FREYSSINET- Seismic Devices
The protection of structures subject from the risk of earthquakes only really began in the middle of the 20th century.
In the majority of cases, structures used passive protection such as shear walls in buildings or even protection based on the plastification of elements chosen in advance for bridges. Even though this type of protection allows the structures to resist design earthquakes and ensure the protection of human lives, after a high-intensity earthquake it often meant major repairs to the damaged protective elements were needed.
Modern society no longer accepts that the consequences of earthquakes are inevitable and the current trend is to equip structures in seismic zones with special devices that absorb or limit the effects of earthquakes on structures. This is positive protection.
Using its involvement and experience in the field of construction and works equipment, Freyssinet has contributed to the development of these seismic devices and today offers a complete range of special products called TRANSPEC®.
These devices can be used alone or in combination to achieve the most efficient and suitable protection for the project. This protection is based on the three fundamental operational modes: isolation, connection and dissipation.
The Freyssinet solutions
Isolation
The principle involves isolating the structure from the movement of the ground by using flexible connectors, mainly structural elastomeric bearings or sliding devices, in order to increase the natural period of the structure, reduce the response to, and protect it from, the seismic acceleration. This results in a reduction of the acceleration of the structures thus equipped by a factor of 2 or 3.
The efficiency of the isolators is directly linked to the transverse rigidity of the structural bearings and leads to significant movements of the structure during the earthquake.
The isolation effectiveness of the structure can therefore be seen in the low natural frequency, slow acceleration and significant relative movements.
Connection
It may be advantageous to limit the seismic movement of the structures to simplify the equipment linking them to neighbouring structures (expansion joints, etc.).
In these cases the designer will use either mechanical structural bearings to transmit directly and in totality the service and seismic loads from the foundations to the structures (passive protection), or seismic connectors. Seismic connectors have the characteristic of only providing very low resistance to slow movements such as those due to temperature and stress-strain variations. In contrast, they provide a rigid link between the superstructure and its supports during rapid movements such as those caused by seismic events.
Another advantage of connectors is the distribution of the major seismic horizontal forces between all the supports (piers) where installed.
Dissipation of energy
The energy developed by earthquakes may be dissipated using dampers to reduce their effects on the protected structures. In the same way as connectors, dampers provide very low resistance to slow movements but are completely efficient during rapid movements (earthquakes, collision fenders, etc.).
These dampers are generally used together with an isolation, produced for example by structural elastomeric bearings, to reduce the movement of structures while limiting the forces to which they are subjected.
Dampers very significantly reduce significantly the overall repair cost of the structures and keep them functional after an earthquake, this is particularly critical in, cases, like, hospitals where an immediate return to full functionality is essential. They also efficiently protect existing structures which were not
originally designed to withstand seismic activity.
