Identifying seismic local collapse mechanisms in unreinforced masonry buildings through 3D laser scanning

The surveys following severe earthquakes show that existing unreinforced masonry buildings are highly vulnerable to local collapse mechanisms. However, their assessment is strongly sensitive to the choice of the mechanism, whose boundary conditions are largely unknown. In the past the mechanism has been selected based on the crack survey alone, because the survey of the deformations is very difficult if traditional tools are used. In the last years advanced survey techniques have been developed, the most powerful of whom resorts to laser scanning. A laser scanner allows the acquisitions of a very large amount of information: building overall dimensions and single elements detailed survey, detection of anomalies, and identification of very limited deformations undetectable with the naked eye. Moreover, contrary to traditional procedures, it allows the survey of the façades without any direct contact with the building, which could be damaged after an earthquake. A laser-scanner survey has been performed in the whole historical centre of Rovere, in the municipality of Rocca di Mezzo, affected by the 2009 L’Aquila earthquake. This survey has been used to study the façades of three different building units, recognising the collapse mechanism triggered by the earthquake ground motion. The mechanisms are fairly different from what suggested by the crack pattern alone and pertain to deformations that cannot be recognised in the photos. Moreover, the faithful geometric models that can be generated from laser scanning allow accounting for deformations and out-of-plumb. Thus, the acceleration activating the mechanism can be estimated much more accurately compared to a perfectly vertical and parallelepiped wall.