Zhuojin Pipe Fitting Co., LtdThermal Debonding and Temperature Stress Analysis of Steel Tube Concrete Truss Arch
Literature Overview
This 2011 paper by Liu Zhenyu and Chen Baochun, conducted with support from the Research Fund of Huaqiao University, investigates the thermal debonding phenomenon and temperature stress distribution in steel tube concrete (CFST) truss arch bridge structures using finite element analysis. The study is grounded in the actual construction of a CFST truss arch bridge and incorporates meteorological data to simulate realistic thermal loading conditions. The findings reveal that thermal debonding between the steel tube and concrete core is a common occurrence in CFST truss arches, particularly on the sun-exposed faces of the chord tubes, and that this debonding significantly alters the stress distribution within the composite member.
Thermal Debonding Mechanism and Analysis
Thermal debonding in CFST members occurs when the differential thermal expansion between the steel tube and the concrete core generates interfacial shear stresses that exceed the bond strength at the steel-concrete interface. The mechanism is governed by the following factors:
| Factor | Description | Effect on Debonding |
|---|---|---|
| Temperature gradient | Differential heating of tube faces | Creates bending moment and interfacial shear |
| Coefficient of thermal expansion difference | α_steel ≈ 12×10⁻⁶/°C, α_concrete ≈ 10×10⁻⁶/°C | Small but cumulative over large temperature ranges |
| Concrete pouring time | Different hydration ages affect bond strength | Younger concrete has weaker bond interface |
| Solar radiation intensity | Asymmetric heating of tube faces | Maximum debonding on sun-exposed face |
| Reference (closure) temperature | Determines baseline thermal stress state | Lower closure temperature increases debonding risk |
The finite element analysis in this paper demonstrates that debonding preferentially initiates at the interface on the sun-exposed face of the chord tube, where the temperature gradient is most severe. The debonded region propagates inward as the temperature differential increases, eventually creating a continuous debonded zone that separates the steel tube from the concrete core over a significant length.
Stress Redistribution Due to Debonding
The paper's most important engineering finding is the redistribution of temperature stresses following debonding:
- Temperature secondary stresses in the arch structure decrease: When debonding occurs, the composite action between the steel tube and concrete core is partially or fully lost, reducing the overall stiffness of the arch rib. This leads to a decrease in the secondary stresses induced by thermal loading on the global structure.
- Temperature self-stress in the steel tube increases: Without the constraining effect of the concrete core, the steel tube expands more freely in response to temperature changes, generating higher self-stresses within the tube wall itself.
- Temperature self-stress in the concrete core decreases: The concrete core, no longer constrained by the steel tube, experiences reduced thermal self-stresses.
This redistribution has critical implications for the structural safety assessment of CFST truss arch bridges. A structure that appears safe under full composite action may experience higher local stresses in the steel tube when debonding occurs, potentially leading to local buckling or fatigue cracking at weld connections.
Welding and Steel Tube Quality Considerations
From a steel pipe fabrication and welding perspective, the thermal debonding phenomenon raises several quality concerns:
- Weld heat-affected zone vulnerability: The longitudinal welds in LSAW or UOE steel tubes are already regions of reduced toughness and elevated residual stress. When thermal debonding causes increased self-stress in the steel tube, the weld HAZ becomes a preferential location for fatigue crack initiation, particularly under cyclic thermal loading.
- Tube wall thickness and temperature gradient: Thicker-walled tubes experience more severe through-thickness temperature gradients under solar radiation, increasing the likelihood and extent of thermal debonding. Engineers specifying steel tubes for CFST truss arch applications should consider the trade-off between structural capacity (which favors thicker walls) and thermal debonding resistance (which favors thinner walls).
- Surface finish and bond development: The internal surface condition of the steel tube directly affects the bond strength at the steel-concrete interface. Tubes with smooth internal surfaces (such as cold-drawn tubes) develop weaker bonds than tubes with rough internal surfaces (such as hot-rolled tubes with retained mill scale). The choice of tube manufacturing process should be considered in the context of thermal debonding resistance.
- Corrosion protection and debonding: Internal corrosion of the steel tube can create voids at the steel-concrete interface that act as pre-existing debonding zones. The corrosion product layer also reduces the effective bond strength. Engineers should ensure adequate internal corrosion protection for CFST tubes exposed to aggressive environments.
Design and Construction Recommendations
The study concludes with several recommendations for practical engineering application:
- The closure temperature selection for CFST truss arch bridges involves a fundamental trade-off: a lower closure temperature reduces the thermal secondary stresses on the global structure but increases the probability of thermal debonding at the steel-concrete interface. This trade-off must be evaluated quantitatively for each specific project.
- Finite element analysis incorporating realistic thermal loading scenarios should be performed during the design phase to predict the extent and location of thermal debonding under expected service conditions.
- Monitoring programs should include strain gauges on both the steel tube and concrete core at multiple locations along the arch rib to detect debonding initiation and progression during service.
- For bridges in regions with extreme temperature variations, the use of internal shear connectors or mechanical interlocks at the steel-concrete interface may be necessary to maintain composite action under thermal loading.
The research highlights an important gap in current design codes, which typically assume full composite action between the steel tube and concrete core throughout the service life without accounting for the potential degradation of this composite action due to thermal effects. Engineers involved in the design and assessment of CFST truss arch bridges should incorporate the findings of this study into their structural evaluation methodology.