Analytical Investigation of Progressive Collapse Resistance of Reinforced Concrete Frame Buildings Using Alternate Load Path Method

Progressive collapse, the disproportionate spread of localised failure through a structure, represents a low-probability but high-consequence failure mode that is inadequately addressed in the design of most reinforced concrete frame buildings in Nigeria. This study analytically investigates the progressive collapse resistance of typical Nigerian reinforced concrete frame building configurations using the alternate load path method, as specified in UFC 4-023-03. A parametric study was conducted using finite element models of 4-storey, 8-storey, and 12-storey residential frames representative of typologies common in Abuja and Lagos, with column removal scenarios at corner, perimeter, and interior positions. Nonlinear static and dynamic analyses were conducted to evaluate demand-to-capacity ratios in beams and connections adjacent to removed columns. The influence of structural regularity, tie force continuity, and column span-to-depth ratio on collapse resistance was systematically assessed. Results indicate that corner column removal produced the most critical progressive collapse scenarios in all frame configurations, with demand-to-capacity ratios exceeding unity in 4-storey frames designed to minimum Eurocode 2 requirements without progressive collapse consideration. Interior column removal scenarios were successfully bridged through Vierendeel action in frames with continuous top reinforcement, while perimeter removals highlighted the vulnerability of short cantilever edge beams. The study identifies the minimum reinforcement continuity conditions required to achieve collapse resistance without dedicated progressive collapse reinforcement, providing practical guidance applicable to Nigerian structural engineering design practice. Keywords: progressive collapse, alternate load path, reinforced concrete frames, nonlinear analysis, robustness.