Cruzando la historia: diseño y construcción de la nueva pasarela en el dique de St. Elmo en La Valeta (Malta)

The purpose of this article is to summarise our experience as responsible team for the conception, detailed design and construction of a new footbridge at the unique location of the Valletta Grand Harbour. This exciting project was the result of our first prize in an international design-and-build competition for the construction of a footbridge spanning an existing gap in St. Elmo breakwater. This breakwater was constructed between 1903 and 1909 to convert the Grand Harbour in an all-weather port. A gap near its land end was left to prevent water stagnation and shorten routes for smaller crafts. Accessibility to the breakwater was possible by means of a two-span 2x34-m steel footbridge, erected in 1906. This footbridge was partly demolished in 1941 during World War II. The breakwater and its lighthouse remained isolated until 2012, only accessible by boat. Our design is an asymmetrical footbridge whose main structural element consists of a single 70-m-span classic Pratt arched truss which has similar proportions, in terms of height-to-span ratio, to those pertaining to the original bridge. The vertical truss is aligned along the outer face of the breakwater and the bridge deck cantilevers out from its bottom chord. In essence the transverse section forms an L shape, thus reinforcing both the presence of an existing historic abutment wall on the open sea side and the absence of a parapet wall down the steps of the abutment on the harbour side. The cantilevered deck of the bridge is a continuous unobstructed observation platform facing the Grand Harbour, an astonishing natural and man-made panorama, while the sea-side truss creates a protection feeling. The project successfully solves a complex problem using an innovative design concept: the reconstruction of a piece of Valletta's history, in harsh environmental conditions due its (uncommon) position in a breakwater and with exceptional construction constraints due to inaccessibility to site. The footbridge, with an atypical cross sections that prevents water accumulation over the steel elements and their joints, it is at the same time contemporary, transparent, functional and durable, it has easy maintenance and it is respectful with the history of the city as well as with the existing context and remains. Our experience as designers and contractors of this signature footbridge is summarized in this article, going from conception and advanced structural analysis (static and dynamic) to construction and erection using a specialised heavy-lift cargo vessel.