A simplified procedure for the seismic safety of large and slender rigid masonry blocks

This paper aims at finding the features of an artificial accelerogram which can be used to assess the seismic safety of large and slender rigid masonry blocks. The most important of these features derives from a simple observation: well-known uncertainties on time variations of seismic actions imply that seismic risks analyses must be developed without privileging any detailed description of expected accelerograms. It follows that the best choice to build an artificial accelerogram is to represent it with sequences of instantaneous Dirac impulses. Obviously, in order to be effectively used, the accelerogram must cover the widest range of situations with a few parameters. This implies the need to look at the most unfavourable effects, also because of the large uncertainties that still exist about future seismic actions. With this aim, an artificial limit accelerogram is proposed to represent the most unfavourable effects of the intense phase of an earthquake. This consists of a sequence of instantaneous impulses, all applied right after the impact of the blocks on the ground. A secondary sequence of intermediate impulses is then introduced to reduce the resonance effects and to cover a broad range of conditions. Approximate equations of motion of the rocking blocks are also proposed and the accuracy of the proposed formulations is estimated. Numerical analyses are performed to highlight the most significant aspects of the proposed approach.