Miglioramento sismico di torri snelle in muratura mediante dispositivi isteretici e precompressione parziale

In Italy and in many Countries of the Mediterranean area, minarets and towers in general, brick or stone masonry made, represent one of the most valuable historical, artistic and architectural heritage. Moreover, the preservation and conservation of industrial heritage, of which masonry chimneys are one of the most relevant sign, are taking increasing importance. In slender masonry towers, axial forces resulting from self-weight often are close to the limit value of the masonry compressive strength. When they are located in seismic zones, the coupling of gravitational loads with dynamic actions can cause high damage and collapse even for moderate values of the seismic action. The request of conservation of monumental heritage leads to the need for researching, testing and developing useful interventions aimed to the seismic improvement of slender towers. Indeed, for this problem an effective response have not yet found even for the difficulty of modeling towers behavior considering the material and geometry high nonlinearity. This paper proposes a new solution for seismic upgrading of slender towers. The idea consists in the introduction of hysteretic elasto-plastic devices with inhibited instability (BRAD), coupled with a low level of partial pre-stressing of the tower. The effectiveness of the proposed retrofitting method has been demonstrated with reference to a real bell tower. Non-linear dynamic analysis, using a set of seven spectrum-compatible accelerograms scaled for different levels of PGA, were performed on the tower above mentioned, varying the stiffness and the yield force of devices in order to identify the optimum design of the intervention. For describing the dynamic behavior of the tower a numerical beam model was used. The nonlinearity of the material was described through a damage fibers model, whereas the hysteretic devices were described by means of nonlinear concentrated spring. The retrofitting performance was measured considering the various parameters of displacement, acceleration, and local and global masonry damage.