Monitoraggio e identificazione dinamica del “Manhattan Bridge” di New York

In this paper, dynamic proprieties and results of the identification process on the Manhattan Bridge are described. Accelerations caused by ambient vibrations are recorded using an advanced MEMS-based system, whose main features are the transmission of the data in digital form and the possibility of performing some system analyses directly on-board of the sensors, transmitting synthetic data only to the main computer. 28 MEMS accelerometers are used and 4 different experimental setups are adopted in order to identify a significant number of modes of the bridge. A standard frequency domain identification technique is applied (the Enhanced Frequency Domain Decomposition -EFDD) and the dynamic properties of the bridge (such as natural frequencies, mode shapes and damping ratios) are obtained. Several modes of the main span are identified, with natural frequencies in the range 0.2-1.0 Hz. A Finite Element model is then built and a model updating procedure is also performed using an improved Evolutionary Algorithm is used. After the updating procedure, the numerical modal frequencies and mode shapes match well the experimental ones. This work is part of a research that aims to investigate how real-time monitoring systems can be used to detect damages induced by vibrations and distortions in existing steel bridges.