Seismic vulnerability assessment and retrofitting of local historical unreinforced masonry buildings

Abstract

Malta is situated in a low-to-moderate seismic hazard zone. Regardless, the building density and local unreinforced masonry (URM) construction increase the seismic risk, which is further compounded by a lack of seismic design especially in the case of historical URM buildings. The latter scenario leads to increased uncertainty as to the seismic response of URM buildings. Historic records highlight the inherent seismic vulnerability of URM buildings, further raising the question as to what feasible interventions may be done on such historical buildings of insufficient seismic resistance and their effectiveness in strengthening these structures. Locally, research studies carried out on the seismic resistance of stand-alone contemporary URM buildings (Galdes, 2013) and on the seismic vulnerability of URM historical buildings (Buhagiar, 2019). However, such studies did not consider retrofitting measures to reduce the buildings’ seismic vulnerability. This study intends to determine the seismic vulnerability of historical URM buildings, and propose suitable retrofitting techniques. This dissertation considers two geometrically-distinct structures; the Inquisitor’s Palace and the Civic Clock Tower, both located in Birgu (Vittoriosa) in Malta. Since the latter was destroyed in WWII, a hypothetical analysis shall be carried out as if it were to be rebuilt nowadays. This study primarily maps out the major alterations in structural elements of the Inquisitor’s Palace, whilst researching the original construction materials and geometric composition of the Clock Tower. Thereafter, two seismic vulnerability assessment methods are used: non-linear pushover static analysis using ‘3DMacro’ software to obtain capacity curves of the Clock Tower and linear static analysis using the Equivalent Frame Method (EFM), applied to both the Clock Tower and Inquisitor’s Palace. The EFM results are compared with those from the pushover analysis of the Clock Tower (carried out in this study) and of the Inquisitor’s Palace (carried out by Buhagiar,2019). Comparative results show that, whilst EFM is sufficiently accurate (albeit, slightly more conservative), it fails to indicate the failure mechanisms. 3D Macro’s pushover analysis is more accurate, and provides detailed failure mechanisms and crack patterns, which is useful for the implementation of retrofits. The results of this study indicate that both unreinforced masonry structures investigated, whose large plan density and thick structural walls prevent significant damage, are still vulnerable to surface damages, and both buildings do not satisfy the Damage Limitation Limit State. The latter failure implies irreversible damage to surface friezes, sculptural facades and other features of cultural and historical value in direct contact with structural walls. Using a combined Microstran (stiffness-based software) and 3D Macro approach, numerous retrofits are analysed and compared to attain the most efficient solution for strengthening the Civic Clock Tower. The findings clearly indicate that the utilised strut cross-braced steel frame provides the greatest efficiency with regards to cost, rigidity increase, design flexibility and reversibility. The study concludes that the implementation of such a retrofit on the Civic Clock Tower substantially increases its sway stiffness, in turn reducing lateral deflection demand, such that the Damage Limitation Limit State is also satisfied, ensuring its structural integrity. This study may thus serve as a template for the implementation of retrofits to further safeguard the heritage value of local historical URM buildings against seismic events.