Rocking activation of free standing elements in real conditions: A safe experimentally-based acceleration limit

This paper presents an experimental campaign aimed to characterize rocking conditions of free-standing elements. Focus is given on the minimum horizontal acceleration required to activate rocking. The proposed experimental campaign mainly regards prismatic samples, that differ in size R (ranging from 6.30 cm to 23.0 cm), aspect ratio B/H (mostly 0.20 and 0.33) and material (wood, marble, steel). Small scale specimens are tested since no scale effect is expected in the rocking activation phenomenon. Both harmonic (continuous and pulse) and seismic input signals are applied for a total of more than three hundred tests. Experimental results evidence that, in most cases, rocking occurs for acceleration lower than the value generally assumed as the rocking limit, which is proportional, by acceleration gravity, to the aspect ratio. Therefore, a different value is proposed for the acceleration that triggers rocking phenomenon, with the aim of defining a safe acceleration threshold usable in vulnerability assessment of free standing elements. Actually, this limit takes into account the real conditions of rocking behavior, e.g. element imperfections, not perfectly rigid conditions for both block and supporting floor, not planarity of the contact surfaces.