For large structures subjected to air explosions, there is spatial variation in scaled distance and angle of incidence (α) on the structural surface. Currently, the reflection coefficient Cr is estimated either considering normal reflection or including the effect of α; however, there is no consensus on a single approach, which may lead to a significant difference in the estimated reflected pressure. Hence, the objective of this study is to investigate the effect of the estimation of Cr on the peak displacement of large homogeneous structures subjected to air explosions. In this study, a finite element model of a large structure, a concrete gravity dam without reservoir, with a height of 60 m and a slope of 0.8:1 on one side with a rigid foundation is considered. The Kinney-Graham, Newmark, and CONWEP models for blast parameters are used to estimate the spatially varying load-history. It is found that there is a difference of 66.7% between the peak crest displacement estimated using the two different approaches. Thereafter, the possibility of using equivalent models is investigated to enhance the computational efficiency. The results of the equivalent model are compared with a full-order model to assess the trade-off between accuracy and computational efficiency.