Seismic vulnerability of typical Maltese chapels

Abstract

The vulnerability of Maltese chapels to seismic activity is a significant concern due to Malta's location in a seismic zone. These chapels, constructed with unreinforced masonry (URM), have historical significance but may not be adequately resistant to earthquakes. Despite surviving significant historical earthquakes in 1693, 1743, and 1856 with varying degrees of damage and subsequent repairs, their structural integrity remains uncertain for future seismic events. Key chapels like St. Philip & St. James in San Gwann, St. Marija Maddalena in Dingli, and St. Matthew’s in Qrendi exemplify this architectural vulnerability, requiring assessment and potential reinforcement to ensure conservation and safety against seismic risks. The study examines the seismic vulnerability of chapels by comparing two pairs: San Gwann Chapel and Dingli Chapel, and San Gwann Chapel with Qrendi Chapel. It focuses on how the differences in foundation geology and structural regularity affect vulnerability to earthquakes. San Gwann Chapel, situated on Lower Globigerina Limestone, shows structural regularity, while Dingli Chapel, on Upper Coraline Limestone over Blue Clay, faces different challenges. Similarly, despite both being on Lower Globigerina Limestone, San Gwann Chapel contrasts with Qrendi Chapel due to Qrendi's structural irregularities. These insights underscore the role of geological and structural factors in assessing and mitigating seismic risks for the buildings. This research was conducted under a non-linear pushover seismic analysis using "3DMacro" software to model the three selected chapels. The study focused on their seismic performance under moderate earthquake conditions (approximately 0.1g). All chapels, characterized as single-story buildings with alpha values exceeding 1, demonstrated sufficient safety margins against seismic forces at this intensity level. However, under higher earthquake intensities, the analysis predicted a sequence of failure starting with Dingli Chapel, followed by Qrendi Chapel, and finally San Gwann Chapel. The study examines the seismic safety of several chapels in Malta, focusing on Dingli Chapel and San Gwann Chapel. San Gwann Chapel, built on Globigerina Limestone, shows better safety performance and less displacement compared to Dingli Chapel, which rests on Upper Coralline Limestone over Blue Clay. Qrendi Chapel, also on Globigerina Limestone but with asymmetrical additions, exhibits reduced safety compared to San Gwann Chapel. Despite regular architectural layouts, Dingli Chapel's safety factor is lower than Qrendi Chapel's due to subsoil conditions, emphasizing the critical role of subsoil type in structural integrity over design irregularities. All the three studied chapels, demonstrate resilience against seismic events, with alpha values above 1, indicating no need for retrofitting for an earthquake of 0.1g. This study contributes to understanding the seismic vulnerability of Maltese chapels, essential for preserving these culturally significant buildings.