Research On Nonlinear Effects And Mechanisms Of Self-excited Aerodynamics And Vibrations Of Typical Bridges

With the continuous increase in span length,wind-induced dynamic instability has gradually become a control factor restricting the development of super long-span bridges.Among them,vortex-induced vibration(VIV)and flutter are the two most critical self-excited vibrations that pose serious threats to structures and traffic safety.The key issues of VIV and flutter have entered the research of nonlinear effects,mechanisms and models.Starting from nonlinear effects,this dissertation investigates the aerodynamic nonlinear characteristics of long-span bridges and their effects on VIV and flutter performance using multiple methods,such as free and forced vibration wind tunnel test of sectional model,numerical simulation,and theoretical analysis.To clarify the mechanism for the significant VIV effect of the central-slotted box girder.Free vibration wind tunnel tests of sectional model involving the synchronous measurement of pressure distributions and VIV responses combined with computational fluid dynamics(CFD)are performed.The nonlinear evolution and hysteretic characteristics of the vortex-excited force(VEF)under the torsional VIV of central-slotted box girder are compared and studied.The increase of aerodynamic correlation between the upper and lower surfaces of the downstream box girder caused by the central slot can significantly enhance the overall VEF,which is an important reason why the VIV performance of central-slotted box girders is weaker than that of closed box girders.The phase lag between the VEF and torsional angle is extremely susceptible to the vibration frequency during the lock-in period,and the occurrence of VIV is related to the change of work power of VEF caused by the violent phase fluctuation.To study the nonlinear characteristics of flutter self-excited force(SEF)of bridges.A quasi-flat plate sectional model with an aspect ratio of 62.5:1 is taken as the simplified bridge girder section.Through wind tunnel tests of torsional forced vibration with single degree of freedom,the nonlinear effects of the flutter SEFs of the section under various initial angles of attack(Ao As),vibration amplitudes and incoming wind speeds are examined.The test results show that the nonlinearity of the self-excited lifting moment is stronger than that of the self-excited lift force for torsional motion.In a large amplitude range,it is verified that the main derivatives related to torsional motion are almost single-valued functions of the reduced frequency,which is independent of the vibration frequency.A three-order polynomial nonlinear SEF model considering the effect of motion amplitude under torsional motion with a single degree of freedom is established.To study the large-amplitude nonlinear bending-torsional coupling response features of long-span bridges during flutter,the bridge flutter control equation and energy analysis method based on instantaneous power balance are established by multiplying the mechanical balance equation with the velocity term,and the energy evolution of the whole bridge flutter process considering the nonlinear effect of amplitude is analyzed.Case studies of thin flat plate and closed box girder are carried out to validate the effectiveness of this method in the prediction of critical flutter state.The flutter mechanism of the two sections is analyzed from the perspective of energy,and the reason for the forward movement of the torsional center of the coupled flutter is revealed.Combined with the amplitude dependence of structural damping and flutter derivatives,the full-state evaluation of Humen Bridge from the critical state of linear flutter to the divergence of nonlinear flutter has been achieved,and validated by the free vibration wind tunnel tests of sectional model.To examine the influence of Ao A nonlinearity on flutter of long-span bridges,where the wind Ao A changes significantly.The flutter analysis of Lingding bridge is performed by 2D-2DOFs closed-form solution method and 3D multi-mode method based on the free and forced motion wind tunnel tests of sectional model.The influence of structural motion coordinate system on flutter derivatives is compared.The flutter performance of long-span bridges is susceptible to Ao A,and the influence of additional Ao A induced by static wind increases with increasing initial Ao A.For long-span bridges with dense torsional mode frequencies,more attention should be paid to flutter instability mode transitions due to mode competition at large Ao As.The flutter derivatives are insensitive to the structural motion coordinate system at large Ao As below 10 degrees.