## 2 Presentation of the Safety Chair  
The more adequate occupants' position to protect them from shelter impact phenomena during building collapse was studied [6]. Since the time available to get into the shelter is short [1] and panic situations are highly probable, sitting in the chair should be as quick and simple as possible, which led to the adoption of the horse-back riding posture. In addition, to keep occupants away from the periphery of the shelter, where they are more likely to shock with the structure or with building debris, the safety chairs were positioned in a central location inside the shelter. Considering all these aspects, a safety chair structure was designed, constituted by a triangular frame with pinned connections with sub-horizontal elements where the occupant sits (\"strut\", in Figure 1), a sub-vertical element where their chest rests (\"bridge\", in Figure 1), and a vertical element (\"column\", in Figure 1), completing the triangular layout. Figure 1: Chair structure and body posture. The chair frame elements are square hollow section steel profiles with 40 mm sides and a thickness of 5 mm (\"columns\") or 4 mm (\"bridge\" and \"strut\"). A high steel grade was considered, with a yield stress of about 1.250 MPa to minimize the weight of the chair. The columns of the safety chairs are connected to the shelter structure through a shock-absorber system aimed at softening the occupant's accelerations due to the impacts that may occur during the building collapse, protecting them from severe injuries or death. This system must remain functional during the whole building collapse process, for the various collisions that may occur. This solution is diverse from that used in cars, in which parts of its chassis are used as shock-absorbers to slow down the cabin [7]. In this case, the structure of the chair was designed to remain in the elastic regime, while the reduction of the impact effects is guaranteed by the shock-absorber system composed of a viscous damper and a set of rubber springs (Figure 2). The viscous damper is installed between the chair's structure (bottom connection) and the shelter structure (top connection) with pinned connections at both ends to avoid bending forces. A commercial solution was adopted, with a maximum stroke of 600 mm, axial force capacity of 50 kN and an adjustable damping coefficient. The top connection to the shelter structure has an additional steel U profile (UPN 100), welded to the shelter columns. This and other U-shaped profile have two bumpers (Figure 2, detail A), with a global stiffness of 1 kN/mm and capability to shrink 40 mm, aimed at preventing the chair to move outside the shelter limits. To complete the system, a spring set of three pairs of neoprene blocks are installed (Figure 2, detail B), one pair at the top, one intermediate and one at the lower connection point. The springs make use of the high distortional capacity of neoprene, as seen in bridge bearing pads or in earthquake-based isolation systems. The neoprene blocks present a shear modulus (G\({}_{\rm neoprene}\)) of 0.3 MPa and a damping coefficient (\(\xi_{\rm neoprene}\)) of 10%. The blocks at the middle and bottom positions are cylinders with \(\mathcal{O}\)\(60\times 100\) mm length, while the neoprene blocks of the upper position present a square section of \(67\times 67\) mm\({}^{2}\) and a length of 160 mm. Aimed at reducing the movement of the upper fixation point of the chair in the horizontal direction to an acceptable level, the effective (free) length of the top pair of springs was reduced by 50%. Besides the shock-absorber system, additional retention devices will have an important role in complementing the occupants' protection. The retention system will include cushion devices that not only reduce the body's acceleration but also avoid the direct contact with the steel elements of the chair structure, conveniently accommodating the human body before any impact. Airbag-type devices were not a choice, given the fact that successive impacts may occur.