Natural hazards such as earthquakes and strong wind events place large forces on tall, slender structures and also on long bridges. The structural system usually can be described by a Lagrangian model. In view of numerous uncertainties due to model errors, stress calculations, material properties, and load environments, the system is uncertain. Here, the Lagrangian structural system is modeled as an uncertain state space model. The paper develops a robust active control approach with uncertainties in the system, control input, and disturbance input matrices. Robust active control provides both robust stability and FL. disturbance attenuation. The H©. norm of the transfer function from the external disturbance forces (e.g., earthquake, wind, and etc.) to the observed system states is restricted by a prescribed attenuation index. Considered uncertainties are norm-bounded unstructured uncertainties. Results may be further extended to structural uncertainties. Also, the results may be applied to robust control analysis and design of structural systems.