Obtaining evidence of structural safety using a fibre optical monitoring system on the example of the “Wave of Bern”

Abstract

In 2004, the roof structure of the main train terminal in Bern was extended to cope with increasing passenger volume. For this reason, an innovative architectural structure was erected across the passenger platforms. Although the wave-shaped roof with its choice of light materials was designed with diligence and care, there were some remaining uncertainties to the full extent of the roof behaviour under the given wind exposure, which could not be resolved using the conventional theoretical design approach alone. To clear these minor uncertainties, the structural monitoring department of Basler & Hofmann was approached to install a fibre optical structural monitoring system on the most exposed steel/glass roof structure (GD 5) of the train terminal, so that the influence of high wind loads on its behaviour could be monitored. The ridge purlin of the main roof is subject to vertical and horizontal loads from various loads, which sometimes act on the structure with eccentricity. Therefore, the purlin is subject not only to bending but also to torsion stresses. Therefore, it was important to carry out multidirectional vibration monitoring of this ridge purlin. A variety of different sensors was used to register the static structural and dynamic behaviour. The vibrations of the purlin induced by vertical loading were measured using optical strain sensors (System OSMOS); the torsion behaviour was monitored using inclinometers and accelerometers. The latter sensors were also used to register any wind induced wobbling of the outer edges of the steel/glass roof. The correlation between wind speed and vibration behaviour was accomplished by using a wind anemometer which was installed on the roof structure. The aim of the monitoring system was to obtain a clear understanding of the behaviour of the roof, especially when it was subject to strong wind loads. The results were needed to make reliable predictions of the long term aerodynamic stability of the roof. In consequence, structural monitoring can be used on innovative structures to verify the theoretically expected behaviour and gain confidence in the stability of the structure. In the long term, monitoring can be used as effective asset management tool to optimise spending on maintenance and repair. In case of unforeseen events such as very high snow loads, monitoring could act either as alarm system or allow for the structure to being operated safely.