Thesis title: Seismic Vulnerability Assessment of Existing URM Structures through a Simplified Analytical Method
Existing UnReinforced Masonry (URM) buildings are characterized by a high seismic vulnerability, as seen from the consequences of past earthquakes. These damages can be essentially interpreted on the basis of two collapse mechanisms: Out-Of-Plane and In-Plane (when the “box-behaviour” is guaranteed), according to the structural details of the buildings and the direction of the seismic action.
Several numerical tools have been developed for the seismic analysis of URM structures but most of them are very computationally demanding and too complex for the daily use of practicing engineers.
This Thesis proposes a simplified analytical (“by-hand”) procedure for the seismic vulnerability assessment of existing URM structures, from an analogy with the SLaMA (Simplified Lateral Mechanism Analysis) method, developed and extensively validated for reinforced concrete structures. This mechanism-based methodology provides a first approximation of the global seismic behaviour of a building starting from the sub-system level.
Key features of the SLaMA procedure are the evaluation of: a) the moment-curvature/rotation capacity curves at component level; b) the hierarchy of strength at subassembly level and c) the capacity curve of the global mechanism. When dealing with URM structures, the building geometry is defined by means of an Equivalent Frame model and to assess the analytical elements bending capacity a peculiar section analysis, the Monolithic Beam Analogy approach, is implemented.
Experimental in-plane tests on panels, one-storey substructure and two-storey wall, available in literature, are used to validate the SLaMA-URM procedure against macro-mechanical finite element and equivalent frame modelling.
Through a parametric analysis, prototype buildings with different geometric configurations are analyzed by the SLaMA-URM method, where the interaction between the In-Plane and the Out-Of-Plane performances of walls is taken into account, thus obtaining the global structural performance.
Some retrofit strategies are then applied and their effect is evaluated through the procedure, in order to achieve a higher capacity, both in terms of the ultimate performance (%NBS) and the expected economic losses (EAL). To further investigate the different response of the structures, two seismic intensities (high and medium) of the construction site are considered.
At the end, a cost-benefit analysis is performed to facilitate the selection of the most appropriate retrofit strategy, not only through the total intervention costs but also through a useful parameter that defines the cost of one percentage point of NBS (1%NBS cost).
In conclusion, the SLaMA-URM method, being totally implemented by simple spreadsheets, wants to be an effort in the process to put the designer at the center of the seismic performance assessment and of the design of retrofit interventions, allowing him to acquire a greater sensitivity towards the obtained results.