Thermal Environmental Effects on Modal Parameters and Health Monitoring of Bridge Structures Mahendra P. Singh Program Director National Science Foundation Arlington, VA, USA mpsingh@nsf.gov Mahendra P. Singh, Preston Wade Professor at Virginia Tech, received his Ph.D. degree in Civil Engineering from the University of Illinois in 1972. Currently he holds the position of Program Director at National Science Foundation in USA. Harsh Nandan Graduate Student, ESM Dept., Virginia Tech Blacksburg, VA, USA hnandan@vt.edu Harsh Nandan received his Bachelor of Technology degree in Civil Engineering from the Indian Institute of Technology. He is currently a graduate student in the Department of Engineering Science and Mechanics at Virginia Polytechnic Institute and State University in USA. Surot Thangjitham Associate Professor, ESM Dept., Virginia Tech Blacksburg, VA, USA thangjitham@vt.edu Surot Thangjitham received his Ph.D. degree in Engineering Mechanics from Virginia Tech. He is currently an Associate Professor in the Department of Engineering Science and Mechanics at Virginia Polytechnic Institute and State University in USA. Summary Often changes in the frequencies extracted from the vibration response are used for the detection of damage. However, these structural characteristics are also affected by the outside thermal envonment; this effect could be large enough to mask the changes caused by the damage. So it is important to understand and predict the effect of the environmental variations on these characteristics to separate them from those caused by the damage. In this paper, we examine the trends in the frequencies variations of a reinforced concrete box girder bridge structure subjected to a nine-year long record of measured temperature, radiation, and wind speed. It is observed that the major trends in the frequency variations are parallel to the major trends in thermal environment. The paper then presents and validates an identification approach to estimate the system frequencies directly from a few measured temperature values without a detailed finite element analysis. Keywords: Structural Health Monitoring; Bridges; Structural frequencies; environmental effect; system identification. 1. Introduction For health monitoring of bridge structures modal parameter-based methods have been considered for use wherein the changes in the parameters are used to predict the damage. However, it is well known that the modal parameters are affected not only by the damage but also by environmental temperature and humidity changes. It is, thus, necessary to separate these two effects for damage detection. In this paper, we first examine how the environmental thermal variations affect the modal properties and then examine if these trends can be estimated by using simple models. 2. Thermal-Structural analysis Environmental thermal conditions affect the modal properties due to changes in material properties such as Young’s modulus and also due to internal thermal stresses that cause geometric stiffness effects. To incorporate these effects here, we first calculate the temperature distribution inside the structure by heat conduction analysis and then use it to calculate the temperature dependent material properties and the pre-stress effects due to differential temperature distribution. For a civil structure with irregular boundaries, it is necessary to solve the heat conduction problem by discretization methods such as finite element method with due consideration of radiation and convection effects