Controlling the geometry and forces of a hybrid cable-net and fabric formwork for thin concrete shells

The construction of anticlastic, thin-shell concrete structures can be efficiently achieved through the use of flexible formworks. Such formworks allow a departure from the traditional hyperbolic paraboloid shells, where its ruled surface serves as the basis for straight-line timber formworks. In addition, as forces are carried by the tensile system to the outer boundaries, the amount of falsework and their foundations, is heavily reduced, leading to economy in material, transportation and storage. This paper presents a prototype hybrid cable-net and fabric formwork used for the construction of two shell structures with identical boundary conditions. The second prototype, which is the main focus of this paper, was constructed after introducing several constructional variations and improvements. Moreover, it was constructed to test more accurate and flexible approaches to measuring both the geometry and internal forces of the cable net. This, in turn, allowed a higher degree of control of the applied prestresses, thus leading to lower tolerances between the digital form-finding model and the physical, as-built geometry.