Construction-Stage-Dependent Support-Path Evolution and Structural Dynamic Response During Frame-Bridge Jacking Beneath a Heavy-Haul Railway

Frame-bridge jacking beneath an operating heavy-haul railway progressively transfers vertical support from the existing subgrade to the advancing bridge top slab. This project-specific numerical case study compares four representative support states, three train-operation scenarios, and five discrete speeds (45–65 km/h) using a coupled train–reinforced-track–frame-bridge model with a fixed 0.005 s integration step. Midas Civil was used only for a cross-platform low-order modal consistency assessment and does not establish physical accuracy. The first lateral-bending frequency remained nearly constant at 6.688–6.689 Hz, whereas the first vertical-bending frequencies were 7.365, 8.964, 8.019, and 7.122 Hz in Stages 1–4, respectively. Among the four modeled stages, Stage 3 produced the largest principal acceleration responses: 0.53 m s−2 vertically in the reinforced system and 0.11 and 0.82 m s−2 laterally and vertically, respectively, on the top slab. Stage 4 produced the largest vertical displacements, reaching 1.73 mm in the reinforced system and 1.66 mm on the top slab. These discrete cases indicate that acceleration and deformation are controlled by different representative support states; they do not locate a unique maximum along the continuous jacking path. Within the modeled assumptions, the results suggest project-specific monitoring emphasis on eccentric load transfer and acceleration during the Stage 3 support state and on vertical deformation and top-slab action during Stage 4.