Engineering analysis of strong ground rocking and its effect on tall structures

Two, the most intensive 6-dof ground motion records were selected from a database of a program of acquiring seismic aftershocks at Kefalonia Greek Island as reported by Perron et al. in 2018 [1]. The maximum Modified Mercalli (MM) intensity VII and peak ground velocity 18.4 cm/s reached during these seismic events made it possible to investigate substantial structural response under combined translational and rotational excitations. Respective horizontal-rocking effects are studied first on a simplified cantilever model of a slender tower. Next, the seismic responses of two 10 and 30 story tall buildings and a 160 m, reinforced concrete industrial chimney under the joint rocking and horizontal excitations are investigated in detail. A complicated pattern of translational-rocking seismic effects on slender structures was observed. It was noted that sometimes the presence of seismic rotations about horizontal axis increases the overall response (up to 30%), sometimes it decreases the combined seismic response. The net effect depends primarily on two, the most important structural parameters: fundamental natural period and effective height of the analysed structure. The rocking response spectrum proposed in Eurocode 8 part 6 for slender structures was compared with seismic time history response analyses of the r/c chimney, yielding its substantial conservatism compared with the time history response computations using the chosen records of 6-dof strong ground motion. It is also concluded that the application of the familiar SRSS rule to combine the horizontal and rocking effects may underestimate the overall seismic response. This may be important for future calibrations of the seismic codes including rocking ground motion effects.