Seismic Assessment of Unreinforced Masonry: High Frequency / Low Energy Seismic Waves

Traditionally, the high frequency components of earthquake loading are disregarded as a source of structural damage because of their small energy content. This argument is correct only if we consider macroscopic out-of-plane bending failures and in-plane shear failures, which indeed require a large energy input from earthquakes. In common cases where the wall is made up of two external masonry skins, or wythes, containing a loose rubble infill, I argue that the higher frequency waves travelling through stiff masonry structures can cause two types of failure mechanisms that have not yet been taken into account. First, the high frequencies can cause small vertical inter-stone vibrations that result in irreversible relative displacements of the stones. These relative displacements occur because in the majority of stone masonry structures, the stones are not cubic shaped, but are instead pyramidal, with their bases oriented toward the exposed part of the wall, and the tips pointing inwards. The conical shape of the stones facilitates the downward sliding of the stones, which leads to a relative displacement of the masonry units. As a consequence of this relative movement of the stones, the wall becomes deformed and unstable, ultimately crumbling under its own weight. While the trigger may be high frequency components of vibration, the actual energy needed to cause this deformation and failure mechanism comes from gravitational forces. Hence, small energy, high frequency seismic waves can trigger the collapse mechanism for poorly built walls. A second failure mechanism, which is also as yet unaccounted for, is associated with the outward thrust from the compaction of the loose inner core of the wall. In masonry walls the inner core is mostly made up of loose sand and gravel that has the tendency to compact and undergo shear fluidification when experiencing high-frequency vibrations. The lateral thrust caused by compaction of loose soils can be as high as the weight of the compacted soil column, and this thrust will push the unstable masonry units outward causing the collapse of the exterior masonry skins. This failure mechanism will compound the effect of the inter-stone displacement elicited by the high frequency motion components.

The investigation of these two new failure mechanisms will be done in three phases. The first phase of the research will include an in-depth review of the state-of-the-art methods to analyze masonry failures and the construction of physical models to identify and quantify the parameters that control the failure. In the second phase, these failure parameters will be used to create a computational model using discrete elements, which I will use to systematically investigate the effect of changing a number of wall parameters 3½4slenderness ratio, block size, joint thickness, opening distribution, excitation frequencies3½4 on the two new failure mechanisms. The discrete element model will also be used to study different options to mitigate the effect of higher frequency waves on masonry, and ways to effectively retrofit masonry for this kind of loading. The final phase of the research will involve building and testing a series of specimens to corroborate the results obtained with the discrete element models. These two new failure mechanisms will then be integrated with other known masonry failure mechanisms to define a more complete picture of the stability and peformance of masonry structures under seismic loads.

A series of brick specimens are being built and tested on a vibrating table of fixed frequency of 60Hz. These specimens are made up of two independent leaves of brick masonry with the core space equal to the brick with (9cm) filled in with sand or gravel. The infill material will be contained, first in a plastic bag and then by the masonry itself. The in-plane boundary conditions will vary depending on how the infill is contained. In the case of the core material being contained in a plastic bag, the plastic bag itself externally supported by bricks will be the boundary condition. Where the core material is not contained in a plastic bag, the lateral boundaries will be made with cardboard and supported with bricks.