Pulse Identification Of Near-fault Earthquake Ground Motion And Analysis Of The Seismic Damage Of Bridge Considering Wave-propagation Effect

With the rapid development of economic construction in China,more infrastructure projects will be constructed.China is an earthquake-prone country,with different sizes of earthquake faults distribute across the vast land.Thus,the probability of building a bridge near or cross the near-fault region is much higher than before.The large amplitude and long period distinguish the near-fault earthquake motion from the far-field earthquake motion,which imposes a new challenge to the seismic safety of bridge engineering.The damage or even collapse of a bridge in an earthquake event will cause great loss of lives and properties,the forthcoming rescue to the epicenter may also be blocked.To study the damage mechanism of the bridge under the excitation of pulse-like near-fault earthquake motion,the characteristic of the pulse-like near-fault earthquake motion should be firstly investigated.Though the quantified identification method for the pulse-like near-fault earthquake motion based on wavelet analysis has been proposed for many years,the multiple pulse component identification method still needs to be studied.In addition,human subjectivity still influences the selection of mother wavelet,the selected mother wavelet can not adaptively change according to the characteristic of the earthquake motion.In recent years.new signal analysis methods have been proposed,which may be combined with the identification of the pulse-like near-fault earthquake motion.When analyzing the seismic response of the structure,the seismic response of structure is assumed to be synchronous,this assumption is not suitable for a large-span bridge since the transmission of the seismic wave inside the structure needs some time.In the seismic risk evaluation of engineering structure under the excitation of pulse-like near-fault earthquake motion,the earthquake motions recorded at the near-fault seismic station are often adopted directly.Even when the pulse-like pattern is considered,great human subjectivity is involved with the selection of qualified pulse-like input earthquake motion,which can not guarantee the continuity and reliability of the analysis result.This thesis is aimed to study the identification method for the pulse-like near-fault earthquake motion,the practical method to simulate the transmission of the seismic wave inside the structure,and the seismic risk under the excitation of pulse-like near-fault earthquake motion.Finally,the damage procedure of the bridge under the excitation of pulse-like near-fault earthquake motion is investigated based on the previous research.The contents of this thesis are as follows:(1)The near-fault earthquake motion recorded at a seismic station may contain multiple pulse-like components,the identification method based on DB4 mother wavelet can continuously extract the multiple dominant pulses,finally,the multiple pulse-like components can be obtained.The characteristic of pulse component embedded in near-fault earthquake motion changes drastically,the use of one fixed mother wavelet to identify the pulse-like earthquake motion with a different pattern of the waveform may involve great human subjectivity.The selection of optimum wavelet procedure can adaptively choose the most suitable mother wavelet according to the unique pattern of the target earthquake motion waveform,which can minimize the difference between the mother wavelet and the target seismic motion.The hybrid method combined with ESMD can identify and extract the multiple pulse-like seismic motions with neither the manually selected sifting time as needed in the EMD nor the manually selected frequency range as needed in wavelet transform,which involves the lowest amount of human subjectivity in the process of identification and extraction of the pulse-like near-fault earthquake motion.(2)The seismic response of a structure is the result of the transmission and reflection of the seismic wave inside the structure.The motion of the upper structure is excitated by the resisting force from the bottom structure under the excitation of earthquake motion.To reasonably simulate this kind of seismic excitation,this thesis proposes an explicit fiber beam-column(EFBC)element and its analytical framework,which enables the input seismic motion is transmitted within the structure in a chain manner.Three numerical experiments are carried out to show that the EFBC element and its analysis framework can capture the propagation pattern of seismic response under the excitation of earthquake motion.In the seismic response analysis procedure,it is suggested to model the structure with the EFBC element to reasonably simulate the propagation pattern of the seismic response.(3)Due to the characteristic of near-fault earthquake motion,a great amount of seismic energy can be inputted into the structure within a short period of time,which can trigger the severe damage of the structures.This thesis tries to investigate the difference of the destructive capability between the pulse-like near-fault earthquake motion and that recorded at the seismic station by seismic risk analysis based on the annual expected loss ratio.By analyzing the seismic risk of pier and bridge system under the excitation of pulse-like near-fault earthquake motion based on the EFBC element,it is found that the seismic risk for the seismic motion recorded at the seismic station is dangerous-prone in a statistical sense.Thus,the identified pulse-like seismic motion owing the largest pulse energy is recommended to analyze the seismic response and the seismic risk for safe.(4)To investigate the difference in realistic structure seismic analysis between the EFBC-based model and the vibration-based model,the seismic response pattern of a three-span continuous girder bridge based on EFBC element is analyzed,the result indicates that the result of EFBC-based model is quite different with that of the vibration-based model.The error of the displacement response between EFBC-based model and OpenSEES-based model increases with the increasing nodal elevation.At the initial excitation stage,the pier shear in the longitudinal direction from the EFBC-based model is much larger than that from the OpenSEES-based model.At the vibrating stage,though the profile of the pier shear response from the two models is close to each other,the peak resisting force from the EFBC-based model is larger than that from the OpenSEES-based model in most time.(5)The damage procedure of the key components in a three-span bridge under the excitation of the identified near-fault pulse-like earthquake motion based on the EFBC element is analyzed.The results indicate that the small supporting length of the abutment is the weak point of the whole bridge system under seismic excitation,increasing the supporting length at the abutment can postpone the early failure of the abutment.The plastic hinge length at the pier foot develops reasonably during the seismic excitation.The bearing stiffness can greatly influence the failure mode of the whole bridge system.Increasing the bearing stiffness before the arrival of principle seismic wave can isolate the failure time of the critical components,making the failure of the bridge system into a gradual process,which can prevent the sudden collapse of the bridge to some extent.