Experimental Study Of Nonlinear Vibrations Of Long-span Cable-stayed Bridge

Nonlinear vibration in long span cable-stayed bridge is one of the hottest topics in civil and mechanical engineering. Long span cable-stayed bridge is a combination of beam, pylon and cables. The pylon is relatively rigid compression element, the beam is relatively semi-rigid bending element, and cables are flexible tension-only elements. There are significant differences in mechanical properties and strong mutual coupling resonance behaviors, especially the cables are prone to large amplitude nonlinear vibrations. In order to explore the mechanism of the coupled vibration in cable-stayed bridge, large amplitude nonlinear vibrations of cables and other related issues, statistical analysis, theortical research, finite element method and experiments are carried out in this paper. The main contents are as follows:The mechanical parameters of girders and cables in cable-stayed bridges are statistical studied to ascertain their distribution laws and statistical relationships among those parameters. Dozens of domestic cable-stayed bridge construction drawings were collected with mainspan larger than 300 m. Mechanical parameters of girder and cables are studied to found the well-obeyed distributions and statistical relationships between those parameters. The 95% confidence intervals of those various parameters are found. That work provides practical basis for further theoretical analysis and experimental design. Research has shown that the parameters can well obey certain statistical distribution. For the girder, different material has obvious different relative axial stiffness and weight-per-metre, while not significantly different relative vertical and horizontal bending stiffness. The longer the span is the smaller the stiffnesses and weight-per-metre would be. Thus, structure is more soft featured which make it easier to produce a sharp vibration. The girder stiffnesses and weight-per-metre are disproportion among each other as change of span. The increase of cable-stayed bridge spanning capacity mainly relies on cable support system. However, just relying on this system would face the shortages of axial compression strength and stability problems of girder. To further enhances spanning capacity need to adopt new structure systems.Nonlinear equations of a cable with both ends of three directions excitations were set up. Based on the equations and combine with common modes in long span cable-stayed bridge, the nonlinear vibrations of longitudinal drift vibration, girder lateral excitation and level excitations on both ends are studied respectively. Research has shown that a small longitudinal girder drift can effectively reduce vibration, while fully consolidated or large amplitude longitudinal drift would sharply increase it. In the view of nonlinear vibration reduceing, large span cable-stayed bridge should designed as floating or half-floating system as well as reasonable longitudinal restraint stiffness of girder. Forced vibration and parametric excitation are two basic incentive modes, if sums of excitations along axial component and horizontal component remain the same; responses of cable would remain the same.The nonlinear vibration characteristics of free and forced vibration of a cable are experimental studied. Gravitational effect makes the first in-plane frequency slightly large than the out-of-plane one, however, it has little effect on the second order frequencies. Nonlinear jumping process of cable was directly observed. Spatial movement patterns and laws of this process were analyzed. Results show that: three stages were undergoneing when jump happens, rather than merely a sudden change in amplitude. Meanwhile, resonance inconsistency of in-plane and out-of-plane was observed.Many coupled vibrations of cable and beam were observed by experiment. The in-plane frequencies are larger than corresponding out-of-plane frequencies at the affect of the beam, however, this effect were never took into consideration in the tension measurement. Coupling vibrations and energy transfers happen between different modes of cable beam structure in both free and forced vibrations. There is a certain time lag between the vibration of beam and cable. In-plane vibrations of beam deviate from the central line at the effect of structural nonlinear vibration.An elaborate cable-stayed bridge experimental model was designed. Many nonlinear coupling vibrations among the beam, pylon and cables were observed. Three forms of vibration, namely, the overall vibration, partial vibration and full-bridge coupled vibration would occur at different excitation frequencies. It was observated from the experiment that adjacent cables were about to collide with each other. A new form of nonlinear resonance with full-bridge complex couple was found. The cables, beam and pylon were in large amplitude vibrations as the resonance occurs. Two response frequencies were aroused by an excitation frequency. The sum of the former two frequencies is exactly equal to the latter one. Further sweeping experimentes were executed, it was found that the larger amplitude vibrations of beam and pylon would change the nonlinear vibration characteristic of some cables, making it became soft spring from hard spring.