Reducing the seismic vulnerability of prefabricated structures with the new SAFE+ anti seismic devices

Safe+ Model A and Model B by Fibre Net are a patent developed in collaboration with the University of Bergamo. CE marked, the Safe+ seismic devices allow for the restraint of structural elements, improving the local and global seismic response of prefabricated buildings. The fast and secure installation does not interfere with production activities and does not require structural modifications.

Recent earthquakes that have affected the national territory have highlighted, similar to other types of structures, that the ability of prefabricated buildings to withstand seismic actions depends primarily on the presence of connections capable of transferring loads between various structural elements, and only secondarily on the strength of beams, columns, foundation systems, and cladding panels.

Analysis of the warehouse subject to seismic actions: collapse modes The Guidelines for local and global interventions on single-story industrial buildings not designed with seismic criteria, prepared by the Civil Protection, RELUIS, CNI (National Council of Engineers), ASSOBETON, and the Regional Federation of Engineers of Emilia Romagna, summarize the collapse typologies of these buildings based on the damage observed following the earthquake that struck the Emilia Romagna region in May 2012. Specifically: loss of support, rotation of beams out of their plane, absence of floor diaphragm, damage to temperature elements, and damage to columns.

Loss of support

The loss of support of the horizontal structures of the support elements was one of the most frequent causes of damage to prefabricated buildings and occurred either due to a lack of connection elements or, if a connecting pin was present, due to concrete failure, expulsion of concrete cover, and shear key.

Rotation of beams out of their plane

In the presence of tall main beams, a rotation mechanism may be triggered near the supports, leading to the loss of beam equilibrium.

Absence of floor diaphragm

The absence of sufficient stiffness floor diaphragms to ensure a uniform distribution of horizontal forces to the various vertical load-bearing elements can result in a disorganized functioning of the structure.

Damage to cladding elements

Depending on the construction period of the building, the cladding panel can be made of masonry for older buildings or prefabricated concrete panels for those built in the last 40 years. The collapse modes can affect only the cladding panel if it is connected to the main structure through inadequately sized brackets to withstand horizontal actions, or even the entire structure in the case of panels braced to the structure that can function as bracing elements, sometimes leading to the failure of the short pillar (or stub pillar).

Damage to pillars

In single-story buildings, vertical elements are responsible for supporting both vertical and horizontal actions and are designed considering a cantilever static scheme. In some cases, due to horizontal forces, a loss of verticality has been observed due to rigid rotation at the base caused by either foundation rotation or damage to the reinforced concrete structure supporting the pillar (footings, pile caps, etc.).

In other cases, the main issue with these structural elements is the formation of a plastic hinge at the base of the pillars. In the presence of large deformations, cyclic actions, and a lack of an adequate number of stirrups in critical dissipative zones, premature failure occurs due to instability of the compressed bars, leading to a reduction in section ductility and, consequently, of the structure.

Analysis of the warehouse subject to seismic actions: beam-column connection

Let’s imagine analyzing the behavior of a single-story prefabricated structure subjected only to horizontal actions and isolating the behavior of a portal composed of two pillars and a beam connecting them in simple support. The lateral deformability of the portal depends on the interaction between the foundation footing and the ground, the inertial characteristics of the pillar, and the type of joint connecting the pillar to the beam.

The joint must prevent the mutual translation between the elements it connects, thereby avoiding the loss of support of the beam on the pillar without preventing the rotations that occur due to the lateral displacement of the portal between the beam and the pillar.

The connection must also be able to withstand the actions induced by the vertical component of the earthquake, which can lead to a sudden displacement of the beam. In many prefabricated buildings designed without seismic criteria, the main vulnerability of the structure lies in the beam-column joints where the connection relies solely on the friction of the support. It is easy to understand how the combination of horizontal seismic force and vertical forces renders this type of connection ineffective.

In this regard, the current Technical Standards for Construction, in paragraph 7.4.5.2, highlight how the connections between prefabricated elements influence the static behavior of the structural system and its response to seismic actions, and they must be designed to possess greater displacement capacity and resistance than the corresponding demands.

Understanding how the support functions, it is clear that the intervention strategy for existing buildings can follow two paths:

Designing over-resisting connections that do not compromise the plasticization of dissipative zones (e.g., using steel plates and bolts) Implementing seismic devices capable of dissipating part of the seismic energy.

Both solutions are valid but require careful evaluation by the designer of the possible secondary effects on the structure.

In the first case, which involves using rigid connections, the plates must be installed to allow the structure to experience displacements due to thermal actions and should not alter the original support scheme unless intentional (transforming the portal into a frame).

In the second case, the choice of the seismic device should be calibrated based on the resistance and rotational capacity of the pillar section and the lateral deformability of the portal (it is important to avoid displacements between the beam and pillar that are too large, which could lead to the displacement of non-structural elements such as installations).

The above concepts are summarized in paragraph 7.4.5.2 of the Technical Standards for Construction:

“…In prefabricated elements and their connections, the possible degradation due to plastic deformations must be considered. When necessary, the design strength of the prefabricated connections, evaluated for non-cyclic loads, must be appropriately reduced for verifications under seismic actions.

In the case of connections between non-monolithic prefabricated elements that substantially influence the static behavior of the structural system and, consequently, its response under seismic actions, the following three situations are possible, each of which must correspond to an appropriate design criterion:

Connections located outside the designated dissipative zones, which therefore affect the dissipative capacities of the structure; Connections located near the designated dissipative zones at the ends of prefabricated elements but oversized to avoid compromising the plasticization of the dissipative zones themselves; Connections located within the designated dissipative zones at the ends of prefabricated elements, equipped with the necessary characteristics in terms of ductility and dissipable energy quantities…”

How Fibre Net seismic devices work

Based on the study of vulnerability in prefabricated structures, Fibre Net, in collaboration with the University of Bergamo and with the initial idea of engineer Giovanni Bulferetti, has developed and engineered two families of seismic devices called SAFE+, in compliance with both the NTC 2018 and the EN 15129 Standards for Seismic Devices, obtaining the corresponding CE marking.

The SAFE+ systems have the dual function of mechanical connection between structural and non-structural elements, as well as seismic energy dissipation. In particular, the SAFE+ Model A device has been developed for beam-column restraint, and the SAFE+ Model B device is used for connecting roof tiles to main beams and for joining cladding panels to the supporting structure, thus preventing overturning.