Long-term hygro-viscoelastic creep and stress redistribution in a flax-fiber-reinforced polymer bridge: Field monitoring and numerical investigation

The long-term structural performance of bio-based composite bridges under varying environmental conditions remains insufficiently understood, particularly the influence of humidity-dependent creep on stress evolution and serviceability at the structural scale. This study investigates the hygro-viscoelastic creep behavior of a full-scale flax fiber-reinforced polymer (FFRP) pedestrian bridge through a combined experimental and numerical approach. A three-dimensional orthotropic viscoelastic model based on a generalized Kelvin–Voigt formulation is developed and implemented in Abaqus via user-defined material subroutine (UMAT) to capture humidity-dependent anisotropic creep behavior. Model parameters are calibrated using humidity-controlled creep tests on FFRP laminates and validated against long-term in situ monitoring data, showing good agreement. Parametric simulations over a 10-year service period show that environmental humidity significantly accelerates creep deformation, with a pronounced nonlinear increase above approximately 75% relative humidity. While global deflection and laminate strain increase monotonically with humidity, internal stress exhibits a non-monotonic trend. Ply-level analysis shows that different fiber orientations respond differently to humidity-dependent stiffness degradation, leading to stress redistribution within the laminate. This behavior is quantified using a load redistribution indicator, highlighting a soft redundancy mechanism associated with time-dependent stiffness contrasts among plies. The results demonstrate that long-term structural performance cannot be assessed based on deformation alone, as humidity-induced stress redistribution plays a critical role. These findings provide insights for serviceability assessment and durability-oriented design of bio-based composite bridge structures.