Daily and seasonal effects of environmental temperature and humidity on the modal properties of structures

In Structural Health Monitoring understanding the effect of the environment on the modal properties of structures is essential. In this study, we evaluate the influence of changes in ambient conditions on the natural frequencies of three real large-scale structures: A 9-story reinforced concrete (RC) building and two sixteenth century churches of adobe masonry. For these structures, both environmental parameters (ambient temperature and relative humidity) and modal parameters were continuously monitored for a combined period of 8-years (RC building) and 2-years (for each of the adobe churches) in order to assess the effect of daily and seasonal variations of environmental parameters on the modal response. The results of the seasonal comparison indicate a negative correlation between ambient temperature and natural frequencies for the concrete structure, while for adobe structures a positive correlation between humidity and natural frequencies was observed. For daily variations, an out-of-phase and lag response of natural frequencies to variations in the environment was observed. A change on the daily lag was observed depending on the time of the year. Both seasonal and daily comparisons show the existence of a strong seasonality in frequency variations, where the form of response of this to environmental effects is different depending on the season. The magnitude of the daily variability is smaller in comparison to the seasonal variations. Finally, it was determined that the variation in natural frequencies depends on several factors such as the predominant material of the structure (namely reinforced concrete and adobe), the type of environmental exposure (temperature or humidity), and the characteristics of the structure (dimension of main elements).