A first insight into towers' behaviour: geotechnical and structural mechanisms, leaning instability, long-term behaviour.

During the first stage of construction historic towers could have been not so far from a bearing capacity collapse, due to lack of strength of the soil, and safely survived thanks to some delay or interruption of the building process, that allowed the foundation soil to improve its strength and the tower to be successfully finished. Then, a more subtle and often unforeseen danger was represented by leaning instability, that increases if there is lack of stiffness of the soil. And, because of creep phenomena, a tower, even if initially stable, can attain with time an instability condition (asymptotic leaning instability). There also are aspects of structural nature, linked to the masonry behaviour as a unilateral material (Heyman, 1992), changes of geometry, reduction of cross section and stress concentration on masonry contact points, that deserve special attention, as proved by the collapse of the Campanile in Venice and the Civic Tower in Pavia. And there are aspects related to seismic vulnerability of towers, examples being unfortunately offered by the recent seismic events in Emilia Romagna and Central Italy. This Chapter is intended to be a deliberately short introduction to these problems, in order to provide an unifying framework to the contributions that Authors of this volume are dealing with in subsequent Chapters.