To enable acceptable seismic displacement demands to be estimated, the displacement capacity of a structural system needs to be known. This is controlled by selected performance criteria. It is postulated that the assignment of fractions of the required seismic strength of a system to its components may be arbitrary. This enables the displacement capacity of buildings to be evaluated without the knowledge of the magnitude of seismic strength. A redefinition of some traditionally used structural properties is a prerequisite for applications. The following study of reinforced concrete mixed systems illustrates rationale and extreme simplicity of application.