'Franka' roofs : can they be a viable alternative to the typical concrete roof?

Abstract

When designing contemporary roofing structures, section active systems which primarily rely on bending action tend to be the system of choice. This structural choice manifests itself through the use of reinforced flat concrete slabs. Upon reflection, three observations pertaining to material, geometry and construction emerged. Firstly, concrete production is resource and energy intensive, accounting for up to 8% of the worlds CO2 production. Using locally sourced materials could help reduce its demand by encouraging the recycling of local materials and their use in new products such as reconstituted stone. Secondly, unlike reinforced concrete which can resist both tension and compression resulting from the reinforcement, stone is a brittle material that is only capable of resisting compressive loading. This leads us to the second point being geometry. Maltese architecture has its roots in masonry construction. The above mentioned fundamental properties guided the development of roofing systems on the Maltese islands, resulting in an architecture founded on the use of funicular structures such as arches, vaults and domes. This was one of the primary sources of inspiration for the adopted geometry of the roofing structure devised in this dissertation study. Lastly, whether a typical cast in-situ concrete roof or an arching masonry system is adopted, both require temporary formwork. Hence, what if the temporary formwork is integrated within the arch, serving as formwork during construction and later becoming an integral part of the structure? This dissertation adopts the concept of active-bending as the primary method through which an initially flat formwork is actively bent into an arched form corresponding to the Elastica curve which is geometrically similar to an inverted catenary curve. Through the filling of the formwork with voussoirs, the formwork geometrically transforms from the Elastica curve into a segment of a circle. By using physical models which were 3D printed in PLA, this concept was widely explored by testing several different solutions for the integrated formwork, resulting in geometric forms from active-bending and the incorporation of the formwork in the final curved structure. Such geometries ranged from deep arches to shallower ones, eventually translating into the third dimension by creating a vault. The study proceeds with a finite element analysis of the selected integrated formwork filled with the voussoirs to understand the behaviour of the formwork during the filling process along with the final state of the completed arch, the impacts of material thickness on the formwork integrated arch along with a comparative study between a formwork integrated arch and a traditional masonry arch. Results from Finite Element Analysis showed that a steel integrated formwork could offer the required flexibility for active bending whilst being stiff enough to withstand the load from the filling process with voussoirs. It could also be noted that an increase in rod thickness of the integrated formwork had a positive impact on the load bearing capacity of the formwork integrated arch, resulting from the activation of the steel formwork which aids the arch in carrying load via bending. When comparing the formwork integrated arch with a traditional masonry arch with mortar joints, it could be concluded that a significant increase in the load bearing capacity of the arch was noted for the formwork integrated arch along with the improvement in failure mode which was ductile in this case. Such mode of failure is preferred over brittle failure which is sudden. Through the use of the Funicular Polygon Method, the importance of geometry on the load carrying capacity of the arched roofing structure could be illustrated along with the use of this method as a simplified approach for the analysis of curved structures whose results could then be compared with those from Finite Element.