Blast resistant modular steel (BRMS) buildings are prefabricated cost-effective and explosion-proof structures used to shelter personal near hazardous-material handling or storage facilities, in facilities handling high-pressure processes, and within designed blast zones in mining, petrochemical, and military industries. Typically, BRMS buildings are designed to be fixed (anchored) to their foundation. However, this design approach can result in very large anchorage and foundation reactions, which result in very large and high-cost foundations. Therefore, an alternative foundation approach which minimize foundation reactions and known as unanchored or free-to-slide foundation has recently attracted much attention for BRMS buildings. In this study, dynamic behavior of a typical BRMS building under blast loading with unanchored foundation was determined using 3D nonlinear dynamic finite element and a simplified numerical analysis method. The developed numerical analysis method is based on nonlinear numerical integration scheme, and it was verified using the finite element results. Applicability of the proposed numerical analysis method was evaluated using finite element results for a typical unachored BRMS building subjected to blast loads. Several values of coefficient of friction between the foundation and BRMS building were considered. The computed building sliding, horizontal velocity, acceleration, and horizontal foundation reaction histories were used to evaluate applicability of the proposed numerical method. The overall results show that the proposed simple numerical analysis method based on rigid-body dynamics is suitable to predict building sliding and sliding velocity, but unconservative in terms of foundation vertical reaction and horizontal acceleration.