Possible Precursors of Pombalino Cage

Abstract

Pre-Pombalino buildings in Lisbon nowadays represent a rare alive witness to a major historic European earthquake event: the 1755 Lisbon earthquake with tsunami followed by a great fire. More substantial interest is however shown by researchers in the so-called Pombalino cage [1, 2], a structural typology created after the 1755 earthquake to rebuild the city, and considered by many authors, as a first engineered solution designed specifically to be resilient to earthquakes. The Pombalino cage is a composite timber-framed masonry construction with a wide use of St Andrews crosses and specific connections. In comparison very modest attention has been paid to clearly identify in history possible precursors of this technological solution, despite the many examples of vernacular masonry infilled timber framed buildings, which are common to many Mediterranean regions exposed to seismic hazard. An additional difficulty in tracing with certainty the historic development of the Pombalino cage is the lack of original drawings for this construction typology. This paper attempts to determine the conditions at the time, in terms of existing technical know-how and current construction methods that would have made the development of the Pombalino cage, possible. The work carried out in this research encompasses in situ surveys of 18th century buildings, documentation from several archival libraries, analysis of ancient treatises, and previous research by the authors. This manuscript presents first a review of: (i) traditional timber-framed construction systems belonging to earthquake-prone regions; (ii) then connects these to influential contemporary knowledge on the 1755 Lisbon earthquake; (iii) evidence from Portuguese legislation, regulations, and notices published after the earthquake; (iv) and finally the academic education and training of Portuguese military civil engineers and architects (before and after the earthquake) who developed the Pombalino Cage; (v) the paper concludes with a discussion of whether these elements can be identified as the necessary precursors of the Pombalino cage.