Research On Dynamic Response And Collapse Of Power Transmission Tower-line System Under Coupled Horizontal-rocking Earthquake Ground Motion

Highly flexible structures such as transmission towers have large heights and low lateral stiffness.The rocking component of the ground motion has a great influence on it.Therefore,in the seismic design,ignoring the effect of the rocking ground motion may lead to great security risks to the transmission tower-line system.While in existing studies on seismic response and collapse of transmission tower-line systems,the rocking component is rarely considered.One reason is that it has not received enough attention,but the more important reason is due to the lack of actual record of rocking component by seismograph.Aiming at above problems,this paper studies the extraction of the rocking component of earthquake ground motion,the dynamic response of the transmission tower-line system and the collapse mechanism under the coupled horizontal-rocking component of earthquake ground motion.Main research contents and corresponding conclusions are as follows:(1)Based on the spectral ratio method proposed by Graizer and the newly introduced frequency spectrum similarity between rocking displacement of earthquake ground motion and vertical velocity of earthquake ground motion,a new method for extracting the rocking component of ground motion using multiscale wavelet transform and threshold processing is established and the rocking component mixed in the original horizontal seismic record of the 1994 Northridge earthquake in Pacoima site is extracted.The residual tilt of the extracted rocking component is basically consistent with the actual records observed by California strong earthquake observation team after the earthquake.Besides,based on the two-point difference theory and multi-dimensional shaking table test,the feasibility and accuracy of this new extraction method are further verified.(2)A three-dimensional shaking table test of the three-tower-two-line system scale model under the excitation of coupled horizontal-rocking component of earthquake ground motion and other excitation conditions is conducted,and the results show that the rocking component will significantly amplify the seismic response of the structure.Moreover,due to the asymmetry of the rocking displacement of the ground motion,the structure produces an asymmetric additional P-Δ effect,which leads to an asymmetric offset of the tower top horizontal displacement timehistory curve.(3)The longitudinal and lateral nonlinear vibration equations of the transmission tower line system under the excitation of coupled horizontal-rocking component of earthquake ground motion are derived.In the equations,the total P-Δ effect of the structure is divided into the conventional P-Δ effect and the additional P-Δ effect caused by the rocking displacement.Besides,two dynamic response calculation theories with and without considering the additional P-Δ effect caused by the rocking displacement are proposed and the dynamic response is calculated.By comparing with the shaking table test results,the equations derived in this paper which consider the additional P-Δ effect caused by the rocking displacement is verified.(4)The criterias for determining member failure and structural collapse are discussed,and a member failure control program is written based on VUMAT,ABAQUS user material subroutine.Based on ABAQUS nonlinear dynamic timehistory analysis and the extracted rocking component of 1994 Northridge earthquake ground motion,the collapse process of the ZC27153 large-span fourtower-three-line system under the excitation of coupled horizontal-rocking component of earthquake ground motion as well as horizontal earthquake excitation is simulated,and the influence of rocking component on collapse mechanism of transmission tower-line system is fully discussed.Indexes such as the failure of the members and local deformation of the tower,the interlayer displacement angle,and the relative displacement of the tower top indicate that the collapse of the transmission tower-line system under the excitation of coupled horizontal-rocking component of earthquake ground motion is significantly more serious,and the rocking component exacerbates the collapse of the transmission tower.(5)Under the excitation of coupled horizontal-rocking component of earthquake ground motion,the structure produces an asymmetric additional P-Δ effect due to the asymmetry of the rocking displacement.This effect continues to accumulate,which ultimately leads to the overturning collapse of the transmission tower-line system,while no overturning collapse occurs under pure horizontal earthquake excitation.