The concept, design and experimental validation of the new Avançon Bridge in Bex, Switzerland, are described. The lightweight glass fiber-reinforced polymer (GFRP) sandwich bridge deck adhesively bonded to steel girders reduced the traffic disruption period by approximately 40 days or 80% compared to a cast-in-place concrete bridge and also enabled the bridge to be widened to two lanes. The semi-integral bridge concept allows the application of a continuous asphalt layer across the abutments without expansion joints and thus facilitates and reduces maintenance. The GFRP sandwich deck with structural balsa core fulfils all the requirements concerning serviceability, ultimate limit state and fatigue.