Structural assessment can be performed using semi-probabilistic methods, as is the case in design codes. However, these codes are primarily calibrated for the design of new structures and do not account for existing structures parameters, which need to be determined more precisely. To adapt the formalism of these codes, adjustment of partial factors is possible through the updating of the distribution of random variables. Challenges in reliability analysis of existing structures notably include data collection and the comprehensive analysis of structures. This study focuses on a family of small-span reinforced concrete bridges, known as Open Frame Underpasses (OFU), designed with the CHAMOA-P calculation model. This study particularly analyses the adjustment of partial factors using sensitivity factors obtained by reliability calculation rather than the default values used in the Eurocodes. Although this approach is intended to provide a more rigorous evaluation for the selected OFU existing structures, it presents challenges related to the analysis of these structures. The effectiveness of the parameter adjustment is evaluated by comparing the results with case studies and existing codes.