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Bridges represent key structural elements of transportation systems. They are exposed to different natural and environmental effects that may cause damage representing a potential threat mainly in disturbing the process of transportation of people and goods. The short- and long-term road closures might have a tremendous impact on regional economic and social development. Therefore, the definition of their dynamic characteristics as are natural frequencies by experimental measurements is very important for fast and early assessment of the current conditions. For that purpose, the method of experimental modal analysis is used. This method involves measurement of structural response under ambient conditions. Presented in this paper is investigation of environmental effects upon dynamic characteristics of RC frame bridges. The investigation was carried out on two overpasses with approximately the same geometry. Both structures have two spans each and are in straight direction, but are placed under a skew angle of 58° and 67.8°. The overpasses were constructed in 2016, but have still not been put into operation. They are situated along “Friendship” high-way in N. Macedonia and the distance between them is 3km. Three measurements were performed for each overpass, namely the first measurements were done in October 2017 when dynamic tests were performed, the second were done in March 2020 and the third were done in May 2022. Modal identification of the overpasses was effectively carried out using the enhanced frequency domain decomposition method in frequency domain and stochastic subspace identification method in time domain. The identified dynamic characteristics were compared with each other and with the environmental effects. The results from the analysis show that the identified natural frequencies effectively indicate change of dynamic characteristics of the overpasses due to environmental effects. Greater difference in identified natural frequencies is observed in longitudinal direction, while the least difference occurs in vertical direction. |
