Wind-resistance Research On Long-span Steel Truss Arch Bridges Under Construction

With the rapid development of the bridge technology,the span of steel truss arch bridges has exceeded 500 m.However,long-span steel truss arch bridges during construction are subject to weaker constraint than those in operation.With the proceedings of the construction,the cantilever length of the main girder and main arch rib elongates,resulting in the decrease of overall stiffness of the structure and increasing the risk of wind-induced vibration.Therefore,it is of great need to assess the wind resistance performance of such long-span arch bridges under the most unfavorable construction state.The Guangzhou Nansha Mingzhu Bay Bridge with a span arrangement of(94.8+164+436+164+96+58.8)m is the longest steel truss arch bridge under construction.The bridge is located in a strong wind area along the coast of Guangzhou,which is characterized by high wind speed as well as high probability in typhoon occurrence.These characteristics make it more vulnerable to wind excitations.Taking the Mingzhu Bay Bridge as project background,the current paper performed a systematic investigation on the dynamic characteristics and wind resistant performance of bridge under construction state with the aid of the numerical simulation technique,the theoretical analysis and the wind tunnel test.The main research contents and conclusions are as follows:(1)Seven typical construction states of the bridge were chosen according to the construction process.The finite element models for the arch bridge under each construction conditions were established using ANSYS,and the dynamic characteristics were also investigated.The results show that as the length of the girder and arch cantilever increases during construction,the frequency of the bridge generally shows a downward trend;for the same cantilever construction length,the vertical bending frequency of the bridge will increase when the buckle cables are applied,while the transverse bending frequency and torsion frequency seem independent of the cables.Hence,the most unfavorable construction state should be the state when bridge with the maximum cantilever length,when the maximum single cantilever length is 213 m.The corresponding first-order horizontal bending frequency and first-order vertical bending frequency are 0.329 Hz and 0.432 Hz,respectively.(2)The buffeting calculation method proposed by Davenport was utilized to perform buffeting analysis in time-domain.Meanwhile,the effects of three arrangements of wind-resistant cable on vibration reduction were studied.The maximum buffeting displacement of the main girder and arch rib occurred in the condition(2)having the maximum cantilever length of main girder,where and the vertical peak displacement of the cantilever end of the main girder reaches 47.3cm,the peak displacement of the cantilever end of the arch rib is 45.7cm.Then,three wind-resisting cable arrangements were proposed,namely option 1,2 and 3.Option 1,2 and 3 have the same arrangement with two wind-resistant cables crosswise.One end of the cables for both 3 options are all anchored to the top of the arch-foot pier,while the other end of the cable for Option1 is stretched on the lower chord at the 1/2cantilever of the main girder,on the lower chord of the cantilever end of the main beam for Option 2 and on the lower chord at the cantilever end of the arch rib for Option 3.For working condition(2),the use of anti-wind cable scheme 3 can effectively reduce the vertical displacement of the main girder,and the peak value is reduced from 47.3cm to 35.1cm,which is about 25%.The effect on the displacement of the transverse and forward bridges is small,but it can’t amplify the displacement of the transverse bridge.(3)Aerodynamic coefficients of the buckle cable section were obtained through CFD simulation.Based on the Den Hartog quasi-steady galloping criterion,the galloping performance of the three cable sections was studied.Results indicated that the A-type cable(The height is 0.228 m and the width is 0.228m)has the risk of galloping occurrence under wind attack angles near-4° and 7°,while the B-type section(The height is 0.311 m and the width is 0.228m)and C-type section(The height is 0.228 m and the width is 0.311m)have little possibility of galloping occurrence at the range of wind attack angles(-12°～+12°).Therefore,it is recommended to use B-type and C-type buckle cables in practice,and A-type buckle cable should be avoided.(4)Based on the most unfavorable condition(2),a 1:100 aeroelastic model was manufactured to carry out wind tunnel tests to test the aerodynamic performance of the bridge during construction period,and the experimental results were compared with the finite element simulations.It is found that: the buffeting response with wind deflection angle is relatively smaller than that of 0 degree wind deflection angle;under the design wind speed at 0° wind attack angle,the root mean squares of vertical displacements for the cantilever end of the arch rib and the main girder are 53.6mm and 47.1mm,respectively.The analysis results showed that the structural strength and stability meet the safety requirement.The test displacements are lower than the FEM results(132mm and 128mm),Through further verification,it is found that the main reason is that the damping ratio of the test model is too large.In this paper,a long-span steel truss arch bridge under construction is taken as the background,the wind resistance performance changes during the construction period are studied.The proposed wind resistant cable scheme can reduce the maximum buffeting displacement by about 25%.The research results can provide reference for similar projects.