Structural assessment of the blast resistance of Maltese unreinforced masonry construction

Abstract

The events at Ronan Point in the 1960s triggered a worldwide endeavour to research further into the capacity of buildings to resist the effect of blast or impulse loads, which previously had not been investigated extensively. Unfortunately, the extent of this research with respect to the local context is rather limited, and there is no definitive quantification of the structural behavior of Maltese buildings in response to blast loads. Of interest is the behavior of the most predominant building type in Malta: the unreinforced masonry (URM) building. This method of construction remains the most common and favoured building method, despite the shift in typology from two-story terraced houses to multi-story structures, including basements and around five floors above street level. The use of unreinforced masonry locally has remained prevalent, but it is inherently problematic when response to lateral loads is considered. Unreinforced masonry walls are notoriously weak under the application of lateral loads, such as seismic actions, and subsequently also blast loads. The scope of this dissertation is to determine the behaviour of local unreinforced masonry walls subjected to accidental out-of-plane loads due to blast using published blast strength data and analytical methods. This study seeks to determine the magnitude of the blast loads that would cause structural failure of URM walls with and without pre-compression. In the light of recent events, where gas leaks were reported in Maltese households throughout the past years, the risk associated with the occurrence of such leaks has been considered in this dissertation, as it is a very real and possible accident that may occur in Maltese households, and has indeed occurred in the past. Previous research related to the local context has considered different typologies and blast sources, and this dissertation continues to build on that body of work and other research related to other dynamic loads. The general blast analysis carried out in this dissertation is coupled with a parametric analysis in order to investigate the behaviour of URM walls with varying masonry units, and mortar strengths. Viable and feasible alternatives to the current method of construction have also been analyzed in order to determine the improvement in the behaviour of URM walls subjected to lateral loading. Research has shown that URM walls in isolation are not able to sustain the pressures and moments resulting from the blasts, for both masonry materials considered and varying mortar strengths. URM building analysis has shown that Globigerina Limestone walls are significantly stronger than Hollow Concrete Block walls in sustaining blast pressures, with the early failure of individual walls within the structure proving to be beneficial to the overall stability of the building. Increasingly asymmetrical typologies have also resulted in worsened strength capacities of the URM building. The use of reinforced masonry walls has proven to be significantly stronger and is therefore recommended for locally constructed masonry buildings. The results of this study have shown that the current local URM construction methodology, combined with increased building heights, has resulted in a building typology which is extremely vulnerable to the occurrence of accidental blast loads. Residential buildings are more at risk due to accidental blasts than intentional blasts. This study also highlighted the observation that local URM buildings are more vulnerable to overall collapse when subjected to seismic loading than when subjected to blast loading, which nevertheless would still cause local damage at the location of the blast loading.