A novel numerical model for fire analysis of post-tensioned concrete beams with curved unbonded tendons

This paper presents a novel numerical framework for the fire analysis of post-tensioned concrete beams with curved unbonded tendons. The framework consists of three sequential steps: (i) fire development in the structural surroundings, (ii) hygro-thermal analysis of the cross-section, and (iii) mechanical analysis, where the reinforced concrete beam and prestressing tendons are represented as separate interacting components. The core contribution lies in step (iii), where a new beam-type finite element formulation is proposed. Unlike conventional shell or solid models, the approach substantially reduces the number of degrees of freedom and thereby the computational cost. Additional efficiency is achieved by interpolating strains instead of displacements, which eliminates numerical locking often present in alternative formulations. The model further accounts for material nonlinearity (allowing implicit or explicit consideration of transient and creep strains) while the kinematics are linearized, enhancing robustness. These features collectively enable efficient analysis of large structural systems rather than isolated members. Validation against experimental data confirms accuracy and numerical stability, while the modest computational demand supports routine application on standard personal computers. Finally, the model is applied in a parametric study investigating whether the EN 1992-1-2 stress–strain model for concrete implicitly accounts for creep effects in fire-exposed prestressed beams.