Solar Radiation Gradient Temperature Effect on Concrete-Filled Steel Tube Arch Ribs

Literature Overview

This paper, published in the Journal of Chongqing Jiaotong University (Natural Science Edition) (Vol. 42, No. 5, 2023, pp. 16-24), authored by Guo Zengwei, Zhang Yali, Yang Yifan, and Zhou Shuixing from Chongqing Jiaotong University, investigates the gradient temperature effects induced by solar radiation on concrete-filled steel tube (CFST) arch ribs in bridge structures. The research was funded by the National Natural Science Foundation of China (Grant 51878106), Chongqing Natural Science Foundation (cstc2019jcyj-msxmX0818), Chongqing Municipal Education Commission Science and Technology Research Project (KJZD-K202100704), and Chongqing Graduate Joint Training Base Construction Project (JDLHPYJD2020023). The study is based on real-time temperature measurements from the Jianwei Min River Grand Bridge in Sichuan Province, China, making it a field-informed investigation with direct engineering relevance.

Solar Radiation and Temperature Field Characteristics

Solar Radiation Mechanism on Steel Tube Surfaces

Solar radiation impinges on the exposed upper surface of the steel tube arch rib, creating a non-uniform temperature distribution across the cross-section. The upper surface of the steel tube, being directly exposed to solar radiation, experiences significant temperature elevation, while the lower surface, shielded from direct radiation, remains at ambient temperature. This creates a radial temperature gradient across the steel tube wall thickness and extends into the concrete core. The temperature gradient induces differential thermal expansion, leading to thermally induced stresses in both the steel tube and the concrete core.

Radial Temperature Distribution

The study reveals that the radial temperature distribution within the CFST cross-section is highly non-uniform under solar radiation. The temperature gradient is most severe near the upper surface of the steel tube, where the temperature changes rapidly over a short radial distance. The gradient decreases with increasing depth into the cross-section, approaching a more uniform temperature distribution at the center of the concrete core.

This comparison is particularly significant: the existing Chinese highway bridge design code (JTG D60-2015) and railway bridge design code (TB 10092-2017) underestimate the temperature gradients near the solar-exposed surface of CFST arch ribs. The actual gradients near the steel tube surface are substantially higher than code recommendations, while at mid-depth, the gradients are lower than code values. This suggests that the current code provisions may be inadequate for accurate thermal stress analysis of CFST arch ribs in high-solar-radiation environments.

Diurnal Temperature Variation Pattern

The temperature field within the CFST arch rib varies significantly over a 24-hour cycle. Solar radiation intensity peaks around midday, leading to maximum temperature gradients in the afternoon hours. The temperature gradient evolves dynamically as the sun moves across the sky, with the maximum gradient typically occurring in the early to mid-afternoon when solar radiation is strongest and the thermal mass of the concrete core has not yet fully equilibrated.

Thermal Stress Analysis

Steel-Concrete Interface Stress Behavior

The study established a thermomechanically coupled finite element model that accounts for the contact state at the steel-concrete interface. The interface contact stress exhibits a "tension-compression" alternating pattern over a 24-hour period. This alternating stress pattern is a critical finding: during the heating phase, the steel tube expands more than the concrete core due to the higher thermal expansion coefficient of steel, creating compressive stresses at the interface. During the cooling phase, the steel tube contracts more rapidly than the concrete, potentially creating tensile stresses at the interface.

The peak tensile stress at the steel-concrete interface reaches 0.8 MPa at 14:00 and 15:00, which are the peak solar radiation hours. This tensile stress is significant because it can lead to interface debonding, concrete delamination from the steel tube, and ultimately void formation (脱空) within the CFST section. Once voids form, the composite action between steel and concrete is compromised, reducing the overall structural capacity and potentially initiating progressive damage.

Gradient Temperature Formula and Characteristic Parameters

The study provides a radial gradient temperature calculation formula and characteristic parameter values for use in engineering design. These parameters are derived from field measurements and validated against finite element simulation results. The formula enables engineers to estimate the temperature gradient distribution within CFST arch rib cross-sections under solar radiation, which is essential for accurate thermal stress analysis and structural design.

Engineering Practice Implications

Design Considerations

The findings have direct implications for the design of CFST arch bridges in regions with high solar radiation. The current code-recommended temperature gradients (JTG D60-2015 and TB 10092-2017) are insufficient for accurate thermal stress analysis near the solar-exposed surface. Designers should consider using the revised gradient temperature values from this study, particularly for CFST arch ribs in high-altitude or arid regions where solar radiation intensity is high.

The steel tube surface finish and coating selection are also critical: reflective coatings or light-colored paint can reduce solar radiation absorption and mitigate temperature gradients. Additionally, the steel tube wall thickness affects the temperature gradient profile: thicker walls provide greater thermal mass and reduce the temperature gradient across the wall thickness, but at the cost of increased material usage.

Construction Considerations

During construction, the concrete pouring temperature and curing conditions must be carefully controlled to minimize thermal stresses at the steel-concrete interface. Post-pouring, the development of expansion compressive stress in the concrete core exceeding 0.8 MPa is recommended to prevent interface debonding and void formation. This can be achieved through controlled post-tensioning of the concrete core, the use of expansive admixtures in the concrete mix, or the application of thermal pre-stressing during the curing phase.

The quality of the steel tube welds is also important: weld defects such as lack of fusion, porosity, or slag inclusion can create stress concentrations that exacerbate the thermally induced interface stresses. Comprehensive NDT, including ultrasonic testing (UT) and phased array ultrasonic testing (PAUT), should be applied to all welds in the steel tube arch rib fabrication.

Steel Pipe Engineering Perspective

From a steel pipe manufacturing standpoint, the selection of steel grade for CFST arch ribs should consider the thermal expansion behavior. Steels with lower thermal expansion coefficients, such as certain austenitic stainless steels or specially designed low-expansion alloys, could potentially reduce thermal stresses, although their higher cost and different mechanical properties may limit their practical application. For conventional carbon steels, the wall thickness should be optimized to balance thermal mass (reducing temperature gradients) against structural efficiency (minimizing weight).

Key Reflections and Conclusions

This field-informed study provides critical data on the solar radiation-induced temperature gradients and thermal stresses in CFST arch ribs, revealing significant discrepancies between actual temperature gradients and current code recommendations. The alternating tension-compression stress pattern at the steel-concrete interface, with peak tensile stresses reaching 0.8 MPa, highlights the vulnerability of CFST arch ribs to thermal fatigue and interface degradation in high-solar-radiation environments. The recommended mitigation strategies, including the development of expansion compressive stress exceeding 0.8 MPa in the concrete core, provide actionable guidance for engineers to prevent interface debonding and void formation. These findings underscore the need for code revisions to incorporate more accurate temperature gradient values for CFST arch bridge design and emphasize the importance of field measurements in validating and improving theoretical models for structural thermal analysis.