Effectiveness of passive seismic control of steel structures

This paper introduces a simple approach in assessing the amount of seismic mitigation of passive seismic protection of steel skeletal type structures. The amount of seismic protection is expressed in terms of reduction in the amplitudes of kinematical parameters associated to seismically induced oscillatory motion of the analyzed structures. The introduced parameter for assessing seismic protection is associated to the mitigation interval of steel skeletal structures equipped with passive protection. This interval is expressed in terms of natural vibration periods of the structure and is considered from the moment when the vibratory motion is initiated until 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) pseudo steady state motion. The paper proposes a time variable parameter that synthesizes all these features of the transitory motion. The length of this interval and its descending slope emphasize the effectiveness of seismic protection. The shorter the interval and the greater its descending slope, the more effective 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 situations: 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.