Method For Determining The Critical Wind Speed Of Long-span Suspension Bridges Under Post Flutter

According to the classic linear theory of bridge flutter,once the critical wind speed is reached,then a fatal structural failure occurs on girder with a divergent self-exited vibration,thus the critical flutter wind speed is easy to determinate.However,several wind tunnel tests have confirmed the existence of nonlinear post-flutter LCOs(Limit Cycle Oscillation)of bridges recent years,in these cases the critical wind speeds become indeterminate due to the nondestructive limited amplitudes,and there is no accurate method or uniform standard to estimate the critical wind speed of nonlinear post flutter.In this paper,a method of Post-flutter Pushover Analysis based on wind tunnel test using seismic Pushover Analysis was proposed.On the basis of elastic-plastic limit of structures during nonlinear vibration,precise critical wind speed and amplitude threshold of a long-span bridge under post-flutter LCOs were evaluated through the method of nonlinear static elasto-plastic analysis,the critical collapse behavior of the bridge was investigated.In order to realize this numerical method,the following work were carried out in this paper:1.Using the Pushover Analysis method in seismic engineering for reference,the possibility of applying pushover analysis method on post-flutter limit state of bridges was proposed.Combined with the wind-induced characteristics of long-span bridges,an improved Pushover Analysis for post flutters was proposed considering the similarities and differences between structural behavior under post flutter and earthquake.Further the detailed calculation steps were given and a corresponding analysis program was compiled base on finite element software ANSYS.2.Taking a long-span suspension bridge as engineering background,a series of sectional model tests on a typical closed-box bridge deck were conducted,in which the typical coupled bending-torsional post flutter LCOs were observed,further the typical post-flutter characteristics were investigated to obtain key parameters of post-flutter Pushover Analysis calculation.3.Based on the elastic-plastic limit of bridge structures,the criterion of post-flutter critical wind speed was modified.Then a numerical example was calculated to evaluate the critical wind speed of a long-span suspension bridge according to the result of nonlinear static analysis,the figures were more precise compared with specification,meanwhile the failure mode and weak links of the bridge were predicted.