Seismic Vulnerability Assessment of Aging Highway Bridges Using Fragility Curves

A substantial proportion of the highway bridge stock in Sub-Saharan Africa was constructed during the 1960s–1980s under infrastructure development programmes funded by bilateral donors and development banks. These bridges, now 40–60 years old, were designed to pre-modern seismic codes with limited ductility detailing, minimal capacity design principles, and no explicit consideration of material degradation over the service life. Concurrent chloride-induced corrosion of reinforcing steel — accelerated by tropical humidity, proximity to marine environments, and substandard construction practices — has reduced structural capacity by an estimated 15–30% relative to original design values in many cases. This paper develops time-variant seismic fragility curves for a representative three-span reinforced concrete box girder highway bridge (45+60+45 m, built 1978) on the South Sudan primary road network, accounting simultaneously for seismic demand characterisation, structural capacity degradation from chloride corrosion, and epistemic uncertainty in structural modelling parameters. The analysis employs nonlinear static (pushover) and incremental dynamic analysis (IDA) methods implemented in an OpenSees finite element model using fibre-discretised pier cross-sections with age-dependent material constitutive laws. Twenty ground motion records (10 historic, 10 code-compatible synthetic) are used to characterise the ground motion uncertainty. Fragility curves are expressed as lognormal cumulative distribution functions P(DS >= ds | PGA) with intensity measure PGA (g) and four discrete damage states: Slight, Moderate, Extensive, and Complete. Key findings: (i) at the design-level hazard (PGA=0.18g, TR=475yr), the aged bridge (t=45 yr) has P(Complete Damage) = 0.08, compared with P