PSI - Issue 64

F.-B. Cartiaux et al. / Procedia Structural Integrity 64 (2024) 285–292 Cartiaux / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Sensor setup and monitoring process 3.1. Sensor setup

The monitoring system was installed by OSMOS Group inside the box girders in February 2022. It consists of a combination of fiber optic strain sensors called Optical Strands (OS) and of MEMS accelerometers. To allow the comparison between the two parallel decks, the sensor setup is the same for both, with twelve OS and eight accelerometers. Four accelerometers are bi-axial, and the four other ones only measure the vertical component. The first two spans of the viaduct, starting from the North end, are monitored on six cross-sections, with different sensor configurations to address specific issues on each cross-section, as follows: At mid-span of the first and second spans (Fig. 2): • Four longitudinal OS, two of them on the lower flange and two of them under the upper flange, to monitor the main bending of the span and the local bending of the upper slab under the effect of the traffic, • Two accelerometers on the lower flange, to monitor vibrations in the transverse plane. Translations in two directions and rotation are deduced from the combination of the two vertical and one transverse acceleration measurements.

Fig. 2. Cross- section of one deck with sensor arrangement: longitudinal optical strands “CO” and accelerometers “ACC” .

At a distance to the first pier which is around one third of the span length, for the first and second spans: • Two accelerometers on the lower flange, to monitor vibrations in the transverse plane. The setup is like the one at mid-span. The location at one third of the span length theoretically allows to record vibrations according to even modes (which have a node at mid-span). Near to the first pier, one meter before and after: • Two diagonal OS on the box webs, to measure the principal tensile strain due to the shear in the webs under the effect of the traffic. 3.2. Monitoring process In addition, the ambient temperature is also measured inside and outside the box girder, for each deck. In total, the monitoring system includes 24 OS, 16 accelerometers (on 24 channels) and four temperature gages. All sensors of each deck are connected to one Expert Data Acquisition System (EDAS), which is an industrial computer equipped with a 4G router. The EDAS performs the data acquisition at a sampling rate of 100 Hz. This recording is not continuous all the time: to avoid useless data, it is triggered by a threshold value that was configured after the first days of operation.