Seismic response of braced frames with and without friction dampers

The dynamic response of friction damped bracing systems and ordinary cross-bracing systems inserted into surrounding frames without lateral stiffness is analysed in order to compare the typical behaviour of the two different bracing systems. The cyclic force-displacement relationship in the two cases is accurately modelled on the basis of experimental and related numerical models which are available in the literature, and the structural dimensionless parameters defining the shapes of the cycles are stressed. Systems having the same average period of vibration in the elastic phase of the response are assumed to be comparable with one another, which leads to a relationship linking the structural parameters of the two kinds of system. Extending the ductility concept to the friction devices, by a step-by-step integration of the equation of motion for the two systems the dimensionless structural responses are derived with variations in the strength level factor. Computing the average values of the maximum responses to five accelerograms artificially generated to be compatible with the normalized elastic spectrum proposed by Eurocode 8, the behaviour factor versus period curves are also deduced with variations in the available ductility. These curves show the different reliability levels provided by the two kinds of bracing systems considered and they are useful for practical applications. Two meaningful numerical examples, carried out by assuming an input accelerogram corresponding to an actual seismic record (EI Centro earthquake, 1940) confirm these design suggestions.