BRIDGE VULNERABILITY: GENERALITIES AND A CASE STUDY ANALYSIS

Abstract

Majority of the bridge collapses are due to hydraulic reasons. The Italian technical standards of construc-tions (NTC 2018) provide few indications about the design and verification criteria of bridges with respect to river processes. In the last year, a working group on the "Hydraulic Compatibility of Bridges" (sites.google.com/view/gii-ponti) was set up, with the aim of formulating proposals for good practices and guidelines for assessing the bridge hydraulic compatibility, as a basis for both bridge safety and flood risk analysis. The working subgroup on "Vulnerability" aims to define the methodology for assessing the impacts on the bridge, in or-der to verify the safety, in terms of forces on the bridge (piers, abutments and deck) and erosion (general-ized, contraction and localized). The present paper provides a summary of the activities carried out by the working subgroup, with the main considerations and comments and an example of the evaluation of the impacts on the bridge for a case study. Different formulas on the localized scour of the piers and abutments were analyzed. Literature formulas notoriously overestimate the localized scour depth because they consider the scour depth reached at equi-librium condition and because they are derived from the upper envelope of the experimental data. In addi-tion, different approaches were analyzed to estimate the force on the piers, abutments, and deck. An anal-ysis of the formulas was also started to evaluate the scour depth and the actions generated by accumula-tions of plants and floating materials. From the analysis carried out by the working subgroup, several important aspects emerged, which need to be developed and clarified, not only for the drafting of guidelines, but also for new research activities: i) influence of the duration and shape of the floods in the estimate of the scour depth; ii) evaluate how to combine the depth of localized scour with the contraction scour; iii) methods for evaluating the forces ex-erted on the bridge; iv) hydrodynamic (forces) and morphodynamic (scour) aspects under pressure or submerged bridge conditions; v) maximum scour of existing bridges as a function of the floods to which they are subjected; vi) scour generated by bridge protection systems (slab bridge foundation, bridge piers protected by rocks, etc.).