Structures made of FRP composites have been shown to provide efficient and economical applications in bridges and piers. They are being increasingly used due to their several advantages when compared to traditional materials, namely, the lightness, strength, good insulation properties, low maintenance and improved performance when submitted to aggressive environments. However, fire behaviour has been recently identified by several authors as the most critical gap for these materials to be fully exploited. In bridge construction, decks are the most vulnerable element in the bridge system due to its exposition to the direct actions of wheel loads, chemical attack, temperature/moisture effects, fatigue and fire. In addition, composites used in civil infrastructure pose an unusually high hazard since polymers are highly flammables and release copious amounts of heat, smoke and fumes when they smoulder and burn. Furthermore, when they are exposed to high temperature (typically over 100ºC) the polymer matrix will soften, and this can cause distortion, buckling and failure. Within this general context, and using as a case study the new Avançon Bridge in Bex (Switzerland), this Master thesis delves into the thermal, mechanical and thermomechanical response of a GFRP-balsa sandwich bridge deck at elevated temperatures. By the development of a numerical model using the finite element (FE) program Abaqus, the thermomechanical response will be studied considering the non-linear response of the structure with temperature dependent material properties.