ABSTRACT
The measurement of static vertical deflection in concrete box-girder viaducts with large spans and large height is a complex task, especially if a measurement error below 0.5 mm is required.
Market provide us a wide variety of technical solutions, but none of them fulfils all the needs.
For example, the traditional solution of displacement transducers requires an external fixed point below the target point of the viaduct, and this is not possible in most of these cases. Laser-based systems (distanciometer and interferometer, for example), required an external fixed point but not below the target point of the viaduct, and it is always possible. But its accuracy strongly depends on the distance to target. In the case of large distances, error usually is over 2 mm. Laser DopplerVibrometers (LDV) measure velocity of the target point and get the displacement by integrating the velocity – time record. In the case of static vertical deflection, the target point show extremely low velocity values and the integration process induces a very significant uncertainty. Also LDV sensors are expensive and the simultaneous measurement of several target points (using various LDV sensors) is only reserved to very singular bridges. General view of a concrete box-girder viaduct during the load testing. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig319_1.jpg"/>
The use of GPS sensors provide error over 10 centimeters. Finally, the use of Vision Based Displacement Systems is up to date a complex technology and its accuracy depends on pixel dimension. In the case of large distance between target point and camera, the error is actually over 1 mm.
Digital levels remain as a viable solution since they provide error around 0.5 m, also they provide discrete measurement.
In the particular case of box-girder viaducts, it is possible to install sensors inside the box-girder. However, the solution is not free from difficulties. It is possible to obtain a fixed reference line using a wire. However, the almost null stiffness of the wire makes impossible the use of contact sensors (as displacement transducers).
This document shows a comparison between different measurement techniques to obtain the vertical deflection in concrete box-girder viaducts by using non-contact sensors placed inside the box-girder. Also a comparison between other conventional systems is exposed.
