Seismic Retrofit Strategy Selection and Prioritisation for Existing RC Road Bridges

The accelerated deterioration of reinforced concrete (RC) road bridges built in the post-World War II period, combined with increasingly stringent seismic-performance requirements, calls for structured and scientifically grounded procedures for selecting retrofit strategies. This paper proposes an integrated four-phase methodology combining (i) calculation of the verified safety index (ISV) according to the VAL4 procedure; (ii) estimation of the discounted life-cycle cost (LCC) over a 100-year horizon; (iii) GIS analysis of alternative traffic routes; and (iv) a two-level simple additive weighting (SAW) multi-criteria analysis, supported by five deterministic weighting scenarios. The framework is applied to a three-span Gerber-type RC road bridge (L = 78 m), classified in the highest urgency category (attention class CdA 1) according to Italian bridge-management guidelines, with an initial seismic safety index ISV = 0.06. Three intervention alternatives are compared: (A) in-situ structural upgrading with carbon-fibre-reinforced polymers (CFRP) and concrete jacketing; (B) partial demolition of the central span with FRP upgrading of the retained spans; and (C) full demolition and reconstruction. Alternative C obtains the highest base-case SAW score and ranks first in four of the five weighting scenarios examined and in about 99% of 20,000 randomly sampled weight vectors; Alternative A prevails only when traffic impact receives a weight close to its upper bound and seismic safety a weight close to its lower bound. At the base real discount rate of 2%, the 100-year LCC values are EUR 3.98 M for C, EUR 4.45 M for A, and EUR 5.84 M for B, preserving the same ranking as the undiscounted estimates. The case study therefore shows that full replacement may be both structurally and economically preferable. The proposed framework offers a transparent and replicable decision-support procedure for public authorities managing seismically vulnerable bridge assets.