Monitoring the structural properties of cables with small bending stiffness

Résumé du projet

The Walloon Public Service department has recently decided to improve the way they monitor axial forces inside stays and hangers in the bridges they are responsible for, because of two main limitations: their current identification procedure (1) assimilates all of these structural elements to taut strings, even though their flexural rigidity is not always negligible, and (2) is based on sparse on-site measurements of their natural frequencies, taken at best once a year by a skilled worker.

Being nondestructive, vibration-based methods are commonly used for monitoring purposes. They typically rely on an accurate model of the cables which is able to predict the main features of their dynamics for a given set of structural parameters. The unknown ones can therefore be updated until the outcomes of the modal analysis correspond to the experimental results measured in practice.

The cables studied in this project are highly tensioned, but their flexural rigidities are most often not negligible, although small. The predicting capabilities of the taut string model are therefore not sufficient to provide an accurate estimation of their internal tensile forces. In addition, their boundary conditions appeared to affect their modal characteristics as well, by being somewhat flexible in translation and also far from ideal hinges or clamps in rotation.

Based on these observations, the cables are supposed to be correctly represented by pre- stressed Euler-Bernoulli beams anchored to flexible supports. In this context, our team derived simple analytical formulas for their natural frequencies and mode shapes which allowed to provide some guidelines in order to set up an efficient identification procedure for the tension, obviously, but also for the bending stiffness and the end restraints.

Since cables are most often slender, lightweight and slightly damped, they are particularly prone to vibrate in ambient conditions. Remote measurements of the natural frequencies and possibly the associated mode shapes of the element can therefore be collected much more frequently by means of wireless sensors, without requiring any human intervention. The design of such a monitoring system has been assigned to the company V2i, which is specialized in this field, and intends to solve the second problem of the current method.

Publications

On the axial force identification in Euler-Bernoulli beams with unknown boundary conditions, 2020, November 23

On the identification of axial force in stay cables anchored to flexible supports, 2020

Development of a general monitoring program for bridge stays and hangers in Wallonia, 2020

A low-order analytical model to monitor tension in shallow cables with specific end conditions, 2019, March 15

On the identification of the axial force in stay cables with unknown boundary conditions, 2019