A nonlinear robust control is developed for active mass damper system subject to external perturbation. This nonlinear controller is composed by the sum of a linear term plus a chattering component. The linear term is designed using linear matrix inequality (LMI) theory. Then, the chattering term is added to improve controller performance. Lyapunov theory is used to validate our control design. According with experiments, where a flexible two levels building with active mass damper and external perturbation is employed, they show that this chattering term improves controller performance. However, when a fault occurs, this chattering term is complaining.