Study On Dynamic Characteristics Of Kaifeng Po Pagoda

Po pagoda in Kaifeng is a hexagonal brick tower;as the typical representative of masonry pagoda in China transiting from square solid tower to octagonal hollow tower,it implicates a quite unique architectural structure.Founded in AD 974,suffering from thousands of years' severe test of wind and rain,as well as over 60 times violent and slight earthquakes,it still stands upright in the ancient capital city of eight dynasties,Kaifeng,under well-preserved conditions.In the north of Po pagoda,250 m away lies the Longhai Railway.The daily frequent passing by of trains makes vibration effects on Po pagoda.At present,the research on dynamic characteristics of Po pagoda is limited,therefore it has great significance to make in-depth study on its dynamic characteristics.The purpose of this thesis is to research on the dynamic characteristics of Po pagoda,including its natural vibration properties,seismic resistant performance and the vibration effects by trains,in order to provide references for the future protection work on it.Through site investigation and consulting related references,the parameters of pagoda itself is settled.Then numerical simulation method is applied to make modal analysis,response spectrum analysis and time history analysis on the pagoda.The main content of this thesis as follows:(1)Static analysis and modal analysisThrough the data comparison between field measurement and reference records,the constructional dimension of Po pagoda is confirmed.The software ANSYS is applied to build up the finite element analysis model of Po pagoda,make static analysis and modal analysis,and obtain its natural vibration period and vibration mode.(2)Response spectrum analysisThe internal force and displacement response of Po pagoda under 7 degree frequent earthquake,fortification earthquake,rare earthquake are analyzed.And the seismic response of Po pagoda after inputting response spectrum in X(east-west)direction,Z(north-south)direction and Y(vertical)direction is analysed.(3)Time-procedure analysisThe seismic responses of Po pagoda are calculated respectively,under three conditions of minor,medium and major earthquakes;three kinds of seismic waves of El-centro wave,Taft wave,Shanghai artificial earthquake wave;and two directions of X(east-west)direction and Z(north-south)direction input,in total 18 conditions.Meanwhile,the seismic responses of Po pagoda are also calculated,under major earthquake and three kinds of seismic waves on Y(vertical)direction input.The seismic responses under minor,medium and major earthquakes but same wave are compared;the seismic responses under three kinds of different seismic waves but same design intensity are compared;and the outcomes of response spectrum analysis and time history analysis are compared.(4)Analysis on vibration effects on Po pagoda by trains passing by on the Longhai railway.The main conclusion of this thesis as follows:(1)The seismic responses of Po pagoda on two horizontal directions share no major discrepancies,indicating the stiffness of pagoda displays evenly on the horizontal direction;the displacement of structure increases as the raising of height,the displacement on the spire and doorway is larger;the second floor and spire of the Po pagoda are weak floors because of obvious decline on quality and stiffness;under major earthquake,horizontal seismic waves bring severer damages on Po pagoda,compared to vertical seismic waves.(2)Under minor earthquake,the main damage of Po pagoda is the tensile damage on the bottom of the first floor;under medium earthquake,tensile damages are seen on wall of the first floor and part wall of the second floor,tensile and compression damage appear on doorways;under major earthquake,tensile damages appear on walls of the first and second floor,the tower eaves of the first floor and junction of spire and the third floor,the most severe damage are seen on spires and then doorways;different seismic waves respectively lead to different seismic responses of Po pagoda,the closer the predominant period of seismic waves are to the natural vibration period of pagoda,the greater the seismic response is;for Po pagoda,the second floor and spire are the easiest to be damaged;the response spectrum analysis is consistent with time history analysis.(3)The running of trains has relative slight vibration effect on Po pagoda,the peak acceleration occupies only 1/10 of that under minor earthquake,the causing maximum displacement response is only 1/100 of that under minor earthquake;however,the vibration velocity of spire and top of the third floor both surpass allowable vibration velocity,so partial vibratory fatigue generates on the top of Po pagoda if trains pass by frequently.