Observations after strong earthquakes show that out-of-plane failure of unreinforced masonry elements probably constitutes the most serious life-safety hazard for this type of construction. Existing unreinforced masonry buildings tend to be more vulnerable than new buildings, not only because they have been designed to little or no seismic loading requirements, but also because connections among load-bearing walls and with horizontal structures are not always adequate. Consequently, several types of mechanisms can be activated due to separation from the rest of the construction. Even when connections are effective, out-of-plane failure can be induced by excessive vertical and/or horizontal slenderness of walls (length/thickness ratio). The awareness of such vulnerability has encouraged research in the field, which is summarized in this article. An outline of past research on force-based and displacement-based assessment is given and their translation into international codes is summarized. Strong and weak points of codified assessment procedures are presented through a comparison with parametric nonlinear dynamic analyses of three recurring out-of-plane mechanisms. The assessment strategies are marked by substantial scatter, which can be reduced through an energy-based assessment.
Lagomarsino, Sergio;Magenes, Guido
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