An Innovative Shell Structure in Codogno (Italy). Evaluation of Structural and Seismic Performance

The saddle-shaped shells, or hyperbolic paraboloids, often joined together to form a pitched roof or an inverted umbrella, were used by many pioneers in structural architecture, such as Felix Candela, which introduced a very innovative use of reinforced concrete in thin layers or together with some reticulated ribs. An innovative semi-prefabricated building system was developed in Italy in the years ’30 of XX cent by a very active brick factory near Piacenza, RDB: the SAP system, that allowed building curved surfaces by prefabricating light elements. After WW2, this technique was applied also for the new structures covering wide spaces for the developing industry or also for public leisure, using prefabricated panels of the desired length. A particularly interesting application was the BISAP (double-SAP) panel that could be adapted for building large shells. In Codogno (LO), Italy, the BISAP panels were employed to cover a large sports hall, spanning about 37 × 26 m, without intermediate supports, resting (mainly) on the four corner pillars. Border pitch beams sustain at the top two crossed beams that separate (and support) the four hypar fields. On the four sides, two rafter beams are connected by horizontal prestressed tie beams, in order to minimize displacements and assure the preservation of the original shape. The first aim of the structural analysis was to assess the static conditions of the roof under the service loads assigned by Italian code for SLS, and then to evaluate seismic vulnerability at ULS of the whole sports hall, being a public space subjected to particular safety provisions. The FE code used (Straus7) allowed a very careful discretization of the orthotropic slab with the correct inclination and twist of the ribs, giving a reliable forecast of the behavior also in seismic conditions: the dynamic analysis of the modal shapes gives a satisfactory response of the shell, which maintains nearly unchanged his shape during free vibration modes. The seismic safety of the structure can be then increased by simply augmenting the stiffness of the four corner supports, where shear action is concentrated, by adding ribs to the L-shaped sections to form cross shaped ones. In this way also the slenderness (and weakness) of additional intermediated pillars could be overcome.