Seismic risk evaluation of historic masonry structures in an Algiers district, Algeria, using fragility and vulnerability functions: A case study of Belouizdad district

The seismic vulnerability of Algeria’s historic urban fabric, particularly its unreinforced masonry (URM) buildings, underscores the urgent need for effective risk-mitigation strategies. These culturally significant yet structurally fragile buildings are highly susceptible to earthquake damage. This study integrates fragility functions with a vulnerability index (VI) approach to evaluate the seismic risk of URM buildings in Algiers, with a focus on the Belouizdad district. By analyzing seismic scenarios with intensities ranging from Modified Mercalli intensity VII–X, the research develops and calibrates vulnerability and fragility curves, offering a probabilistic assessment of structural damage. Quantitative results indicate that about 80% of the surveyed URM buildings fall into the moderate vulnerability class (VI = 0.20–0.60), nearly 10% are classified as highly vulnerable (VI > 0.60), and approximately 9% show low vulnerability (VI < 0.20). Under a scenario consistent with the 2003 Boumerdes earthquake (PGA = 0.27 g), 29% of the building stock is expected to exceed the mean damage ratio (MDR) of 0.20 and 11% to exceed an MDR of 0.50. The findings confirm that the majority of URM buildings in the study area are moderately to highly vulnerable, largely due to their age, lack of maintenance, and structural modifications. The proposed methodology – implemented within a Geographic Information System for spatial analysis – facilitates the identification of priority areas for intervention and supports targeted risk-reduction policies. Ultimately, this work provides data-driven evidence for safeguarding Algeria’s architectural heritage and enhancing urban resilience against seismic hazards.