Mitigation assessment of passive seismic protection

The intended contribution proposes a versatile approach of assessing the efficiency of seismic mitigation of passive seismic protection via supplemental damping - of steel multi storey type structures. The efficiency of seismic mitigation is expressed in terms of reduction in the amplitudes of kinematical parameters (top lateral displacements, in this case) associated to seismically induced vibratory motion of the analyzed structures. The proposed approach may be applied to other parameters (story drifts, induced velocities and accelerations, ductility coefficients) describing static and kinematical states of steel skeletal structures seismically acted upon structures. The proposed parameter for assessing efficiency of seismic protection is associated to the mitigation interval of steel frame type structures equipped with passive protection (viscous dampers). This interval is expressed in terms of both, time and fundamental natural period of vibrations of the structure and is considered from the moment when the vibratory motion is initiated till the moment the motion reaches its steady state. During this time interval, a clear picture of vibratory motion is exhibited: length of the interval, number of vibratory cycles, amplitudes, their variation / decrease in time, the end of transitory motion and the beginning of the (short, nevertheless) induced pseudo - steady state motion. The intended contribution proposes a time variable parameter that synthesizes all these features of the transitory motion. The length of this interval is expressed in number of natural fundamental periods of vibrations and together with its descending slope emphasizes the effectiveness of seismic protection. The shorter is the interval and the greater is its descending slope, the more effective is the associated seismic protection. Time history type analyses have been carried out on several sets of skeletal steel structures. The structures are analyzed in two cases: without seismic protection (reference structure) and equipped with passive seismic protection (viscous dampers). The numerical results of time history analyses are presented and discussed with reference to the proposed parameter assessing seismic mitigation. The time variation of the proposed parameter is presented graphically for a better and immediate "physical" perception. The effectiveness of the seismic passive protection is discussed in terms of proposed parameter.