Dynamic Characteristics Of Typical Civil Engineering Structures Under Ambient Excitation Conditions

As an integral part of the national infrastructure,civil engineering structures play an important role in people's daily lives and social and economic development.Field measurement provides an effective way to understand the dynamic characteristics of civil structures under operating conditions,and to ensure their safety and applicability.Modal parameters are involved as basic information in analyzing the dynamic characteristic of a structural system,and have been used widely in structural health monitoring,damage identification,windresistant design and so on.Thus,studies on modal parameter identification become increasingly important.The output-only modal identification method under operating conditions has been developed in recent years,but it has been utilized widely in civil engineering.Compared with traditional methods that are based on both input and output signals,this method has many advantages,including operational convenience,ability of working without interruption of normal operation of the structure,providing more credible identification results for real situations.Through comparison,this paper first investigates the working performance of different modal identification methods under the context of two typical kinds of civil structures(i.e.,bridge and building).The limitations of the Stochastic Subspace Identification(SSI)method are stressed,and an improved SSI approach is proposed.Then,an integrated analysis method is established to further explore the dynamic properties of the structural system,via a combined consideration of both the instantaneous and statistical characteristics of structural dynamics in the time-frequency domain.Finally,the above methods are employed to analyze the modal parameters and dynamic characteristics of two types of civil structures subjected to ambient excitations.As an advanced output-only modal identification method,the SSI approach has certain requirements for the features of excitations and the quality of response signal,but these requirements cannot be well satisfied in practice,which results in a number of problems such as false modes and mode omissions.To this end,this thesis presents an in-depth study on the limitations of the traditional SSI method.It is shown through numerical examples and engineering cases that the imbalance of energy distribution between different modal responses of the system is an important reason for the occurrence of the above problems.Based on this finding,an improved SSI algorithm is proposed by using zero-phase filtering techniques.The output signals from the dynamic system are separated into a series of modal component groups according to the spectral energy,with each group consisting of a few modal components whose spectral energies are in comparative order of magnitude.Traditional SSI method can be the utilized to identify the modal parameters for each modal component group.The validity of the proposed method is verified through both a numerical example and an engineering example.Relevant results can be used to improve the identification reliability of the SSI method.When analyzing the dynamic characteristics of a system,most current studies deal with the statistical characteristics and instantaneous characteristics separately,while less attention has been paid to the internal relationship between the above two aspects.In this paper,an integrated analysis method is established through a combined consideration of both the instantaneous and statistical characteristics of structural dynamics in the time-frequency domain.The dynamic characteristics of some high-rise buildings subjected to typhoons are further analyzed by using the proposed method.A damping model is established based on the instantaneous frequency fluctuation intensity.Through comparison,it is found that the proposed damping model is more accurate and applicable than the one based on structural vibration response.Related results in this part provide a new way to further understand the nonlinear characteristics of structural modal parameters,especially of the damping ratio.By using the aforementioned improved SSI method and the integrated analysis method for structural dynamic characteristics,this thesis presents a systematic study on the dynamic characteristics of two typical kinds of civil structures under ambient excitation conditions.The wind-induced vibration responses of high-rise buildings and their dependence upon wind speed are analyzed,and the comfort level is assessed against several criteria based on collected acceleration response signals.The dynamic characteristics of urban bridge and pedestrian bridge under vehicle load and earth pulse excitations are also studied.Results from this part provide useful reference for further understanding the dynamic characteristics of typical civil structures and for updating the finite element models as well.The outcomes from this thesis are expected to facilitate the improvement of structural dynamics theory and modal identification method to some extent,and to provide important reference and useful information for further understanding the dynamic characteristics of bridges under ambient excitation conditions and the wind effect of high-rise buildings subjected to typhoon.