Effects of thermal environment on structural frequencies: Part I - A simulation study

Vibration based structural health monitoring methods often use the changes in the modal parameters to identify damage. However, the modal parameters are not only influenced by damage but also by environmental factors including thermal and humidity conditions. The environmental effects can be large enough to mask the changes caused by damage, especially in the bridge structures that are always exposed to environmental elements. In this paper, a simulation study is conducted to examine the variations in the modal frequencies of concrete box girder and T-beam bridge structures caused by the ambient temperature, solar irradiance and wind speed, utilizing the environmental data recorded at a site in North Carolina, USA. The study includes the effects of temperature dependent material modulus, thermal gradients and prestressing effects on the modal frequencies. Like the environmental temperature variations, the bridge temperatures and model frequencies variations have strong yearly (seasonal) and diurnal trends. The study provides temperature-frequency data at hourly intervals for each bridge structure for future studies and the companion paper where, recognizing the observed seasonal and diurnal trends, the models are proposed to estimate frequencies across seasons from the measured temperature values of a bridge structure.