Seismic retrofit strategies for friction beam-to-column connections of industrial RC precast structures

The seismic retrofit of precast structures requires specific criteria for the selection of the best solution: often, it is not possible to apply the strategies and the techniques used for reinforced concrete structures because of the peculiar issues related to the structural typology. Typical structural deficiencies of the cast in situ reinforced concrete elements, such as the lack of transversal reinforcement area, inadequate overlapping lengths, discontinuities for longitudinal bars, etc., can be avoided. However, the precast structures can exhibit very poor seismic performance of the mutual connections between the structural elements. In particular, they can show: (a) a very poor reliability of beam-to-column connections, i.e. friction connections, or without adequate seismic details which lead to ductile responses; (b) the lack or the inefficiency of the mutual connections between the roof elements, which reflect in flexible roof plans and in plane irregularities; (c) the presence of mutual constraints characterized by high stiffness, which are not compatible with high displacement requests and induce dangerous phenomena of dynamic interaction. In this paper, a real case study is presented which refers to an existing one-story precast building sited in Mirandola (Emilia region) with friction connections for both the beam-to-column and the roof-to-beam connections. The seismic assessment is performed by means of dynamic non-linear analyses, on a refined FEM 3D model, and retrofit strategies for the beam-to-column friction connections are presented, based on the application of external mechanical passive devices, which activate for relative displacements between the structural elements. The analyzed solutions can provide a mechanical restrainer to improve the shear resistance and the lateral stiffness of the resisting frames but also a dissipating system to damp the external energy provided to the structure during a seismic event.