Radial Temperature Difference Action Modes of Concrete-Filled Steel Tubes in Natural Environment

Research Context and Significance

This study addresses a critical but often overlooked aspect of CFST structural performance: the development and distribution of radial temperature gradients within the composite cross-section under natural environmental conditions. Unlike uniformly heated members (such as those in fire scenarios), CFST columns and beams exposed to solar radiation, ambient temperature fluctuations, and wind cooling develop complex three-dimensional temperature fields that can induce significant thermal stresses and deformations.

Understanding these radial temperature difference action modes is essential for predicting long-term structural behavior, fatigue life, and serviceability performance of CFST members in outdoor applications such as bridge piers, transmission towers, and industrial structures. The research provides valuable insights for optimizing thermal protection strategies and adjusting design parameters to accommodate thermal effects.

Thermal Action Mechanisms

The radial temperature difference in CFST members arises from the differential thermal properties of the steel tube and concrete core, combined with the asymmetric heating patterns caused by solar radiation and environmental conditions.

The solar heating mode produces the most severe radial temperature differences, with surface temperatures reaching 60-80°C above ambient conditions on sunny days. The steel tube, with its high thermal conductivity (approximately 50 W/m·K), rapidly transfers heat to the concrete core, but the concrete's low thermal conductivity (approximately 1.5-2.5 W/m·K) creates a significant temperature gradient between the tube wall and the core center. This gradient can reach 20-40°C for typical CFST cross-sections with diameters of 400-800 mm.

Quantitative Analysis of Thermal Stresses

The thermal stresses induced by radial temperature differences can be calculated using the fundamental relationship σ = E·α·ΔT, where E is the elastic modulus, α is the coefficient of thermal expansion, and ΔT is the temperature difference.

The thermal stress in the steel tube can approach or exceed the yield strength for large temperature differentials, particularly in members with thin walls and large diameters. This poses a significant concern for fatigue performance, as cyclic thermal stresses contribute to crack initiation and propagation over the service life of the structure. The concrete core, while developing lower absolute thermal stresses due to its lower elastic modulus, experiences differential shrinkage and expansion that can lead to interface debonding between the steel tube and concrete.

Temperature Distribution Patterns and Time-Dependent Behavior

The temporal evolution of the radial temperature field follows a characteristic pattern governed by the thermal diffusivity of both materials. The steel tube responds almost instantaneously to surface temperature changes, while the concrete core exhibits a significant time lag due to its lower thermal diffusivity.

The inversion of the radial temperature gradient during nighttime cooling is particularly significant because it reverses the thermal stress direction, creating alternating stress cycles that contribute to fatigue damage accumulation. Over a typical service life of 50 years, this results in approximately 18,250 full thermal cycles, each of which may contribute incrementally to fatigue crack growth.

Engineering Countermeasures and Design Recommendations

Based on the analysis of radial temperature difference action modes, several engineering countermeasures can be implemented to mitigate thermal effects on CFST members.

From a welding and fabrication perspective, thermal effects must be considered in the design of field-welded joints. The thermal expansion of CFST members during hot weather can cause significant misalignment at butt welds, requiring careful fit-up procedures and possibly temporary restraint systems. Post-weld thermal stresses superimposed on welding residual stresses can lead to premature fatigue failure if not properly managed through stress-relief procedures.

Study Insights and Long-Term Performance Implications

This research highlights that radial temperature differences represent a persistent and significant loading condition for CFST members in natural environments. Unlike transient loads (such as seismic or wind events), thermal effects act continuously throughout the service life and contribute to both immediate structural response and long-term degradation mechanisms.

The implications for steel pipe manufacturing are substantial. Pipes used in CFST applications exposed to significant thermal gradients should be manufactured with attention to dimensional accuracy and surface finish, as these factors influence the thermal response and stress distribution. Additionally, the selection of steel grade should consider fatigue performance under cyclic thermal loading, favoring grades with higher fatigue strength and improved toughness at elevated temperatures.

For future research, I recommend investigating the interaction between thermal effects and other degradation mechanisms (corrosion, fatigue, creep) to develop comprehensive durability models for CFST members. The integration of thermal monitoring systems with structural health monitoring networks would provide real-time data for validating analytical models and informing maintenance decisions.