Quantitative Risk Analysis of Building Collapse in Dense Urban Residential Neighbourhoods of Ibadan, Oyo State

Building collapse in densely populated urban residential neighbourhoods of Ibadan in the South West geopolitical zone is a recurring event with significant life safety consequences, yet the quantification of structural failure risk at the neighbourhood scale has not been formally undertaken. This study presents a quantitative risk analysis of building collapse across three high-density residential wards in Ibadan, employing a probabilistic framework that integrates structural fragility characterisation with building stock exposure modelling and consequence assessment. Field surveys were conducted to classify the structural typology, construction age, storey height, and observable condition of 620 buildings, enabling development of a neighbourhood building inventory. Structural fragility functions were derived from Monte Carlo simulation of structural capacity and demand for the dominant building typologies, calibrated against collapse records from the Ibadan Urban Flood Management Project database. Risk was expressed as annual probability of collapse per building and as expected fatalities per decade across each ward. Results indicate that the mean annual probability of structural collapse for the most common building typology, a one- to three-storey unreinforced masonry structure constructed before 1990, was 0.0032, corresponding to an expected return period of approximately 310 years. However, across the 620 buildings, this risk aggregate translates to an expected 1.8 structural collapses per year in the surveyed area, consistent with historical records. The study identifies buildings with visibly distressed foundations and inadequate lateral bracing as the highest individual-risk group. Recommendations include targeted structural assessments of pre-1990 buildings, a building condition disclosure register, and community-level risk communication programmes. Keywords: quantitative risk analysis, building collapse, urban residential, Ibadan, structural fragility.