Episodes like impact of traffic vehicles on existing bridges occur very frequently manly due to the increase of traffic volume and vehicles dimensions respect of actual roads and overpass geometry. An interesting case of FRP strengthening on existing deck bridge has been applied to one overpass of state road SS36 (Lecco, Italy). Heavy damage at both intrados and webs of I girder prestress beams was caused by impact of a vehicle higher then the vertical clear dimension between main road and crossing overpass. Extensive damage to concrete, subject to spalling and cracking, and complete failure of ordinary reinforcement and prestress tendons was recognized by detailed inspection. Main concern of strengthening strategy was driven by absence of original detail design project, because lack of project documents as drawings or calculation reports from road owner and maintenance office. For this reason it was necessary to apply “back analysis” strategy of strengthening, assessing original strength contribution (both in flexure and shear) of each bridge girder portion which showed evident damage or failure. After repairing of damaged concrete by specific mortar, to recover original element geometry and inertia, carbon FRP sheets was designed and applied at free surface to increase both flexure and shear prestress concrete girder capacity. Main advantages of this strengthening application were shown by minimize time and cost of repairing without closing of main road.