Distributed fibre optic strain sensors were installed at the bottom and top flange of the steel girder, as well as at the side surface of the concrete. Such a sensing system setup allowed for detailed analysis of the interaction between these two materials and validation of a new design approach. Distributed measurements taken during load tests (locomotive load) prior to commissioning enabled the detection of all microcracks along the girder length. However, most of them were closed after removing the load, which was very important information for the engineer responsible for the long-term safety and durability of the bridge.