Vibration analysis for non-destructive evaluation of large-scale civil engineering structures often rely on ambient vibrations as excitation sources. In this case, the input force is typically not available for establishing the transfer functions for vibration analyses. Conventional technologies dealing with random vibration signals, such as correlation analysis and/or random decrement method, may yield less accurate results when the signal-to-noise ratio of the response measurement is not sufficiently high. In this paper, a method called lag-superposition is introduced that provides a better response spectrum and more accurate results with reasonable computation speed. Formulations of Single-Degree-of-Freedom (SDOF) and Multi-Degree-of-Freedom (MDOF) approaches are presented and verified by experimental tests and numerical examples. Comparisons with other methods are also made.
In recent years the authors have engaged in the development of a special technology to optimally modify a structure's physical parameters (mass, damping and stiffness) in time- dependent loading conditions. Specific efforts have been devoted to reduce structural responses under seismic loading. In this regard, there are many other research activities dealing with different devices and control algorithms generally classified in the following categories: passive control, active and hybrid control, adaptive control, semi-active control, etc. All together, this newly emerging field of structural control has generated significant momentum of development. Real-time structural parameter modification (RSPM) is a different approach from the categories mentioned above. A detailed explanation of RSPM is given in the references. In this paper, we discuss a number of fundamental issues facing the development of various structural control technologies. These issues are summarized from our development of RSPM, and are examined based on the view that structural control devices may be considered from only two distinct types: material type and mechanical type. Such a view helps to classify some advantages and disadvantages of different structural protective systems including RSPM.
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