## 1 Introduction  
Fear of earthquakes is usually associated with the risk of building collapses. When buildings are not designed to prevent those collapses, their occupants may die or suffer severe injuries when an earthquake occurs.  
Seismic retrofitting of vulnerable buildings is the most appropriate solution, but its costs do not allow for intervention to most of them. Once the main concern in the seismic codes was people's safety, a proposal for a seismic shelter was presented in [1], which works as a \"cocoon\" to protect the occupants of seismically vulnerable buildings in case of earthquake occurrence.  
Other concepts for seismic protection systems have been proposed without any further development, neither numerical nor experimental [2; 3; 4]. The shelter structure was designed to resist, without significant deformation, the effects of the impacts to which it is subjected [5]. There, the life-saving capsule was designed to ensure higher chances of survival for its users, and soon it was understood that specific equipment to protect the human bodies from the shock effects was needed. In this sense, safety chairs were designed to be connected to the main shelter structure by a system of springs and dampers and equipped with anatomical restraint devices. It was also realized that it is difficult to restrain ourselves to a safety chair in the reduced amount of time available, even if an Earthquake Early Warning System (EEWS) is installed with the life capsule, as presented in [1]. Thismeans that the chair and the human body interact and have differentiated behaviours to respond to the collapse shock accelerations and corresponding inertial forces on the human body. The life capsule structure responds with strength and stiffness to the collapse shock loads, ensuring its integrity. A specific study that assumes this scenario was developed and is now presented in this paper.  
In case of building collapse, the shelter and the building debris will interact with each other in a series of shocks until the final collision on the ground. The design of the safety chairs was quite complex and challenging, involving diverse scientific fields such as biomechanics, ergonomics and engineering. The safety chairs had to ensure the following main conditions: being able to oscillate in turn of its initial position, in order to reduce the accelerations of the human body to acceptable levels; being able to withstand the deformations associated with that oscillation; being relatively light; allowing for an easy and immediate occupation upon arrival of shelter occupants; ensuring that retention devices are immediately operational; and having low functional and aesthetical impact, given its preferential location in the entrance hall or corridors of the dwellings [1; 5].  
Besides the mechanical effects of fall and impacts, the safety chairs were conceived considering different aspects (that will subsequently be validated in experimental testing), such as the psychological reaction to the earthquake alarm, the need for a prompt settling, the adequacy and redundancy of the restraint devices, and the ease to withdraw from the chair after the building collapses.  
The present paper focus on the analytical study carried out to design and analyse the mechanical behaviour of the safety chairs.