Effect of steel collars on the seismic behaviour of granite and heavy stone columns

Granite and heavy stone circular columns could be often find in the cultural heritage among ancient churches and historical buildings in all the Mediterranean area. Their good strength properties allowed to carry great load values, while their bright colours and aesthetical characteristics have been used by a lot of ancient architects to achieve structural solutions with great visual impact. Even if such materials have great compressive strength values, the environmental effects, such as long term effects and thermal loads, could damage the structural members, by cracking them. In this way the slenderness of the column increases, and the presence of an imposed ground motion could be very dangerous, since the column will be more vulnerable to the rocking motion, which in the critical case could lead to the overturning or to material crushing due to the stress concentration. This paper focuses on the behaviour of granite and heavy stone columns subjected to rocking motion due to a ground shaking or to free oscillations. In this field, the effect of the presence of circular collars is analyzed and discussed and a simplified design method is proposed. The analysis is made in the dynamic field and the effects of the normal force and of an initial imperfection are taken into account.