Study On Dynamic Responses Of Bridges With Twin Circular-cylinder Piers Subject To Water Flow

The Dalin Bridge across the Brahmaputra river in Tib et,is a pre-stressed concrete bridge continuously supported with several twin-circular cylinder piers,it’s span arrangement is 35m+5×30m+35m.In July 2018,the bridge piers and the deck showed significant vibration under the action of current.The vibrat ion direction of the bridge deck is perpendicular to the incoming flow,which is a typical fluid-structure interaction problem.Although a large number of studies have been carried out on the vibration of flexible structures under the action of wind and cu rrent,reports and studies on the flow induced vibration response of large-rigidity and large-mass concrete beam bridges such as Dalin bridge are very few.The current research on pier and bridge under the action of water flow is mostly based on scale mode l tests and numerical simulations,and the content is mostly about the dynamic water pressure,scouring,characteristics of bridge itself and so on.In order to analyze the reason of vibration,this thesis carried out a field measurements and numerical sim ulations of dynamic response of the Dalin Bridge.The vibration mechanism is analyzed and the influences of the damping ratio,the center distance of the piers and the scouring depth on the flow field and the flow induced vibration are also discussed.The main work and conclusions are as follows:(1)The effect of water current on bridges and the causes and effects of flow induced vibration is reviewed.The current research of flow around cylinder/twin cylinders and fluid structure interaction problems of tande m cylinder are reviews and introduces.Combined with the flow induced vibration of the Dalin bridge in Tibet,the research background,significance and contents of this thesis are given.(2)The field measurement was carried out in the flood season of Brahmapu tra river,and the vibration time history of the bridge is collected by accelerometers and laser displacement meters when the flow velocity is 4m/s.The measure data show that longitudinal vibration of bridge deck is a beat vibration dominated by its funda mental mode,the longitudinal maximum acceleration is about 0.08m/s~2,acceleration RMS is0.016m/s~2,maximum displacement is about 1.81mm.While the lateral vibration is random vibration,its maximum acceleration is about 0.05m/s~2,acceleration RMS is0.009m/s~2.From the power spectrum,it can be found that the longitudinal natural frequency of the bridge is 0.91Hz,lateral natural frequency of the bridge is 1.22Hz,which is in good agreement with the calculation results of ANSYS.Using Stochastic Subspace Identification(SSI)method to analyze the time history data of longitudinal bridge,the identification results of damping ratio is 0.34%～3%,which is discrete.(3)The two-dimensional SDOF vibration equation of the piers twin cylinder along the longitudinal direction under the action of water flow is established.Based on computational fluid dynamics,the k-ωSST turbulence model was used in Fluent to perform a two-dimensional numerical simulation of the double cylinder for flow velocity between 2 and 10m/s(U_r=1.69～8.45、Re=2.6×10~6～1.3×10~7).The relationship between displacement of bridge pier and flow velocity under three damping ratios ofζ=0.01,0.02 and 0.03 are obtained.The results show that the Vortex-induced vibration is observed from flow velocity between 3 and 6m/s in all three conditions.With the increase of damping ratio,the maximum amplitude of vortex vibration becomes smaller but the lock-in range is basically unchanged,and when U_r=3.6,the amplitude of VIV reaches the maximum value.The results also indicate that the increased lift forces is found on downstream cylinder due to interference effect of upstream cylinder.At the same time,the relationship between the displacement of the pier and the flow velocity under different damping ratios was calculated.The results show that the vortex-induced vibration of a double cylinder is observed under all three damping ratio conditions.With the increase of the damping ratio,the maximum amplitude of the vortex vibration becomes smaller,but the lock-in range is basically unchanged.Considering piers own three-dimensional vibration mode and assuming the distribution of the velocity is inverted triangle,the two-dimensional results are modified by three-dimensional effect.It is found that When U=4m/s,ζ=0.01,the amplitude of VIV is 1.60mm,which is in good agreement with the measurement.In addition,due to the wake effect,the amplitude of the vortex excitation force of the downstream cylinder is 2.2 times that of the upstream cylind er,which also has a significant enhancement effect on the overall vortex induced vibration of the twin cylinder pier.Furthermore,the influence of the center distance between two cylinders on the piers displacement is studied.The result indicate that wh en the center distance increases,the bridge pier displacement response will decrease.When vortex vibration occurs,the structure of the wake flow is similar under the three conditions of the center distance L=4.6m,4.8m,and 5.0m(L/D=3.54,3.70,3.85).(4)The long-term effect of water flow on bridges may cause scouring problem at the piers and abutments,so the HEC-18 formula proposed in the AASHTOLRFD is used to predict the scouring depth at the pier.It is found that with the increase of scouring depth,the displacement of the bridge pier will increase sharply,which has a very negative impact on the bridge.(5)The three-dimensional simulation of double columns of piers is carried out by Fluent,and the lift force and displacement response of piers under sev eral specific velocity are discussed in detail.The results show that the vibration response of the bridge pier under the three-dimensional simulation has strong three-dimensional effect,and the lift component has multiple frequencies which is complex.Wh en U_m=6m/s,vortex induced vibration occurs on the pier.Due to the distribution of the velocity of the flow,the locking region of vortex induced vibration obviously moves backward.The three-dimensional vortex structure is more complex,so does the flow field structure.With the increase of flow velocity,the fluid will gradually fall off behind the upstream cylinder and form a vortex,and the vortex shedding velocity behind the downstream column will be faster and the shedding velocity will change along the cylinder.