Research On Flutter Characteristics Of Double-deck Truss Girder Suspension Bridge With Long Span And Four Main Cable

Due to the high flexibility and low damping ratio,long-span suspension bridges are very sensitive to wind load,flutter stability is the primary problem to be solved in wind-resistant design.With the increase of traffic volume and the improvement of carrying capacity demands,double-deck truss girder with four cable is widely used in the design of long-span suspension bridges.Compared with single-deck truss,the flow characteristics of double-deck truss girders are more complex.Meanwhile,the difference of flutter characteristics between four cable system and common double cable system also needs to be carefully investigated.Therefore,it is of great value to study the flutter characteristics of a typical suspension bridge with double-deck truss girder and four cables.Taking a long-span suspension bridge with double-deck truss girder and four cables as engineering background,the following research work was carried out through wind tunnel test,finite element analysis and CFD numerical simulation method:(1)By the segmental model wind tunnel test,the flutter stability of the double-deck truss is tested,and the effects of central slotting,vertical stabilizers,horizontal flaps and fairing on the aerodynamic stability of double-deck truss girder are studied.Through comparing the effect of aerodynamic measures with that of single-deck truss girder,the combined pneumatic measures to improve flutter stability of double-deck truss are recommended.(2)Finite element models corresponding to different cable systems of a long-span double-deck truss suspension bridge are established respectively,and the effects of different cable systems on flutter stability of suspension bridges were studied by dynamic characteristics analysis and a 3D flutter analysis method.By analyzing the differences of the results among the cable systems,it can be seen that increasing the cable distance in a certain range can improve the critical flutter wind speed of long-span double-deck truss girder suspension bridge,while increasing the rise-span ratio of cable will reduce the critical flutter wind speed.(3)Using CFD method to analyze the wind characteristics of canyon terrain,defining a wind velocity amplification coefficient to description the relationship between the wind speed distribution along bridge axis and the inlet boundary,and applied it to the 3D flutter analysis of long-span suspension bridge,to explore the effect of canyon topography on aerodynamic performance of four cable truss girder suspension bridge at different construction stages.Compared with the flutter analysis results in uniform flow,In the canyon wind field,the flutter critical wind speed is higher when the stiffening girder is 18% to 77% completed,while the flutter critical wind speed is lower when the stiffening girder is 88% completed to the finished bridge state.It can be seen that for the canyon terrain,the uneven distribution of wind speed along the bridge axis will have a different influence at different construction stages of the truss girder.(4)By using CFD numerical simulation method,the initial sections and optimized sections by aerodynamic measures of single-deck and double-deck truss girder were analyzed respectively to obtain the air flow around the stiffening girder,so as to reveal the microscopic mechanism of aerodynamic measures affecting the flutter stability of the sections.It is found that the aerodynamic measures can suppress the large aerodynamic lifting moment and increase the vertical pulsating lifting force,thus changing the energy distribution between different degrees of freedom to improve flutter stability.