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

Literature Overview and Research Significance

This topic investigates the radial temperature difference patterns that develop in concrete-filled steel tubes (CFST) members exposed to natural environmental conditions. Temperature gradients across the cross-section of CFST members create complex thermal stresses that can significantly affect structural performance, fatigue life, and long-term durability. Understanding these temperature action patterns is essential for accurate structural analysis and design, particularly for large-diameter CFST members in outdoor or semi-outdoor applications.

From my perspective as a metallurgist and welding expert, thermal effects are always of paramount concern. Temperature gradients induce differential expansion between the steel tube and concrete core, and between different regions of the steel tube itself, creating complex stress states that can initiate cracking, accelerate corrosion, and reduce fatigue resistance.

Core Technical Content: Temperature Distribution Patterns

The study characterizes several distinct radial temperature difference action modes that occur in CFST members under natural environmental exposure:

Solar radiation mode: Uneven heating of the tube surface due to directional solar radiation creates a temperature gradient from the sun-facing side to the shaded side
Ambient convection mode: Natural convection currents around the tube surface create temperature variations depending on wind direction and speed
Internal concrete hydration mode: In newly constructed members, residual heat from concrete hydration creates a radial gradient from the center outward
Seasonal cycling mode: Long-term temperature cycling between day and night, and between seasons, creates progressive thermal fatigue

The research demonstrates that the temperature gradient is not uniform across the tube wall thickness due to the thermal inertia of both the steel and concrete materials. The concrete core, with its lower thermal conductivity, acts as a thermal mass that dampens rapid temperature fluctuations at the tube inner surface compared to the outer surface.

Temperature Action Mode	Peak Radial ΔT (°C)	Primary Stress Component	Dominant Structural Effect	Critical Location
Solar radiation	15–35	Hoop and radial	Thermal bowing, eccentricity	Sun-facing tube wall
Ambient convection	5–15	Axial and radial	Minor thermal stress	Windward tube surface
Concrete hydration	20–40	Radial (internal)	Internal cracking, differential expansion	Tube inner surface
Seasonal cycling	10–25	Multi-axial	Thermal fatigue, creep	Entire cross-section

Technical Analysis of Thermal Stress Development

The thermal stress development in CFST members follows a predictable pattern that can be analyzed using the principles of thermoelasticity. The key parameter is the temperature difference between the outer and inner surfaces of the steel tube, which drives the development of thermal bending moments and axial forces. For a tube with outer diameter D, wall thickness t, and radial temperature difference ΔT, the thermal hoop stress can be estimated as:

σ_thermal ≈ E_s × α_s × ΔT × (t/D)

where E_s is the Young's modulus of steel, α_s is the coefficient of thermal expansion, and the (t/D) factor accounts for the curvature effect.

From a welding quality perspective, the thermal stresses induced by temperature differences can interact with residual stresses from the welding process. If a CFST member is welded at ambient temperature and then exposed to significant temperature variations, the superposition of thermal and residual stresses can create localized stress concentrations that exceed the material's fatigue limit. This is particularly critical at weld joints, where the HAZ already has reduced fatigue resistance due to microstructural changes.

The study also highlights the differential thermal expansion between steel (α ≈ 12 × 10⁻⁶ /°C) and concrete (α ≈ 10–14 × 10⁻⁶ /°C). While these coefficients are relatively close, the difference is sufficient to create interface stresses at the steel-concrete boundary under significant temperature gradients. These interface stresses can lead to debonding at the steel-concrete interface, reducing the composite action and the confinement effectiveness.

Engineering Practice and Quality Control Implications

For engineers designing CFST structures in outdoor environments, several practical measures should be considered:

Thermal analysis in design: Finite element thermal analysis should be conducted for large-diameter CFST members to predict temperature gradients and associated stresses under realistic environmental conditions.
Surface protection: Thermal insulation or reflective coatings on the tube outer surface can significantly reduce solar-induced temperature gradients. The choice of coating should also consider corrosion protection requirements.
Welding sequence optimization: When welding CFST members in the field, the welding sequence should be planned to minimize interaction between welding-induced residual stresses and anticipated thermal stresses.
Monitoring provisions: For critical structures, temperature monitoring sensors should be installed at multiple locations around the tube circumference to track actual temperature gradients and validate design assumptions.
Material selection: Steels with lower coefficients of thermal expansion or better thermal fatigue resistance should be considered for applications with severe temperature cycling.

Key Questions and Reflections

A fundamental question raised by this research is the long-term effect of thermal cycling on the steel-concrete bond. Repeated temperature cycling causes differential expansion and contraction at the interface, which may progressively degrade the bond strength over time. This degradation would reduce the confinement effect and potentially lead to premature structural failure. The study does not fully address this long-term degradation mechanism, which warrants further investigation through accelerated thermal cycling tests.

Another important consideration is the interaction between thermal effects and corrosion. Temperature gradients can create differential aeration cells on the tube surface, accelerating localized corrosion. In marine or industrial environments, this thermal-corrosion synergy can be particularly damaging. The study's focus on thermal effects alone, without considering corrosion, represents a limitation that engineers must keep in mind when applying the findings to real-world conditions.

Summary and Study Insights

This research provides valuable quantitative data on radial temperature difference patterns in CFST members under natural environmental exposure. The key finding is that solar radiation is the dominant driver of temperature gradients, creating thermal stresses that can be significant for large-diameter members. Engineers should incorporate thermal analysis into the design of outdoor CFST structures and consider appropriate mitigation measures such as surface protection and thermal insulation. The interaction between thermal stresses, welding residual stresses, and long-term material degradation represents a complex multi-physics problem that requires careful consideration in the design and maintenance of CFST structures. Future research should focus on the coupled effects of thermal cycling, corrosion, and fatigue to provide more comprehensive guidance for long-term structural performance assessment.