Research on the Dynamic Characteristics of Long-Span Cable-Stayed Bridges During the Construction Process

Long-span cable-stayed bridges undergo substantial changes in mass distribution, boundary conditions, cable forces, and load paths during erection, yet their stage-dependent free-vibration characteristics are less documented than those of completed bridges. This study establishes a refined finite element (FE) model of the Liulu Yongjiang Extra-large Bridge and evaluates the frequency evolution over 26 construction stages. Detailed modal interpretations are provided for the maximum double-cantilever, maximum single-cantilever, and completed-bridge configurations. The subspace iteration eigensolver is used for modal extraction, and ambient-vibration measurements at the three representative stages provide an independent frequency check. The calculated fundamental frequencies are 0.399, 0.417, and 0.436 Hz for the three configurations, respectively. The governing mode changes from antisymmetric vertical girder bending at the maximum double-cantilever stage to lateral girder bending at the maximum single-cantilever and completed-bridge stages. Across CS1-CS26, cantilever extension generally reduces the governing frequency, cable installation produces a diminishing vertical-stiffening effect, and the modeled 390 t form-traveler mass lowers both lateral and vertical frequencies. Field tests indicate that the measured frequencies at three stages deviate less than 9.6% from the FE values, confirming the reliability of the model. The revealed evolution laws and influencing mechanisms of dynamic characteristics during construction provide theoretical support for vibration control and safety assurance of similar bridges.