Solar Radiation Temperature Distribution in Dumbbell-Shaped CFST Arch Bridges

Literature Overview

This paper by Peng Yousong, Qiang Shizhong, and Li Song from Southwest Jiaotong University, published in China Railway Science (2006, Vol. 27, Issue 5, pp. 71-75), addresses a critical but often overlooked aspect of steel tube concrete (CFST) arch bridge design: the non-uniform temperature field induced by solar radiation on dumbbell-shaped CFST arch ribs. Funded by the National Natural Science Foundation of China (Grant No. 50278079), the study establishes a heat conduction-based analytical model and employs ANSYS finite element software to compute solar radiation temperature distributions under various parametric conditions.

Core Technical Content

The fundamental challenge in CFST arch bridges lies in the fact that the steel tube shell absorbs solar radiation directly while the interior concrete is thermally insulated, creating a significant cross-sectional temperature gradient. The dumbbell-shaped cross-section—consisting of two steel tubes connected by a concrete web—exacerbates this issue because the two outer tubes receive differential solar exposure depending on the bridge's orientation and the time of day.

Analytical Model Framework

The model integrates several physical domains:

• Heat conduction theory: Governs the transient temperature field within the composite cross-section, accounting for the thermal conductivity mismatch between steel (approximately 50 W/m·K) and concrete (approximately 1.7-2.3 W/m·K).
• Climate conditions: Solar irradiance intensity, ambient temperature variation, wind speed (convective heat transfer), and atmospheric emissivity (radiative heat loss).
• Geometric and orientation factors: Bridge azimuth, latitude, cross-section dimensions, and the shadowing effect between the two outer tubes.
• Material properties: Absorptivity of the steel tube surface, thermal diffusivity, and specific heat capacity of both steel and concrete.

Key Findings

The study demonstrates that solar radiation can induce cross-sectional temperature differences exceeding 25°C in dumbbell-shaped CFST arch ribs. This is a remarkably large gradient for a structural element where thermal expansion differential between steel and concrete can generate significant secondary stresses. The parametric analysis reveals that the surface absorptivity of the steel tube is the single most influential parameter governing the magnitude of the temperature gradient.

Engineering Practice Implications

From a steel pipe manufacturing and structural engineering perspective, several practical considerations emerge:

Surface Treatment Strategy

The finding that absorptivity is the dominant factor has direct implications for steel pipe surface treatment in bridge applications. Standard bare steel or dark-colored coatings (absorptivity 0.6-0.9) will maximize heat absorption. Conversely, applying high-reflectivity white or light-colored coatings (absorptivity 0.2-0.3) can reduce peak cross-sectional temperature differences by 40-60%. This is a straightforward and cost-effective mitigation measure that should be incorporated into design specifications for CFST arch bridges in sunny regions.

Thermal Stress and Structural Response

A cross-sectional temperature gradient of 25°C or more in a dumbbell-shaped section creates differential thermal expansion between the sun-exposed outer tube and the shaded interior concrete. For a steel tube with a typical yield strength of 345-460 MPa (Q345-Q345 grade per GB/T 1591), the thermal strain alone (α ≈ 12×10⁻⁶ /°C) represents approximately 300×10⁻⁶, which, when restrained by the concrete core and the opposite tube, can generate significant secondary bending moments. Engineers must verify that the combined effect of permanent loads and thermal gradients does not exceed the plastic hinge capacity of the section.

Design Recommendations

1. Conduct detailed thermal analysis for all CFST arch bridges in regions with high solar irradiance and low wind exposure.
2. Specify high-reflectivity coatings for the sun-exposed surfaces of steel tubes as a standard design practice.
3. Include thermal gradients as a load case in structural verification, particularly for the cross-sectional bending moment.
4. Consider the seasonal variation in solar angle when evaluating the most critical temperature distribution patterns.

Reflections on Methodology

The use of ANSYS for thermal analysis is well-established, but the value of this work lies in the systematic parametric study rather than the computational tool itself. The authors correctly identify that the engineering community often neglects solar radiation effects in steel-concrete composite structures, treating them as purely mechanical problems. This is a significant oversight for long-span arch bridges where the temperature-induced secondary stresses can govern fatigue life and long-term deformation.

The study also highlights an important aspect of steel pipe specification: the surface condition of the steel tube is not merely an aesthetic or corrosion protection concern but a structural performance parameter. This insight should be communicated to steel pipe manufacturers, who may not fully appreciate that the surface coating specification directly affects the structural behavior of CFST bridge components.

Study Insights and Implications

The research underscores the interdisciplinary nature of modern steel-concrete composite bridge engineering. A steel pipe manufacturer supplying tubes for CFST arch bridges should be aware that the surface treatment specification is a structural requirement, not merely a corrosion protection measure. The 25°C temperature gradient finding should be treated as a design load case, and the absorptivity parameter should be explicitly controlled in procurement specifications. Future research should extend to cyclic thermal fatigue and long-term creep-thermal interaction effects, which are particularly relevant for the service life assessment of CFST arch bridges in tropical and subtropical regions.