Comparative Design and Cost Assessment of Pad Foundations under Varying Soil Bearing Capacities

Foundation performance and construction economy are strongly influenced by the engineering properties of the supporting ground, particularly allowable bearing capacity. This study comparatively evaluates the structural design and estimated cost of isolated reinforced-concrete pad foundations supporting a one-storey residential building under six allowable bearing-capacity conditions: 50, 100, 150, 200, 250 and 300 kN/m². The original design model was analysed using ProtaStructure and developed using Eurocode-based principles. Foundation dimensions, depths, reinforcement-related design actions, bending moments and shear responses were compared, after which the resulting quantities were assessed using a bill-of-quantities approach with constant study rates. The estimated foundation cost decreased substantially from ₦7,282,222 at 50 kN/m² to ₦3,014,905 at 250 kN/m². A further increase to 300 kN/m² produced a marginal increase to ₦3,015,128. This indicates that once practical minimum footing dimensions and structural detailing requirements become controlling, additional bearing capacity may provide little or no further economic benefit. Within the scenarios investigated, 250 kN/m² produced the lowest estimated cost. The findings demonstrate that foundation optimisation requires integration of geotechnical investigation, structural design and quantity surveying rather than consideration of bearing pressure alone.