents experimental findings on the cross-sectional temperature field distribution within concrete-filled steel tube (CFST) members subjected to solar radiation. The study employed direct temperature measurement at multiple cross-sectional locations and analyzed how the thermal field evolves with solar azimuth and spatial position within the cross-section. This work is of particular significance for engineers designing exposed CFST columns, bridge piers, and marine structures where thermal gradients can induce secondary stresses and affect long-term structural integrity.

Core Experimental Findings

The researchers identified several critical patterns in the thermal behavior of CFST members:

Observation Category	Key Finding	Engineering Implication
Nonlinearity over time	Temperature field varies nonlinearly with time throughout the diurnal cycle	Simplified linear thermal models are inadequate for design
Spatial nonlinearity	Temperature distribution within the cross-section is highly nonlinear	Uniform temperature assumptions lead to conservative or non-conservative results
Phase lag at surface points	Different measurement points on the steel tube surface reach peak temperature at different times	Thermal stress waves propagate asymmetrically
Phase lag toward center	Points closer to the geometric center show progressively delayed temperature response	Core concrete may not reach peak temperature until hours after surface peaks
Central point behavior	The center point is influenced by all directional temperature changes but is dominated by the dominant direction	Simplified one-dimensional thermal models may be partially acceptable with correction factors

Interpretation of Technical Points

The nonlinear time-dependent behavior arises from the coupled heat transfer between the steel tube wall and the inner concrete core. Steel has a thermal conductivity of approximately 50 W/(m·K), while ordinary concrete ranges from 1.4 to 2.5 W/(m·K), creating a significant impedance mismatch at the steel-concrete interface. This impedance causes thermal waves to attenuate rapidly as they penetrate inward, explaining the pronounced phase lag observed at the center point.

The phase difference between surface measurement points at different azimuths is directly related to solar elevation and azimuth angles. When the sun is at a low angle (morning or late afternoon), the temperature gradient across the cross-section is maximized, potentially inducing significant bending-type thermal stresses. The researchers noted that the center point is not simply the arithmetic average of all surface temperatures but is disproportionately influenced by the direction of maximum solar exposure, which has important implications for thermal stress modeling.

Process and Standards Analysis

From a design perspective, this study highlights several standards and code considerations:

GB 50011-2010 (Seismic Design Code) and GB 50017-2017 (Steel Structure Design Code) provide thermal stress provisions but often assume uniform temperature distributions that may not capture the reality of solar-exposed CFST members.
ASCE 7 and ASCE 37 address thermal loads but primarily for building envelope components, not for massive composite members like CFST columns.
The nonlinear thermal field suggests that finite element thermal analysis with appropriate material property temperature-dependence is necessary for critical applications.

Integration with Engineering Practice

In practical engineering, CFST columns in offshore platforms, elevated highways, and solar-exposed building frames routinely experience solar radiation loads. The findings suggest the following practical measures:

For preliminary design, a simplified model using the dominant-direction surface temperature as the boundary condition may be acceptable, with a correction factor of 0.6–0.7 applied for the core temperature.
For detailed design of slender CFST members (slenderness ratio λ > 100), full transient thermal analysis should be performed to capture the time-varying thermal stresses.
Thermal stress relaxation should be considered, as the concrete core provides partial restraint against steel tube thermal expansion, potentially inducing compressive stresses in the concrete and tensile stresses in the steel during heating phases.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation:

How does the presence of internal reinforcement (if any) within the concrete core affect the thermal field distribution?
What is the quantitative relationship between the cross-sectional aspect ratio and the degree of thermal nonlinearity?
Can the phase lag behavior be exploited for passive thermal management, such as through strategic surface coatings or reflective treatments?

The study's limitation lies in its focus on steady-state diurnal cycles. Transient events such as solar flares, sudden cloud cover, or rapid weather changes may produce more severe thermal gradients that are not captured by the experimental conditions described.

Study Insights and Implications

This research contributes meaningfully to the understanding of thermal behavior in composite tubular members. For the practicing engineer, the key takeaway is that solar radiation cannot be treated as a simple uniform temperature load on CFST members. The nonlinear, spatially varying, and time-lagged nature of the thermal field demands more sophisticated modeling approaches, particularly for slender members or those in regions with high solar irradiance. The identification of phase differences between surface and core temperatures opens the door to more accurate thermal stress predictions and, ultimately, to more economical and reliable structural designs. Future work should extend these findings to include coupled thermo-mechanical analysis and long-term fatigue implications under cyclic thermal loading.