Pedestrian underpass structures play a critical role in urban infrastructure by allowing safe pedestrian movement beneath highways and railways. This study presents the structural analysis, design, and parametric study of a reinforced cement concrete (RCC) box culvert designed for pedestrian use and vehicular loading as per IRC Class AA standards. The investigation emphasizes soil-structure interaction (SSI) effects on bending moments, shear forces, and deflections using STAAD.Pro. Varying subgrade modulus values simulate different soil types, and parametric analysis reveals the significant impact of wall and base spring supports on structural performance. The study establishes design considerations to optimize stability, safety, and economy in underpass construction.