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Title: Structural control and condition assessment with substructure method
Authors: Ding, Yong
Degree: Ph.D.
Issue Date: 2012
Abstract: Structural condition assessment is a major component in a structural health monitoring system. A large scale structural system may have complex boundary conditions and uncertainties due to the discreteness of geometric and material properties. Models on the boundary conditions and any innovative vibration control device for seismic protection in a large scale civil structure may not be accurate. Another obstacle for large scale civil structural condition assessment is that the current damage detection methods are either insensitive to local structural damage or sensitive to measurement noise. It is difficult to conduct structural condition evaluation for large structures partially because of these points. Numerous structural condition assessment methods have been proposed. The structural condition assessment methods in frequency domain always need large number of measured data. The methods in time domain are alternative solutions to structural health monitoring which needs as few data as possible. Though a lot of work has been done in this field, there are yet some gaps which limit the application of this kind of method. It is usually difficult to conduct the parameter identification in large scale structural system also due to the computational efficiency, accuracy and convergence. Previous time response sensitivity methods commonly assume the structure is connected rigidly to the base and the base-superstructure interaction is rarely taken into consideration. Most of the sensitivity methods in time domain commonly need the record of the excitation or need an assumption of the function of the external force time history. Also the initial structural responses, e.g. acceleration, velocity and displacement, are commonly assumed to be zero but they are always unknown and non-zero values. Time-variant structural parameter identification is difficult to be identified with the existing time response sensitivity methods. Efficient structural control can ensure the structural reliability of the structure during the severe earthquake or other harsh environmental load. The integrated system of structural control and model updating would make the structural control algorithm more stable and effective. However, only the integrated system with model updating method in frequency domain has been investigated. This thesis aim to propose a framework to conduct the structural condition assessment and structural control based on substructure methods which could improve the computational effort, perform the general out-put only structural condition evaluation, including load evaluation and damage detection, conduct the time-variant structural condition evaluation and implement the smart structure with the integration of structural control and structural health monitoring.
The time response sensitivity method in time domain with substructure method is an alternative solution to large scale structural condition assessment, which needs as few data as possible. Several components of work in structural condition assessment and structural control are completed in this thesis. Firstly, a substructural external force identification method based on the equation in state space with the First-Order Hold discrete and Tikhonov regularization is presented. This method makes good use of this limited but accurate analytical information of the target substructure for the inverse identification of moving or static external force acting on the structure. Secondly, a general response sensitivity method based on the two-stage identification for structural model updating considering the nonlinear support-superstructure interaction is developed. The two-stage identification method proposed previously for substructural condition assessment is illustrated, proved and improved. With the two-stage method the interface forces are identified in the first stage and the local damage is detected in the second stage. In this study, a concept of pseudo structure is constructed for illustration and proof. Furthermore, two new computational methods are proposed to improve the first stage identification. A time window force identification method is presented to reduce the computation effort of the first stage identification. The structural responses of the first time step are always supposed as zero with time domain response sensitivity method in previous research work. However, the initial structural responses are unknown and non-zero practically. In general, and a method for the simultaneous identification of the unknown force and initial responses are also presented for the first stage identification. An adaptive regularization method is employed for the model updating in the second stage. Thirdly, a time-variant structural parameter is proposed based on time window identification method. With this method, the abrupt structural damage during the earthquake could be identified. Lastly, a new combined system of adaptive structural control and structural evaluation is proposed. The structural control system is implemented with the LQG which is an effective control method for the vibration mitigation of structures. The structural control is adaptive with the changes of the structural parameters via the structural evaluation system. A modified adaptive regularization method is used in the solution of the structural evaluation via model updating. The combination of the structural control and evaluation is designed as decentralized autonomous to guarantee the reliability under the harsh environmental excitation. The decentralized autonomous control system explores the substructure method which is more efficient in calculation with smaller mass, damping and stiffness matrices for the structural evaluation. All these proposed methods in this thesis are verified by numerical simulation. The two-stage identification method and time-variant damping identification is also verified by laboratory work. Results show that the proposed methods on the structural condition assessment are effective and perform satisfactorily even there is noise in the measurement.
Subjects: Structural analysis (Engineering)
Structural control (Engineering)
Hong Kong Polytechnic University -- Dissertations
Pages: xxiii, 290 leaves : ill. ; 30 cm.
Appears in Collections:Thesis

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