Propagation of Earthquake Waves in Buildings with Soft First Floor

Simplified response of buildings with a soft first floor and excited by propagating wave motion at the base are studied, so that the physical phenomena associated with phased input excitation can be characterized. Analytical solutions are obtained for two‐dimensional continuous building models, neglecting the soil‐structure interaction, and for monochromatic antiplane excitation. It is shown that the wave energy does not always propagate from the ground into the building, depending on the value of the horizontal phase velocity of the ground motion. There is a possibility that it propagates only into the “soft” first floor, which then acts as an “isolation layer” for the upper floors. However, this is at the expense of very large deformations of the columns of the first floor, that are not considered in the conventional analysis of buildings. Also, the out‐of‐phase motion of the base may excite torsional vibrations with large amplitudes. It is concluded that the design for P‐delta effects in the soft first floor should not ignore those additional wave passage effects.