A large part of the Italian road network relies on reinforced concrete bridges built in the decades after the Second World War. Many of these structures are now close to or beyond their intended service life, and several show clear signs of material degradation. Their condition has made structural assessment a priority, and the 2020 Italian Guidelines for existing bridges set out a multi-level procedure to carry it out. The most in-depth stage of this procedure is the Level 4 assessment, which relies on advanced analysis. Here nonlinear finite element analysis is the main tool, but its reliability rests on calibration against experimental data.