Study On Wind Load Effects On Large-span Bridges Under Downburst

Downburst is a straight-line strong wind generated after local strong su bsidence airflow in thunderstorm clouds subduct to the ground.It has the ch aracteristics of non-stationarity,sudden change and destructiveness.Thunderstorm downburst and downburst-like wind occur in large area of China.Current Wind-resistance Design Specification for Highway Bridges in China(JTG/T3360-01-2018)and European Code,Eurocode 1: Actions on Structures(BS EN 1991-1-4:2005)only indicate the effects of stationary weather and t yphoon conditions,excluding extreme wind effects such as thunderstorms(downbursts)and tornadoes.Compared with the atmospheric boundary layer(ABL)wind field,downburst wind field has different wind spee d profiles and time characteristics.The characteristics of downburst wind and its effects on engineering structures have drawn many scholars’ attention in recent years.Although some achievements of downburst have been made at present,there are still some shortcomings.In the aspect of field measurements,it is difficult to obtain the downburst wind speed data through field measurements because downburst is mutant and has small spatial scale.There are few literatures on downburst field measurements.In t he aspect of downburst experimental r esearch,the existi ng downburst experimental researches mainly focus on buil ding structures and transmission line towers while experimental research on the effects of downburst on bridge structures has not been carried out.In the aspect of numerical simulation,the downburst hybrid stochastic model for calculating the wind load should be studied in combination with the measured d ata.In view of the above problems,this paper mainly focuses on the charact eristics of measured downburst wind,wind tunnel test of a bridge struct ure under the action of downburst wind and wind load simulation of a certain bridge under downburst wind.The research work is given as follows:(1)The empirical model of downburst wind characteristi cs and the theoretical calculation method of wind load w ere reviewed.A high-rise building structure in an existing literature was taken as an example.The dynamic responses of the structure under the action of simulated downburst wind load were analyzed,and the theoretical results were compared with that of t he ex-isting literature.The results show that the calculation results in this paper agree well with the results of the literatures,which verifies the correctness of the theoretical calculation of dow nburst wind load.(2)The characteristics of downburst w ind at sites of the Chishi Bridge and the Sutong Bridge were studied.Field measurements of wind speeds and wind directions at above-mentioned bridge sites were studied.The moving a verage method was used to study the mean and fluctuating wind characterist ics of downburst.Based on the measured downburst wind speed data,the time-varying mean wind speeds of downburst were extracted by wavelet transform(WT)method,and the modulation function with time was obtained.The Kaimal spectrum was taken as the targ et spectrum,and the turbulence wind speed at the bridge site was simulated by harmonic superposition method.The sim ulated fluctuation wind speeds were modulated and the time history of the s imulated downburst wind speed was obtained.Based on the enginee ring background of the Chishi Bridge,the finite element model of the bridge was esta blished by using ANSYS software.The static wind response and nonlinear bu ffeting response of the long-span cable-stayed bridge with maximum double cantilevers under both downburst wind field and ABL wind field were calc ulated,respectively.The results show that when the wind speed at bridge deck level under downburst is as the same as that under atmospheric boundary layer wind,the maximum response value of bridge girder end calculated by the time-varying mean wind speed is about 1.20 times than that calculated by 10-minute mean wind speed.The Root Mean Squre(RMS)values of buffeting responses of main deck end and pylon top of the bridge under downburst wind are larger than that of the bridge under ABL wind field,respectively,and the maximum ratio is about 2.8.(3)A horizontal wind speed simulation test device for downburst based on ABL wind tunnel was designed an d fabricated.The horizontal wind speed pr ofiles of downburst steady wind field were simulated by adjusting the vertical position and dip of the slop plate which was placed in the wind tunnel.The time characteristics of horizontal wind speeds of the downb urst were simulated by controlling the rotating speeds a nd angles of the two vertical plates in wind tunnel respectively.The results show that the horizontal wind velocity profile of the downburst simulated in the ABL wind tunnel by the downburst simulation device developed in present paper agrees well with th e empirical wind profiles of downburst.The time characteristics of the horizontal wind speeds of downburst are also simulated successfully with the assistance of the downburst simulation device in AB L wind tunnel.(4)The CAARC standard building experimen tal model with length scale of 1:200 was designed and manufactured.Wind tunnel tests of CAARC sta ndard model under the downburst transient wind field,downburst steady wind field and the ABL type B wind field were carried out,respectively.The effects on the responses of CAARC standard building model under different incoming wind velocities and wind yaw angles were studied.The displacements along xdirection(along short side)at the top of the mode l calculated by the simulated wind speeds and the horizo ntal wind speed obtained by the test were compared with that measured in the transient wind field test.The results show that the relative error of the maximum displacement along x-direction at buildi ng model top between results calculated by the test wind speed and the measured value under downburst transient wind field is 3.0%.And,the relative error of the maximum displacement along x-direction between results calculated by the theoretical simulati on wind speed and the measured value under downburst transient wind field is-14.8%.(5)Based on the auxiliary channel bridge of the Humen Bridge in Guan gdong,China,the aeroelastic model with length scale ratio 1:200 for the continuous rigid frame bridg e with maximum double cantilevers was designed and manufactured.Wind tunnel tests of the continuous rigid frame bridge with maximum double cantilevers were carried out under transient downburst wind field,steady downburst wind field and B type ABL wind f ield,respectively.The structural displacements of the bridge model under the three types of wind fields were analyzed and compared with each other.The results show that the maximal time-varying RMS values of the transverse displacement under tra nsient wind field of downburst are about 2.2-9.0 times of that of the bridge under B type ABL wind field.The aeroelastic responses of the maximum double cantilever continuous rigid frame bridge under downburst wind speed were calculated theoretically,and the the oretical results were compared with the experimental results.