Nonlinear Dynamic Response and Collapse Assessment of Irregular High-Rise Reinforced Concrete Structures Under Wind-Induced Loading in the Lagos Coastal Environment

Irregular high-rise reinforced concrete structures in Lagos's coastal built environment in the South West geopolitical zone are subjected to wind-induced dynamic loading that generates complex torsional and inter-storey response characteristics insufficiently captured by equivalent static wind force approaches in current practice. This study investigates the nonlinear dynamic response and collapse potential of irregular high-rise reinforced concrete structures under wind-induced loading specific to the Lagos coastal wind exposure environment, employing aeroelastic wind tunnel testing data and incremental dynamic analysis methodology. A series of three irregular 20-storey and 32-storey reinforced concrete frame-core wall building configurations, characterised by L-shaped, T-shaped, and setback plan forms, were modelled in OpenSees using concentrated plasticity nonlinear beam-column elements with degrading hysteretic behaviour. Wind pressure time histories were derived from a 1:400 scale aeroelastic wind tunnel model tested in a boundary layer wind tunnel, generating 72 one-hour equivalent wind pressure records across 36 wind directions. Collapse was assessed using incremental dynamic analysis with wind speed as the intensity measure, and collapse fragility curves were constructed. Results indicate that torsional irregularity in L-shaped and T-shaped plan forms amplified inter-storey drift ratios in the stiff wing corners by factors of 1.6 to 2.3 relative to rectangular buildings of equivalent height and area under the same wind input, creating localised demand concentration that initiated collapse mechanisms. Collapse probability under a 500-year return period wind event was 18 percent for the most irregular configuration, compared to 3.1 percent for an equivalent regular form. The study provides quantitative evidence supporting stricter plan irregularity limits in Nigerian high-rise design guidelines. Keywords: irregular high-rise buildings, wind-induced response, collapse assessment, nonlinear dynamic analysis, Lagos.