Seismic vulnerability of unreinforced masonry building aggregates of variable heights with soft storey basements

Abstract

The aim of this research study was to investigate the impact on the seismic vulnerability of an Unreinforced Masonry (URM) building aggregate, having individual building units with a plan slenderness ratio of 1:4, when the units within the building aggregate have a variable number of floors. A previous study carried out by Borg (2021) explored the overall seismic resistance of URM building aggregates, however, the study assumed that all the individual units that comprised the building aggregate were of equal height. Therefore, by varying the heights of the individual units, this research study aims to achieve results, which may be more representative of the stepped streetscapes within the Maltese Islands. The research methodology employed to reach the objectives of this dissertation was by means of a non-linear static pushover analysis using the numerical seismic analysis macro element computer software 3D Macro. This parametric study required the definition of the various building aggregate types and height configurations, such that sufficient data is provided to formulate noteworthy conclusions. In order to model the building aggregates using 3D Macro, various data regarding physical, geometric and material characteristics, structural elements and ground motion parameters were gathered and input into the software. Finally, after modelling the various building aggregate configurations, several pushover seismic analyses were performed on each numerical model, obtaining a series of safety factors and pushover curves for each building aggregate configuration. The results obtained using 3D Macro helped quantify how the presence of height irregularities within a URM building aggregate affects its seismic vulnerability and behaviour. It was found that the stiffest aggregate configurations were those that contained no height irregularities or, if height irregularities were present, they retained the centre of mass of the aggregate at the centre of the plan across its entire height. Furthermore, increasing the height difference between adjacent buildings significantly reduced the overall sway stiffness of the building aggregate. The results also indicated that the maximum number of floors that may be sustained during an earthquake by an individual URM building may be exceeded when the individual URM building forms part of a URM building aggregate provided, however, that the cumulative number of floors within the URM building aggregate does not exceed the prescribed limit of 5 floors per individual URM building, that was established within this research study. Furthermore, throughout the numerical analyses, the Pushover -Ex + 0.3Ey Acc analysis (as prescribed in Eurocode 8) was the critical seismic load case for all aggregate configurations irrespective of the changes in building heights and arrangements of the individual URM buildings forming part of the URM building aggregate.