Seismic retrofitting of local contemporary unreinforced masonry buildings with soft storey basements subjected to seismic loading

Abstract

One of the main problems, which is often encountered within local methods of unreinforced masonry (URM) construction for masonry buildings is the Soft Storey Effect. A soft storey may be described as the abrupt reduced sway stiffness experienced by one level in a structure compared to the sway stiffness of the floors above and below. When considering the local context, this soft storey is generally found at Ground Floor or at basement level. In order to provide sufficient space for car parking within these buildings, the internal masonry partition shear walls and piers are often removed in the soft storey level. When these URM buildings are subjected to seismic forces, they may be unable to dissipate the earthquake energy due to a lack of robustness and structural ductility. The primary objective of this dissertation is to identify the adverse effects of seismic events upon the local contemporary URM building typology and to propose structural retrofitting strategies to mitigate these negative effects. In addition, the proposed research study will allow the determination of how many storeys may be sustained during the design earthquake for the Maltese Islands by URM buildings with a typical floor plan layout, and which have been retrofitted to obtain the necessary horizontal sway stiffness and strength to resist the applied seismic forces. It is intended to determine whether such retrofitting interventions are feasible and whether they effectively provide the required seismic resistance enhancement. The research methodology used in this dissertation involved both an analytical and a numerical approach. Throughout the first part of the seismic analysis, the EFM (Equivalent Force Method) was used in order to obtain the sway stiffness of each URM transverse wall within the URM building models considered. This sway stiffness was then replaced within the basement level using either structural steelwork or reinforced concrete transverse plane frames within the basement level, which were used as a seismic retrofitting technique to eliminate the Soft Storey Effect. Secondly, a more accurate numerical method, 3D Macro, was then utilised to carry out parametric Non-Linear Static Pushover Seismic Analyses of the URM building models, from which the building seismic vulnerability safety factors were then obtained. The results showed that the negative impact seismic resistance of having an asymmetrical plan was very evident as several differences were observed in the seismic performance of masonry walls in the transverse direction of the URM buildings. In this research study, the comparison of seismic resistance between different URM buildings was carried out mainly with reference to the integer number of floors that can be safely carried by the buildings when subjected to the design earthquake for the Maltese Islands. Although this is a rather course yardstick for comparison, since it is not sensitive to subtle differences in structural behaviour between different URM buildings, the use of the building safety factors allowed a more refined comparison to be carried out. The seismic analysis results also showed that a decrease in seismic performance occurred within those URM buildings, which were retrofitted with excessively stiff transverse plane frames at basement level, causing failure to occur within the ground floor rather than at basement level. Within single unit retrofitted URM buildings, the transverse structural steelwork plane frames at basement level experienced local buckling. This was not observed in the transverse structural steelwork plane frames at basement level within retrofitted URM building aggregates. However, generally, the retrofitted URM single unit buildings and URM building aggregates exhibited similar behaviour irrespective of the structural material used for the transverse plane frames at basement level. It was also observed that, as the number of single units within URM building aggregates increased, there was little additional benefit to the aggregate seismic resistance, since the benefit of additional sway stiffness was cancelled out by the adverse effect of additional seismic weight. Furthermore, it was also observed that weaker subsoils gave rise to larger seismic amplifications, rendering the retrofitting techniques futile in certain cases.