Mass and temperature changes in operational modal analysis

A challenge in identifying the dynamic properties of large structures through operational modal analysis (OMA), whereby the response of a structure is measured under ambient excitation, is the changes in added mass and temperature which may occur during data collection. The effect of these changes may mask any damage to the structure or lead the investigator to incorrectly conclude damage has occurred. Quantification of such effects requires the use of short-time OMA methods, where the structure’s response is assumed to be linear across short periods of time. Two experiments were carried out to identify methods through which such variations can be accurately identified and quantified. The first experiment consisted of a simple I-beam with spring supports to which a small mass was added at a variety of locations. This simulated changes in mass similar to that which may be induced by pedestrian or vehicle loading of civil structures. The changes in the dynamic properties were quantified through the short-time Fourier transform and a variant on the random decrement technique to test their ability to detect small changes in mass. An inversion of the mass change method is used to estimate both the quantity and location of the added mass. The second experiment investigated the efficacy of the same short-time OMA methods in the presence of temperature variation. A flat plate was heated in laboratory conditions using radiant panels. The changes in dynamic properties were measured during heating, steady temperature and cooling of the plate. The results were correlated to the changes in temperature using variations on established relationships between elastic modulus and temperature. The results presented within this paper provide an experimental basis for the viability of short-time OMA methods for quantifying temperature and added mass induced variation in dynamic parameters which may be used to guide future structural monitoring.