Abstract Summary
Authors: Laura Purpura, Hüseyin Güner, Sébastien Hoffait & Vincent Denoël. Nowadays, improvements in design and manufacturing methods and a more efficient use of structural materials make it possible to design slender and lighter structures such as footbridges. However, these lightweight structures are prone to higher levels of vibrations due to ambient forces such as wind or pedestrians. It is not unusual that footbridges are used for marathons where a large group of people would load the footbridge to higher levels causing possible structural disorders. The comfort of pedestrians must also be guaranteed under more usual loading. Therefore, it is just as important to study the behavior of the structure before its construction as to make verifications after construction. Indeed, reality is often different from simulations, and this can be revealed by dynamic testing experiments on the structure after it has been built. There exist several approaches to modal identification. In this work the vibration testing and modal analysis of a footbridge using the Operational Modal Analysis with eXogenous (OMAX) forces method will be performed. The OMAX method takes into account both the unmeasured ambient forces and measured artificial forces. In our case, an artificial force is imposed using an in-house designed shaker. In this method, the ambient forces are considered as a real part of the excitation and not as noise, as it would be in any other experimental modal analysis. In other words, it is a combined operational-experimental method used to identify the modal characteristics of the footbridges. The advantages of this method are (i) that smaller shakers can be used, which is more convenient and less costly, due to the high amplitude of the artificial forces; (ii) moreover, since the modes are excited by the artificial forces it is possible to obtain the modal masses and thus mass-normalized mode shapes, which is not possible with a classic method relying only ambient excitation; (iii) furthermore, ambient forces are usually confined to a narrow frequency band, so that only a small number of modes can be identified. A larger frequency range can be covered using a shaker, (iv) finally, it is known to give higher accuracy in the results compared to other identification methods, (v) last but not least, from a practical point of view, using OMAX for the identification of the modal properties of a footbridge does not require to close the footbridge to the pedestrian traffic, which is a significant advantage for the experimental testing of existing in-use footbridges. The method has been used and verified on a measurement campaign on the Tilff cable-stayed footbridge, and by basing the identification procedure on shaker forces created with a shaker developed at the University of Liège. Application of the OMAX method allowed to improve the quality of the identification up to a certain traffic level. To assess the accuracy of the method, the results of the method applied to ambient vibrations where respectively 1, 5, 8 and 10 persons are crossing the footbridge are analyzed.
