Limit analysis approach for the seismic vulnerability reduction of masonry towers through strengthening with traditional and innovative materials

The paper investigates the possibility and the effectiveness of reducing the seismic vulnerability of masonry towers by means of composite materials and traditional steel bands. Masonry towers are very widespread in Italy, both as bell towers for churches and defense towers in medieval cities and castles. Masonry material, presenting low mechanical properties, is not suitable to withstand significant tensile and compression stresses induced by earthquake loading. The slenderness of these structures is another factor that can reduce the bearing capacity when significant stresses are present in specific structural parts. The seismic vulnerability of masonry towers is very high, as a consequence of both poor material properties in tension and high compression levels at the base of the structure. Moreover, Italy is characterized by a high potential risk to be stricken by moderate/high seismic events, as experienced in the last decades. In such a situation, the seismic upgrading of masonry towers could appear rather important. Seismic upgrading by introducing both traditional steel bars and composite materials as strips or rebars is analyzed in detail for different towers. Based on some a priori assumed failure modes (one proposed by Heyman), simplified models from a limit analysis approach are here discussed and used to define the most suitable retrofitting solution. The retrofitting possibilities consist of: a) horizontal hooping rings; b) vertical pre-stressed tie rods; c) vertical composite strips. A simplified straightforward relationship is found between the retrofitting requirements and seismic hazard. The procedure is applied for a wide range of geometrical properties and appears to be fast and reliable.