Vibration based condition monitoring refers to the use of in situ non-destructive sensing and analysis of system characteristics –in the time, frequency or modal domains –for the purpose of detecting changes, which may indicate damage or degradation. In the field of civil engineering, monitoring systems have the potential to facilitate the more economical management and maintenance of modern infrastructure. This paper reviews the state of the art in vibration based condition monitoring with particular emphasis on structural engineering applications.

A primary challenge to implementing structural health monitoring techniques on civil infrastructure is the differentiation of effects of environmental variables on the behaviour of structures from other causes of structural change. Data from the Z24 Bridge recorded over the course of nearly a year are analysed in this paper. Covariance-driven Stochastic Subspace Identification is applied to the data and a Fuzzy Clustering Algorithm is used to extract parameters indicative of the bridge's state. The main benefit of this approach is the lack of need for mode shape information and thus it's applicability to structures monitored with spatially sparse sensor grids. The method is shown to provide very encouraging results in separating the response data from the Z24 Bridge in normal and damaged states in varying environmental conditions, and the procedure is then applied to a second data set obtained from monitoring a tall building over several years of its early life in order to identify gradual or sudden structural changes.

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A 183m reinforced concrete chimney for a coal-fired power station was instrumented in the latter part of its life during the construction of a replacement chimney. Because of concerns about large-amplitude response induced by interference effects from the new chimney in the prevailing upwind direction, a response monitoring system was installed, quickly followed by a tuned mass damper (TMD) system. As well as providing live display of the chimney response, the monitoring system was also used to check the functioning of the TMD. The monitoring system featured a direct implementation of the stochastic subspace identification procedure in the 'virtual instrument' controlling the system, so that modal damping values for the system were displayed automatically, in real time. The system thus provided an immediate visual indication of increased damping levels during strong winds, showing the correction functioning of the TMD.The paper describes the chimney, the monitoring system and its installation, the data processing and system identification procedure, together with performance data before, during and after installation of the TMD.

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