Empirical Assessment of Punching Shear Failure Risk in Post-Tensioned Flat Slabs in Commercial Buildings in Victoria Island, Lagos

Post-tensioned flat slabs are increasingly adopted in high-value commercial developments on Victoria Island in Lagos State within the South West geopolitical zone, offering span efficiency and reduced floor-to-floor heights. However, the punching shear failure risk at column-slab connections in post-tensioned systems is a nuanced design challenge that depends on the interaction of prestress, concrete tensile strength, and column geometry. This study empirically assesses punching shear failure risk in post-tensioned flat slab buildings using a combination of construction document review and instrumented field monitoring in three recently constructed commercial buildings on Victoria Island. Structural drawings from fifteen buildings were reviewed to assess compliance with punching shear provisions in BS 8110, Eurocode 2, and the ACI 318 post-tensioned slab addendum. For the three monitored buildings, fibre-optic strain sensors were installed at critical column-slab junctions during construction, and stress distributions were recorded through the construction loading sequence and during the first year of occupancy. Results indicate that slab designs in six of the fifteen reviewed buildings contained potential punching shear deficiencies in at least one column-slab connection, principally at re-entrant corner columns where plan irregularities reduced effective perimeter length below code minimum requirements. Monitored buildings showed slab strains at re-entrant column connections exceeding predictions based on uniform prestress distribution assumptions, by factors up to 1.34, suggesting localised prestress loss or eccentricity effects not captured in simplified design models. The study recommends installation of punching shear reinforcement at all re-entrant corner connections and adoption of detailed finite element prestress distribution modelling for irregular flat slab geometries. Keywords: post-tensioned flat slab, punching shear, Victoria Island, column-slab connection, structural monitoring.