TENSION ESTIMATION METHODS FOR NIELSEN-LOHSE BRIDGES USING OUT-OF-PLANE AND IN-PLANE NATURAL FREQUENCIES

Abstract

Nielsen-Lohse bridges are tied-arch bridges, in which braced cables cross each other and are
connected by intersection clamps. In the maintenance of Nielsen-Lohse bridges, cable tension has to be
estimated for safety evaluation. In the current practice for cable tension estimation, the intersection clamps are
removed, a vibration-based cable tension estimation method for single cables is applied to each cable, and the
intersection clamps are then reinstalled. However, the removal and reinstallation of the intersection clamps
take time and labor. To improve the efficiency of the tension estimation procedure, the authors proposed two
tension estimation methods (methods 1 and 2) for two cables connected by the intersection clamp in a previous
study. Method 1 uses the natural frequencies in the out-of-plane direction, while method 2 uses the natural
frequencies in the in-plane direction. Methods 1 and 2 require at least four and six natural frequencies in the
out-of-plane and in-plane directions, respectively. However, collection of the required number of natural
frequencies in each direction and specification of the vibration direction for each natural frequency can
occasionally be difficult. With this background, the first objective of this study is to propose two additional
methods (methods 3 and 4) that allow simultaneous input of both the out-of-plane and in-plane natural
frequencies. The difference between the methods is that method 3 requires the vibration direction for each
natural frequency but method 4 does not. The second objective of this study is to compare the validity of the
four methods (methods 1, 2, 3, and 4), and to examine which method is recommended in which situation
through numerical and experimental verifications. It was concluded that method 1 is recommended if the
vibration direction of measured out-of-plane natural frequencies is assigned correctly, and method 4 is
recommended if the vibration direction of the measured natural frequencies is assigned wrongly. Out-of-plane
natural frequencies were found to be more reliable because out-of-plane vibrationallows are easier to generate.