Seismic vulnerability comparative assessment and retrofitting of historical belfries in Malta

Abstract

Malta’s geographical location puts it at a low-to-moderate risk to seismic activity. This has often resulted in the neglect of seismicity as an integral part of the design process of local buildings. When one considers the fact that local construction methods mostly make use of masonry, it should be assumed that the seismic risk increases. This risk is being enhanced further with the growing urban building density, as well as the construction of taller and more slender unreinforced masonry buildings. While no severe earthquakes have caused significant damage in the recent decades, the earthquake of 1693 with a magnitude of 7.4 on the Richter Scale, has raised issues of concern, which focused on the importance of structural safety and the appropriate achievement of seismic resistance of local buildings. Several studies were carried out to assess building seismic resistance, but the reliability of the methods used is rarely considered. Therefore, this research study seeks to assess two different methods of seismic analysis and to compare their results, hence comparing their accuracy, advantages and limitations. The chosen buildings under study were the belfries of Ta’ Pinu Basilica in Gozo, and St.Paul’s Pro-Cathedral in Valletta. Both structures are tall, slender unreinforced masonry heritage buildings. The methodology used in this research study was to first carry out a non-linear pushover seismic analysis using 3D Macro (3DM)numerical software for both structures, and then to carry out a 2D Equivalent Frame Method (EFM)seismic analysis. The limitations of both approaches were then considered, and their results were also compared. It was concluded that the Equivalent Frame Method is a more conservative approach, since it is only capable of modelling fully-rigid diaphragms. The results also implied that these masonry structures require strengthening in order to gain further resistance to seismic actions. Therefore, retrofitting structural solutions were also proposed, in order to preserve these structures and their structural integrity.