Effective mass is a dynamic property of a structure associated with the modal characteristics; natural frequencies and mode shapes. It is a measure used to classify the importance of a mode when a structure is accelerated via its base. Additionally, a structure under acceleration loads can be affected by the distribution of modal effective masses in different frequencies. Hence, cumulating the effective mass in one frequency may cause a considerable reaction force in the structure. In this paper, effect of structural stiffening in a small satellite structure has been studied. Results show that both the dynamic response and equivalent static loads can be affected by characteristic of stiffening, which is a direct consequence of spreading of modal effective masses among more frequencies. Results of analyses performed on access doors of different geometries, with reinforcement frames having different stiffness, show that for the case of constant satellite structure total mass, the equivalent loads due to random vibrations and shock phenomenon change by 33% and 91%, respectively.