Energy absorption due to the dynamic loads on the moment resisting frame structures often cause damage to elements of the structure. In this study, to minimize the damage that occurs in the moment resisting frame structures, than do the protection of structure such as the addition of additional mass as a external damper expected to give a better response to dynamic load, such as period of vibration shorter, and smaller displacement of structure.In this study conducted experimental studies and analytical portal structure without external damper and the external form of additional mass damper. In the laboratory experiment, the model given impact load. This simulation gives the output in the form of natural frequency, displacement, and the value of the damping ratio of the structure. The results of the study indicate that the external damping in the form of additional mass is not good enough to contribute to the change in natural frequency (fn), but quite effectively provide value changes damping ratio (ξ), the period of vibration (T), and the maximum displacement (δmax) of the structure.