The Impact of High-Frequency/ Low-Energy Seismic Waves on Unreinforced Masonry

Traditionally, the high-frequency components of earthquake loading are disregarded as a source of structural damage because their energy content is small and their frequency is too high to resonate with the natural frequencies of structures. We argue that higher-frequency waves traveling through stiff masonry structures can trigger two types of failure mechanisms that have not yet been taken into account. First, the high frequencies can cause small vertical interstone vibrations that result in irreversible relative displacements of the stones, which may ultimately lead to collapse. The energy needed to cause this deformation and failure comes largely from gravitational forces. Second, the partial fluidification and densification of the loose, granular inner core of some unreinforced masonry walls results in an increase of outward thrust. Preliminary results of a series of static and dynamic tests, as well as numerical models, demonstrate the potentially destructive effects of high-frequency/low-energy seismic waves on unreinforced masonry structures. Based on this new understanding, an improved construction method is suggested.