Interference Effects And Mechanism Study Of Twin Blunt Box-Section Girders On Aerodynamic Force

In recent years,with the highway construction to the mountain areas,continuous rigid frame bridge of large-span and high-pier is widely used.The section of this kind of bridge is single box single room with straight web.As it is a better choice to plan two respective bridge in high way construction,more and more twin blunt box girder bridges are being built in practice.The aerodynamic interference effect can't be neglected under the action of wind owing to small spacing between two boxes.With the increase of bridge span and piers height,this interference effect becomes one of the key problems in wind-resistant design of bridges.In this dissertation,the interference effect and mechanism of aerodynamics of parallel double-bluff body-box girder is reviewed in detail.Then the interference effects of aerodynamic coefficients under different spacing of 3 aspect ratios cross section of H/B(H is the height of box girder.B is the width of box girder)are studied by means of wind tunnel test and numeral calculations.Another variable to be studied is D/B(D is the clearance between two box girder).At last,two results of aerodynamic coefficients are compared to explain interference effects.The main work of this dissertation is as follows:1.Interference factor is defined as aerodynamic coefficients of double-bluff body-box girder to aerodynamic coefficients of single box girder ratio.When interference factor is smaller than 1,it means a reduced effect of aerodynamic coefficients.When interference factor is bigger than 1,it means an increased effect of aerodynamic coefficients.2.The interference effect of upper box girder is reflected in the following aspects:(1)Interference effect can be ignored when D/B?1.5.(2)The interference of drag coefficient and torque coefficient are increase effect.(3)The lift coefficient interference effect of upper box girder is closely related to H/B.Interference is reduced effect when H/B=0.3,however it is increased effect when H/B=0.4?0.6.3.The interference effect of downstream box girder is reflected in the following aspects.(1)The interference of drag coefficient?lift coefficient and torque coefficient are obviously decreased effect:(2)The reduced effect of drag coefficient become more significant with D/B's reducing and H/B's increasing.(3)The reduced effect of lift coefficient and torque coefficient is not obvious with D/B's reducing when D/B?1.5,but it is more significant with H/B's reducing.The reduced effect of lift coefficient and torque coefficient is not obvious with H/B's reducing when D/B>1.5,but it is more significant with D/B's reducing.4.There are obvious vortex between two box girder when the spacing is smaller.And the strength of vortex weakened as D/B grows.This causes the pressure of upper box's web and wing board becoming smaller than single box and the pressure of downstream box's web and wing board getting much smaller than single box.This can give rise to drag coefficient and torque coefficient of upper box increasing and drag coefficient and torque coefficient of downstream box are negative.The smaller D/B is,the bigger absolute value is.5.The windward side wing board of single box is under pressure by wind,and the roof of box is strongly affected by wind suction.The windward side wing board of downstream box is affected by wind suction,and the wind suction of downstream box's roof is not as strong as single box..The change of wind effect at these two part is the reason for the reducing of downstream box's lift coefficient.