Comprehensive Optimization For Wind Barriers On Driving Safety And Flutter Performance Of Suspension Bridges With Steel Box Girders

Wind barriers belong to the subsidiary structure of the bridge,which can be connected with the main girder and placed outside the marginal guardrail,but most of them are directly combined with the marginal guardrail to form a protective whole,so as to increase the height of the marginal guardrail or reduce the porosity.Wind barriers can effectively shield the approaching flow and reduce the wind loads of vehicles.On the contrary,when wind barriers are added,the box girder displays bluff characteristics more,leading to deterioration of the aerodynamic stability.Therefore,it is necessary to optimize the form of wind barriers by comprehensively considering the driving safety and the flutter performance.In the western mountainous area,the angle of attack of approaching flow is large,so its applicability in mountainous area needs to be improved.In this dissertation,such a long-span suspension bridge with steel box girder is taken as the research object,and the following research is carried out:Firstly,the aerodynamic stability of bridge structure is concerned.The flow field characteristics and flutter stability of the suspension bridge with a box girder are studied by CFD numerical simulation,and the flutter mechanism at different angles of attack is explained from the perspective of energy input.Secondly,the safety of vehicles on bridge is concerned.The effects of the wind barriers on the wind field above the bridge deck,the aerodynamic coefficients of the vehicles and the driving safety are studied,and the evaluation index of the wind protection effect of the wind barriers is proposed.The results show that the blunt body characteristics of the cross section of stiffening girder are enhanced after reducing the porosity of the bridge deck protection system,and the approching flow is seriously blocked by the wind barriers,and a vortex is formed behind it.With the increase of the attack angle,the transversal wind speed above the deck decreases.When the height of bridge deck protection system is increased,the windproof height is higher,but the windproof effect at low altitude is not as good as the case of reducing porosity.After installing the wind barriers,the side force coefficient and rollover moment coefficient of the truck are significantly reduced,the critical wind speed of the driving increases,and increases with the increase of the attack angle.This is because the wind barriers have a small shielding effect on the aproching flow at the negative angle of attacks,and the side force coefficient of the vehicle is relatively large,and the most unfavorable attack angle is-5°.Thirdly,the effects of vehicle windproof measures on aerodynamic stability of bridge are studied.The aerodynamic force and flutter stability of the bridge with wind barriers at different angles of attack are studied,and the most unfavorable condition is determined as the index to evaluate the wind resistance performance.The results show that the effect of increasing the height of the bridge deck protection system on the flutter performance of the bridge is very limited.Reducing the porosity of the bridge deck protection system has little effect on the flutter performance at-5° angle of attack,but the flutter performance of the bridge is greatly reduced under 0° and 5° angles of attack.Finally,based on the uniform design method and RBF neural network,the windproof ability and wind-resistant performance of a large number of wind barriers schemes are predicted.On the basis of meeting the flutter test wind speed,the maximum critical wind speed of vehicle driving is found,and the wind barriers type that can give consideration to both driving safety and flutter performance is obtained.The final scheme is: porpsity is 62% and the height is 2.85 m.After installing the wind barriers,the critical flutter wind speed of the optimized bridge is lower at 0° and positive angles of attack,but it is still higher than the flutter test wind speed.However,the critical wind speed of vehicles increases obviously,especially at-5° angle of attack.On the whole,the windproof ability is significantly improved.