Flexibility Identification And Performance Assessment Of Bridge Structures From Ambient Vibration Test Data

Ambient vibration test is one of the most important SHM technologies used for modal parameters identification,it directly uses natural winds,traffic and ground motion to excite a bridge structure.Compared to traditional impact vibration test,ambient vibration test has the merit of easy-performing and low experimental cost because it doesn't need additional excitation equipment to excite a structure.However,existing data processing method of ambient vibration test can only output basic parameters such as natural frequencies,damping ratios and unscaled mode shapes,which cannot be utilized to effectively support structural maintenance management and safety decision.Thus,how to identify structural deep-level parameters from ambient vibration test data is an urge problem.This thesis focus on deep-level parameters identification from ambient test data,such as unscaled flexibility,scaled flexibility,scaled long-gauge strain flexibility and corresponding deflection or long-gauge strain identification.The main research topics and novelties of this thesis are shown below:(1)Existing modal parameters algorithm for ambient vibration test data was firstly overviewed,then detailed introduction of data processing techniques were introduced including data pre-processing from ambient data for free-decay function and data post-processing for modal parameters identification method in frequency domain.(2)The relationship between unscaled Frequency Response Function(FRF)estimated from ambient vibration data and scaled FRF estimated from impact test data was investigated starting from structural dynamic equations of motion,and unscaled flexibility identification method from ambient test data was proposed.This method can be used to predict static deflections under arbitrary loads,which is important for engineers to understand the stiffness distribution.(3)Based on the structural finite element model and structural modal parameters identified from ambient test data,mode shape scaling factors and scaled flexibility was further calculated,which can be used to predict structural absolute deflection under arbitrary static loads.It also has the merit of easy-performing and reducing experimental cost and the effectiveness of proposed method has been verified by a three-span concrete box girder bridge.(4)A scaled flexibility identification method was proposed using mass changing strategy,which directly uses structural responses under ambient excitation of original and modified structure.In this method,structural free-decay responses were firstly obtained using data preprocessing method,such as random decrement technique or natural excitation technique;then structural modal parameters of original and modified structure were identified using frequency domain method;after that,mass-normalized mode shapes and scaled flexibility were identified;finally,structural deflections were predicted by multiplying static force vector and the identified flexibility matrix.The well agreement between measured deflections and predicted deflections of a three-span concrete box girder bridge has successfully verified the effectiveness of proposed method.(5)A scaled long-gauge strain flexibility and dispOlacement flexibility simultaneously identification method was proposed from dynamic long-gauge stain measurements under ambient excitation.This method has the merit of multiple structural parameters simultaneously identification,such as natural frequencies,damping ratios,stain mode shapes,displacement mode shapes,scaled long-gauge stain flexibility and scaled displacement flexibility.After these parameters were identified,they can be used for structural performance evaluation from multiple perspectives and the effectiveness of this method has been verified by a simulated grid structure.The identified flexibility matrix and the predicted deflection are good candidates for structural performance evaluation,which can provide much more detailed information than generally identified basic modal parameters for bridge owners and engineers to understand the safety condition of the in-serves structure.On one hand,the obtained results can be used to predict static deflections or long-gauge strains under arbitrary loads by multiplying static force vector and the identified flexibility matrix.On the other hand,identified structural parameters can be utilized to detect structural damage.