Reliability Analysis of Reinforced Concrete Columns in Typical Nigerian Multi-Storey Frames Under Combined Loading

Structural reliability analysis provides a probabilistic framework for quantifying the adequacy of structural components against failure, offering a more complete picture of safety levels than the deterministic partial factor approaches embedded in current Eurocodes and Nigerian standards. This study conducts a structural reliability analysis of reinforced concrete columns in typical Nigerian multi-storey frame configurations, focusing on combined axial and uniaxial bending failure under the statistical variability of material properties, construction tolerances, and loading representative of Lagos and Kano in the South West and North West geopolitical zones. Resistance models for column cross-sectional capacity were derived from Monte Carlo simulation using distributions of concrete compressive strength, steel yield strength, and geometric dimensions derived from quality control data at Nigerian construction sites. Load effect distributions were modelled using extreme value statistics applied to live load survey data and tributary area analysis. Reliability indices were computed and compared against the Eurocode 0 target of beta equal to 3.8 for a 50-year reference period. Results indicate that columns designed to the minimum requirements of Eurocode 2 using Nigerian site-representative material variability achieved reliability indices ranging from 3.2 to 4.1, with the lowest values associated with columns having the highest slenderness ratios and greatest live-to-dead load ratios. The use of mean in-situ concrete strength 20 percent below design characteristic strength, observed in site surveys, reduced reliability indices by 0.4 to 0.6 units. The study recommends partial factor adjustments for columns in low supervision construction environments and provides column-specific reliability calibration data to support revision of the Nigerian National Building Code. Keywords: reliability analysis, reinforced concrete columns, Monte Carlo simulation, combined loading, safety index.