Creep Effect Analysis of Steel Tube Concrete Arch Bridges

Literature Overview and Creep Phenomenon

The paper by Li Shengyong and colleagues from Fuzhou University investigates the creep behavior of steel tube concrete (CFT) axially compressed members using the age-adjusted effective modulus method. The study derives creep coefficient formulas for CFT members and evaluates three creep coefficient models (CEB-FIP 78, CEB-FIP 90, and ACI 209R) against experimental data. The ACI 209R model is identified as the most reasonable among the three. A case study on a 380-meter-span CFT railway arch bridge demonstrates the practical applicability of the proposed method. Creep is a time-dependent deformation phenomenon that profoundly affects the long-term performance of CFT structures, and from a metallurgical perspective, it is closely related to the stress-strain behavior of the concrete core and the confinement effectiveness of the steel tube.

Creep Mechanism and Material Interaction in CFT Members

Creep in CFT members arises from the viscoelastic behavior of the concrete core under sustained axial loading. The steel tube provides lateral confinement that reduces the creep strain of the concrete by maintaining a confining pressure that counteracts the tensile strains induced by creep. However, this interaction is not instantaneous: as concrete creeps, the steel tube undergoes elastic deformation, and the redistribution of stresses between the steel and concrete leads to a time-dependent change in the load-sharing ratio. The age-adjusted effective modulus method accounts for this interaction by adjusting the concrete modulus to reflect the progressive transfer of stress from the concrete to the steel tube over time. The creep coefficient phi is defined as the ratio of creep strain to elastic strain, and its value depends on factors including concrete age at loading, humidity, temperature, and the stress level relative to the concrete strength.

The ACI 209R model's superior performance in the experimental comparison is attributed to its more comprehensive treatment of the age-adjusted effective modulus, which better captures the nonlinear stress-strain behavior of concrete under sustained loading. For CFT arch bridges, the long-term creep deformation of the arch ribs leads to progressive deflection of the arch crown, increased tie beam elongation, and redistribution of internal forces among structural members. These effects are particularly pronounced in large-span bridges where the sustained load duration exceeds several decades.

Steel Tube Welding Quality and Long-Term Creep Performance

The steel tube in a CFT member is typically fabricated from welded steel pipe, and the quality of the welds directly influences the long-term creep performance of the CFT member. Weld defects such as incomplete fusion, lack of penetration, or slag inclusions create stress concentration points that can accelerate fatigue cracking under cyclic loading conditions. More importantly for creep analysis, the presence of weld defects reduces the effective confinement pressure exerted by the steel tube on the concrete core. If the steel tube develops cracks or loses structural integrity at weld locations, the confinement effect is compromised, leading to accelerated creep of the concrete core and potentially catastrophic failure. The welding of CFT arch rib segments should follow a strict quality assurance protocol that includes pre-weld material certification, weld procedure qualification, real-time welding parameter monitoring, and comprehensive post-weld NDT. For thick-walled steel pipes (wall thickness greater than 20 mm), multi-pass welding with controlled heat input per pass (typically 0.5–1.5 kJ/mm) and interpass temperature control (100–250 degrees Celsius) are essential to prevent microstructural degradation in the HAZ.

Creep Analysis of the 380-Meter CFT Railway Arch Bridge

The case study on the 380-meter-span CFT railway arch bridge provides valuable engineering data for validating creep prediction methods. At this span length, the arch ribs are subject to sustained self-weight and live load, and the creep deformation over the design life (typically 100 years) can be significant. The comparison between ACI 209R and CEB-FIP 78 predictions reveals that the CEB-FIP 78 model underestimates long-term creep deformation, which could lead to unconservative design if used for operational monitoring and serviceability assessment. For railway bridges, the dynamic component of train loading introduces cyclic stress that interacts with creep through the phenomenon of creep-fatigue interaction. The steel tube welds are particularly vulnerable to this interaction because the HAZ typically has lower fatigue crack initiation resistance than the base metal. I recommend incorporating creep-fatigue interaction analysis into the long-term performance assessment of CFT arch bridges, using Miner's linear damage rule or more sophisticated damage accumulation models. The research by Li et al. establishes a solid theoretical foundation for creep analysis of CFT members, but practical implementation requires integration with welding quality data, material certification, and field monitoring results to ensure accurate long-term performance prediction.