Modal Parameter Identification And Model Updating Of Concrete Cable-stayed Bridge

It is more and more attention for the research on vibration based damageidentification of long span bridge recent years. Modal parameter identification andmodel updating of long-span concrete cable-stayed bridge structure is a verycomplicated problem which needs to be solved urgently in bridge health monitoring,and it has not been completely solved yet. Taking the Hesheng bridge as theprototype, a health-diagnosis-oriented concrete cable-stayed model bridge wasdesigned and made with a scale of1:15. A series of works were done in thisdissertation, mainly includes model design of health-diagnosis-oriented concretecable-stayed model, modal testing and modal parameter identification ofcable-stayed bridge, finite element modeling and model updating of cable-stayedbridge. The main investigations are summarized as follows:(1) Design and fabrication of health-diagnosis-oriented concrete cable-stayedmodel bridgeTaking the Hesheng bridge as the prototype, a health-diagnosis-oriented concretecable-stayed model bridge was designed and made with a scale of1:15.In view of thedifficulties in the process of model design, on the premise of meet the productioncondition, the design of the model bridge girder and tower should meet therequirements that the flexural rigidity is similar, and the axial and torsional stiffnessparameters similar requirements could be relaxed, and the simplified method ofcounterweight and stay-cables was given. The initial finite element models of theprototype and model bridge were established. The static and dynamic characteristicsimilarity of the model and prototype bridge was analyzed at undamaged and damagestates. The results show that after a variety of simplification and treatment the staticand dynamic characteristic and damage characteristic of the model and prototypebridge still have a good similarity. It can provide a reference for the model design ofconcrete cable-stayed bridge.(2) Modal test and modal parameter identification of the model bridge werecarried out using hammer excitation method and ambient excitation methodThe first21modal parameters of the model bridge were obtained usingEigen-system Realization Algorithm(ERA) from hammer excitation modal test data,and the first17modal parameters of the model bridge were obtained using ERA combined with Random Decrement Technique(RDT) from ambient excitation modaltest data; Using white noise and the measured non-white noise signal as excitationinput, finite element time history response analysis was carried out, and modalparameter identification from simulated response data show that the measurednon-white noise excitation can bring spurious modes, in addition, the non-whitenoise excitation can reduce the identification accuracy of modal frequency and mode.With the finite element analysis results as prior knowledge, combined with thestability diagram method, the spectral characteristics of excitation signal and highsignal to noise ratio measurement point signal, it can give the order and eliminatespurious modes. Practice shows that the method can accurately identify the modalparameters of the model bridge and eliminate false modals effectively. It provides anew way to eliminate the false modals in modal parameter identification of ambientexcitation modal test.From different parameter identification method and noise simulation based onstatistical Monte Carlo analysis, uncertainty analysis of the parameters identifiedfrom data that obtained from hammer excitation modal test and ambient excitationmodal test was done. The results show that the uncertainty of modal parameterscaused by different identification methods was obviously higher than that caused bynoise simulation, therefore, uncertainty of modal parameters caused by the differentidentification methods can’t be ignored; Uncertainty analysis of noise simulationbased on Monte Carlo method gives statistical uncertainty intervals of parametersidentified from data that obtained from hammer excitation modal test and ambientexcitation modal test. It provides useful information for the research of modelupdating and damage identification.(3) Reason analysis of the difference of modal frequencies identified from datathat obtained from hammer excitation and ambient excitation modal testModal test and modal parameter identification of the model bridge was carriedout using hammer excitation method and ambient excitation method at three caseswhich were before and after the prestress reinforcement tensioned and the girderdamage status, the maximum difference of modal frequencies identified from datathat obtained from hammer excitation and ambient excitation modal test at the threecases were3.64%,1.24%,1.84%respectively, it can be seen that the differencereduces after the prestress reinforcement tensioned, and the difference rises at thegirder damage status. It is speculated that for the reason of concrete micro crack orcracks existence, structural vibration amplitude may impact the stiffness of the concrete girder and tower, and the higher vibration amplitude the lower structurestiffness.(4) Finite element modeling and model updating of concrete cable-stayed bridgeA modified model updating technique based on parameters and objectivefunction classification was present, the modified model updating technique caneffectively improve the ill-condition problems, improve the efficiency of modelupdating and get better results. The initial dynamic finite element models of modelbridge were built by single beam pattern, triple-girder pattern, beam-shell patternand solid pattern respectively, and then the initial finite element models wereupdated by the modified model updating technology. Compared the calculateddynamic characteristics before and after model updating with measured data, thecalculation accuracy and the model updating effect of different finite element modelswere discussed, the error sources of finite element modeling and relevant issues ofmodel updating were analyzed. The results show that the calculation errors of initialfinite element models were mainly caused by modeling errors and parameter errors.Beam element models had limitations in modeling. Model updating shoulddistinguish the modeling errors and parameter errors accurately, then the realisticbaseline finite element model can be acquired. It can provide a reference for thefinite element modeling and model updating of concrete cable-stayed bridge.