Shaking Table Tests Validating Two Strengthening Interventions on Masonry Buildings

摘要

Masonry buildings constitute quite often a precious cultural heritage for our cities. In order to future generations can enjoy this heritage, thence, effective projects of protection should be developed against all the anthropical and natural actions which may irreparably damage old masonry buildings. However, the strengthening interventions on these constructions have to respect their authenticity, without altering the original conception, not only functionally and aesthetically of course, but also statically. These issues are of central interests in the Messina area, where the seismic protection of new and existing constructions is a primary demand. It is well known, in fact, that the city of Messina lies in a highly seismic zone, and has been subjected to two destructive earthquakes in slightly more than one century, the 1783 Calabria earthquake and the more famous 1908 Messina-Reggio Calabria earthquake. It follows that the retrofitting projects on buildings which survived these two events should be designed with the aim to save the life of occupants operating with “light” techniques, i.e. respecting the original structural scheme. On the other hand, recent earthquakes, and in particular the 1997 Umbria-Marche sequence, unequivocally demonstrated that some of the most popular retrofitting interventions adopted in the second half the last century are absolutely ineffective, or even unsafe. Over these years, in fact, a number of “heavy” techniques proliferated, and therefore old masonry buildings suffered, among others, the substitution of existing timber slabs with more ponderous concrete slabs and/or the insertion of RC and steel members coupled with the original masonry elements (walls, arches, vaults). As a result, these buildings have been transformed by unwise engineers into hybrid structures, having a mixed behaviour (which frequently proved to be also unpredictable) between those of historic masonry and new members. Starting from these considerations, a numerical and experimental research has been carried out, aimed at validating two different strengthening interventions on masonry buildings: (i) the substitution of the existing roof with timber-concrete composite slabs, which are able to improve the dynamic behaviour of the structure without excessively increase the mass, and (ii) the reinforcement of masonry walls with FRP materials, which allow increasing both stiffness and strength of the construction. The experimental tests have been performed on a 1:2 scale model of a masonry building resembling a special type, the so-called “tipo misto messinese”, which is proper to the reconstruction of the city of Messina after the 1783 Calabria earthquake. The model, incorporating a novel timber-concrete composite slab, has been tested on the main shaking table available at the ENEA Research Centre “Casaccia,” both before and after the reinforcement with FRP materials. Some aspects related to the definition of the model and to the selection of an appropriate seismic input will be discussed, and numerical results confirming the effectiveness of the interventions mentioned above will be presented.