Influence of Pore Pressure on Natural Frequency Wandering of Structures under Earthquake Conditions

Temporary seismic frequency wandering of buildings during and after seismic events has often been observed and has not yet been investigated, in particular with regards to the effects of the pore pressures generated within the soil mass. This paper investigates the influence of delayed soil response on the observed temporary natural frequency wandering induced by seismic actions. The focus is on the pore pressure generation within the soil mass and its impact on the observed wanderings. Our findings show that the natural frequency of the system is lower shortly after the seismic event, but it returns to its initial value with time, as expected from measurements. The results are shown in terms of generated pore pressure, its dissipation over time, reduced mean effective pressure and nonlinear stress strain behaviour, all of them affecting the temporary changes in natural frequencies. Parametric studies on the influence of soil permeability, soil density and type of input motion are presented. Finally, a time history inclusive of an aftershock is investigated to explore the different soil-structure interaction mechanisms occurring during the main event and the subsequent aftershocks.