Intense underground explosions may cause severe damage to nearby residential, industrial and public structures. The available empirical allowable ground vibration levels to prevent damage to surface structures are usually given in terms of peak velocities (PV). There may be many ways to mitigate the ground shock effects on structures. Among the economical methods, providing a layer of a sand base is expected to effectively filter out high frequency stress waves and thereby reduce the response and damage of structures. However, very few publications are available in the unclassified literature discussing the responses of structures with a layer of a sand base subjected to blast excitations. This study performs the dynamic response analyses of structures resting on a layer of a sand base subjected to blast induced ground excitations. A previously calibrated numerical model for stress wave propagation in a rock mass is used to generate the input ground motions to the structural model. A granular material model for sand and a damage model for reinforced concrete structure are implemented into the commercial software Autodyn3D as its user subroutines for the analysis. Numerical results demonstrate that using a sand base is effective to reduce the structural response and damage to blast induced ground motions in both high and relatively low frequency ranges, even though the isolation effects tend to reduce with decrease of the ground motion principal frequency.