PSI - Issue 44

Devis Sonda et al. / Procedia Structural Integrity 44 (2023) 1188–1195 Devis Sonda et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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1. Seismic vulnerabilities of existing precast RC buildings In Italy, precast structures have been widely used for one-storey industrial buildings since the 70s. Precast RC buildings usually present a very simple structural scheme consisting in single-storey isostatic frame structures with socket foundations to form a fixed connection. The columns support ordinary or pre-stressed RC precast beams that can have different shapes, with hinged connections or more likely friction-based connections, particularly for existing buildings designed without anti-seismic details. The behavior under seismic actions of these structures referring to the 2012 Emilia earthquake (Savoia et al. 2017, Bournas et al. 2013, Liberatore et al. 2013) is typically characterized by great flexibility and large displacements. In fact, most of the mass is at the roof level, columns are very slender, and beam-column connections are at best hinged. In absence of connections between precast elements, that are the key element in determining the seismic performance of this kind of precast buildings, the displacements due to seismic action can easily reach values of tens of centimeters, without determining significant forces on columns except those due to possible friction mechanisms. In dynamic terms, we can assume that the acceleration applied to the structure is transformed in inertial force and displacement, without involving capacity of structural elements.

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Fig. 1. Emilia 2012 Earthquake: (a) beam collapse despite the large support; (b) collapse of a precast roof slab.

When rigid and fixed joint connections prevent relative beam-columns displacements, the problem of low seismic capacity is moved at the column base, usually not designed with adequate ductility and reinforcement in existing buildings. Columns not designed according to anti-seismic provisions present low transverse reinforcement ratio and usually have a fragile behavior. These considerations highlight the need to avoid seismic action to be fully transferred to the columns, and consequently the possibility to install systems able to allow large displacements. Immediately after the events of May 2012 in Emilia, in the first phase of emergency, aiming at rapid socio economic recovery in the short term, the lack of mechanical connection devices between vertical and horizontal elements and between horizontal elements was often solved with the introduction of steel ties, plates or cable restraints in order to avoid sliding of the beams and therefore unseating failures, transferring the problem of seismic capacity at column base. Some years later, in 2017, the Italian code provisions introduced more detailed instructions, stating that “the design of connections must be done according to capacity design, using ductile elements, …” Thus, after a first period during which the solution was often the realization of rigid beam-column joints, the awareness of the advantages of dissipative connections has determined the developing of strengthening solutions and retrofitting proposals for friction-based connections. The need to be low-cost solutions because of the high number of beam and roof elements to be connected brought to the develop of mechanical systems based on metallic elements, with limited capability to dissipate energy, that in most cases, has low influence on the global energy balance of the structure. Basing on large dimensions of structural elements of precast structures, in order to provide an adequate amount of absorbed energy, able to significantly reduce seismic input, fluid viscous dampers should be the solution needed. Their introduction in existing precast industrial buildings can be difficult because of costs and interference