Response Analysis Of Long Span Transmission Tower Structure Under Tornado Wind Loading

A tornado is a rapidly rotating funnel-shaped column of air that spins with high speeds,which can cause extreme damages to buildings and infrastructures.The long span transmission tower structure is a vital part to power system,with characteristics of large numbers and wide distribution,causing it higher possibility of tornado attack than other import facilities such as nuclear power plants.The damage of transmission tower may result in the power failure even fire disaster to large areas,causing severe economic losses.Guidelines for electrical transmis-sion line structural loading from American Society of Civil Engineers have included tornado loading as extreme weather loading,which,however,is not considered in Chinese code for transmission tower design.In order to increase the safety of electical power system in tornado-prone areas,the research on behaviors of transmission tower structure under tornado loading is of great theoretical and practical significance.In this thesis,tornado static and dynamic loads(dynamic loads are due to the translation effect of tornado)were calculated by the method of Computational Fluid Dynamic(CFD)and the wind load formula in design code,with compar-ison of tower structure's responses under typical load cases.Firstly,the CFD simulation of tornado wind fields was conducted by using the commercial program FLUENT.The numerical model was built based on an experiment conducted by using a Ward-type vortex chamber.By comparing the numerical results to the experimental measure-ments and the Rankine vortex theory,the tangential velocity profile showed a good agreement,which demonstrated the validity of the numerical model of tornado.The numerical results were also compared to full scale data from the Spencer tornado of May 30,1998 to determine approx-imate velocity and length scale,which were then used to transform the reduced scale numerical model to full scale one.The proper length scale 4000 and velocity scale 60 were estimated to match the full scale numerical tornado wind field profile with the field measurements.Secondly,the tornado loads acting on transmission tower structure were calculated by the method of one-way fluid-structure interaction(FSI)and the wind load formula in design code.FSI analysis was adopted to simulate the tornado loads acting on the surface of the rigid model of transmission tower,and the resulting CFD pressures on the surface were then converted to concentrated nodal forces of the tower finite element model.A parametric analysis was con-ducted by varying the attacking angle of tornados relative to the tranmission line system(0°,450 and 90°).The results of the parametric study are used to assess the sensitivity of the members'peak forces and displacements with the parameters defining the location of the tornado relative to the line.The maximum axial forces calculated by the method of FSI were smaller than the code method,with the error ratio of peak axial force less than 20%and peak displacement less than 30%.Lastly,the time-history analysis of transmission tower under tornado dynamic loading is conducted by assuming an translation path for the tornado relative to the main tower.Two different directions for the tornado motion are considered,which formed the cases of translation motion parallel and perpendicular to the conductors,respectively.At each time step,the steady-state tornado wind field data were used to evaluate the instantaneous forces acting on the main tower,following by a time-history dynamic analysis to assess the typical dynamic responses.The dynamic variation of the tornado loading showed a peak period lasting about 10 seconds(tornado loads firstly increase then decrease).The peak value of tornado load had high value,causing some impact effects.And the dynamic response of transmission tower showed similar trends as the tornado loads time history.