Objective Pipe bridges are one of the main structural forms for long-distance oil and gas pipelines crossing highways and other complex terrain. Their failure can cause pipeline suspension, deformation, or even rupture. Accurately evaluating the mechanical state of the pipeline after pipe bridge damage is a critical prerequisite for determining emergency repair schemes and preventing further losses.

Methods A pipe bridge collapse accident was used as a case study. An elastic constraint theoretical model for the suspended pipeline is established. The vertical pipe is simplified to have a hinged top end subjected to a bending moment. Two theoretical models are adopted, with the bottom end set as hinged and fixed respectively. The rotation relationship between the vertical pipe and suspended pipeline was derived from deformation compatibility at the top joint. Analytical expressions for the bending moment and vertical displacement of the suspended pipeline were then obtained. A three-dimensional Finite Element (FE) model of the pipeline is developed using CAESAR II FE analysis software. Ultrasonic stress testing is applied to obtain the stress state at key sections of the in-situ suspended pipeline. Theoretical calculation results are compared with FE simulation results, ultrasonic stress test values, and field measured values to verify the rationality and applicability of the theoretical model.

Results The hinged-bottom model agreed better with FE simulations and field measurements than the fixed-bottom model. This held for mid-span vertical displacement, end bending moment, and stress distribution. After collapse, the mid-span vertical displacement was 58.8 cm (theoretical), 58.6 cm (FE), and 60.0 cm (field). The theoretical values deviated from the FE results and field measurements by less than 3％. The theoretical stresses at the mid-span and ends were within 3％ of FE results. They were within 18％ of ultrasonic stress measurements. A strength check was performed using theoretical results, FE results, and on-site ultrasonic measurements. The maximum equivalent stresses were 149.1, 148.8, and 158.3 MPa, respectively. All satisfied code requirements and indicated a sufficient stress safety margin.

Conclusion The rationality and applicability of the elastic constraint theoretical model for suspended pipelines after pipe bridge collapse are verified through comparisons of theoretical calculation values with FE simulation results, field measured values, and ultrasonic stress test values. The proposed elastic rotational constraint model provides a valuable reference for rapid safety state analysis and emergency repair decision-making in similar pipe bridge collapse accidents.