Study Of Flutter And Buffeting Response Of Long-span Asymmetric Suspension Bridge Under Downburst

As a large-span flexible structure,the suspension bridge is more sensitive to wind loads as its span increases.Therefore,studying its vibration response is particularly important.In recent years,various types of asymmetric suspension bridge structures have emerged due to the demand for large-span bridges in complex terrain areas such as mountainous valleys,and the changes in their dynamic characteristics have affected the vibration response of bridges.In order to better study the effect of asymmetric structure on wind-induced vibration response,this paper takes main cable-supported asymmetric suspension bridges and edge-span nonsymmetric suspension bridges as research objects.Based on the energy principle,the Galerkin method was used to derive the approximate calculation formulas for symmetric vertical bending and torsion mode frequencies of non-symmetric suspension bridges.The flutter stability of different types of non-symmetric suspension bridge structures was analyzed under benign wind and transient buffeting wind based on the three-dimensional flutter full modal frequency domain method and time-domain method.The vibration response of the two types of nonsymmetric suspension bridges under two different wind loads was analyzed in the time domain based on the transient aerodynamic model of the bridge.The main research content and conclusions are as follows:(1)The Galerkin method based on the energy principle derived estimation formulas for the fundamental frequencies of vertical bending and torsion,both symmetrically and asymmetrically supported suspension bridges with main cable height differences and edge span differences.The influence factor of asymmetrical suspension bridges was defined and the dynamic characteristics mechanism was investigated.The universal applicability and correctness of the Galerkin method’s fundamental frequency estimation formula for asymmetric suspension bridges were confirmed by comparing it with theoretical solutions in the "Design Code for Anti-Wind of Highway Bridges."(2)By using finite element analysis,we built asymmetrically supported suspension bridges with main cable height differences and edge span differences ranging from 0 to 40 meters.The dynamic characteristics of various asymmetrical suspension bridges were analyzed,and the results showed that compared with symmetrical suspension bridges,the natural frequencies and bending-torsional frequencies of the asymmetric suspension bridges gradually decrease with the increase of asymmetry.(3)Using a three-dimensional flutter full modal frequency domain method and time domain method,the vibration response of different asymmetric suspension bridges with different main cable support heights and edge spans under normal wind and gusty winds were analyzed.The research shows that under normal wind conditions,as the height of the main cable support increases,the critical wind speed for flutter decreases,and the flutter critical frequency gradually decreases,thus reducing the vibration stability and overall stiffness of the bridge.As the difference in the edge span increases,the critical wind speed of the non-uniform edge span asymmetrical suspension bridge gradually decreases.When the difference in the edge span is small,the non-uniform edge span asymmetrical impact factor has little effect on the vibration stability of the suspension bridge,and its effect on structural stiffness and vibration stability of large-span suspension bridges can even be ignored.(4)Using a transient aerodynamic model of the bridge,we analyzed the flutter response of different asymmetric suspension bridges under normal wind and gusty winds in the time domain.As the height of the main cable support and the edge span changed,the flutter displacement response of suspension bridges also varied.The maximum vertical displacement,torsion angle,and lateral displacement occurred at the midpoint of the stiffened beam of the asymmetric suspension bridge.The greater the height difference between the main cable supports,the larger the displacement response at the midpoint of the stiffened beam;the torsion angle basically remained unchanged as the height of the main cable supports increased.Under gusty wind conditions,the flutter results at the mid-span were significantly larger than those under normal wind conditions,and the vertical displacement at the mid-span increased with increasing main cable support height and edge span.