Research On Multiple Pounding Responses And Characteristics Of Bridges Under Near-fault Vertical Ground Motions

With the innovation of encryption techniques and monitoring means of the seismic monitoring network, more monitoring data of near-field earthquakes show that acceleration of the vertical component exceeds its excepted value. Moreover, some bridge earthquake damages, such as circular cracks, bearing fracture and pier partial damage, have been found in Kobe earthquake, Chi-Chi earthquake and Wenchuan earthquake, which cannot be explained by only the traditional horizontal earthquake component. However, these bridge damages may be more reasonablely explained by vertical ground motion.Under strong vertical ground excitations, bridge girder may separate from the supporting bearing and vertical poundings may take place when the girder falls back in contact again with the bearing. However, since dynamic responses of bridge are related to variable contact topologies, pounding wave effect, pier partial damage and nonlinear effect, it is difficult to perform theoretical and numerical studies on multiple vertical poundings.Due to the fact that direct site observation is almost impossible in an earthquake. Evaluations can only be made based on bridge damages and surmise after the earthquake. Even though, the evaluations are still debatable due to the lack of theoretical and numerical results and existence of various interpretations. The necessary reliable analysis results obtained by theoretical methods are rare, and there is no much of fundamental research on vertical seismic response of bridges. Compared to horizontal seismic response of the bridge, the study of vertical seismic response of bridge is rare, lack of the basic understandings of vertical seismic responses, and even lack of the basic theoretical research methods.In this thesis, a theoretical method has been established to investigate the pounding response of bridge with rubber bearing under near-fault vertical earthquakes using the expansion of transient wave functions in a series of eigenfunctions. The basic characteristics of vertical pounding in bridges are studied. The main researches of this thesis are as followings:(1) The bridge is simplified as a continuous beam-spring-rod model in order to investigate the multiple vertical pounding response of bridge under near-fault vertical ground motions. The pounding and separation phase of girder and bearing change alternately and the pounding wave effect in bridge are considered. The theoretical method is established to investigate both the pounding transient response of bridge and seismic response of bridge under all earthquake excitation periods.(2) With the artificial vertical earthquake in form of a sinusoidal excitation, the seismic responses of two-span continuous bridge with rubber bearing are investigated. The theoretical solution of multiple pounding responses of bridge are derived from the expansion of transient wave functions in a series of eigenfunctions. The numerical convergence of the solution on the time-step size and the number of wave modes are investigated. The propagations of transient response waves induced by vertical earthquake and pounding throughout girder and pier are captured. The multiple pounding phenomenon is observed. Several unusual damage in bridge is found to be closely related to pounding response of bridge under vertical earthquake.(3) With the artificial vertical earthquake in the form of a sinusoidal excitation, the number of pounding and maximum of pounding force of bridge are calculated in various seismic excitation periods, bridge natural periods and seismic excitation amplitude. The effect of seismic excitation period, bridge natural periods and seismic excitation amplitude on vertical pounding phenomenon are investigated. With the vertical seismic excitation period changes, there are three intermittent vertical pounding zone, and they appear in the vicinity of the bridge’s first three natural vibration periods, and the occurrence of the vertical pounding zone is related to the excitation amplitude. In addition, The near-field vertical earthquake excitation easily cause the vertical poundings to occur in the vicinity of the bridge’s second and third natural vibration periods, which is significantly different from far-field earthquake excitation.(4) The beam-spring-rod model is more reasonable according compare to beam-rod model. The effect of bridge parameters on basic characteristics of vertical pounding and seismic responses of bridges are studied. Results show that the effects of bridge bearing stiffness, girder span and girder flexural stiffness are significant, and the effects of pier height and elastic modulus are small. However, the fourth pounding zone appears at high-pier bridge. Therefore, rational design of a bridge structure will reduce pounding amplitude and frequency, and improve the ability of bridge to resist the vertical earthquakes.(5) Real vertical earthquake can be modeled as sinusoidal wave superposition throughout Fourier analysis (FFT). With the consideration of bridge bearing, the theoretical solution of pounding responses of bridge under real near-fault vertical displacement ground motion are derived from the expansion of transient wave functions in a series of eigenfunctions. The 3D elastic finite element model is established in order to compare the results of theoretical solutions with numerical solutions calculated from finite element method, and to verify the reliability and accuracy of the theoretical approach. The pounding responses of bridge with bearing under real near-fault vertical ground motion are calculated. The basic characteristics of bridge vertical pounding under real near-fault vertical ground motion are investigated. Results show that the theoretical method in this work can be used to establish the finite element model or check the model results. The pounding phenomenon and multiple pounding phenomenon in bridge structures may occur under real near-fault vertical ground motions. The vertical pounding phenomenon appears in the vicinity of the bridge’s first two natural vibration period. The effect of vertical pounding in the vicinity of the bridge’s second natural vibration period is close or above the vertical pounding in the vicinity of the bridge’s first natural vibration period. Under strong near-fault vertical ground motion, the girder moment will be in the opposite direction of those from the static loading. The piers axial compressive stress will have high-amplitude fluctuations, and even cause the axial tensile stress. These phenomena is in good agreement with the observed bridge structure’s unusual damage phenomena. Calculation results under real near-fault vertical ground motion can verify the vertical pounding characteristics obtained by the harmonic vertical ground motion.