Seismic isolation is one of the most effective seismic risk mitigation techniques. Let's talk about how it works and how it is made.
One of the possible technologies for mitigating the seismic risk of buildings is that of seismic isolation. In the engineering field, this technique involves inserting special devices in specific points of the structure to isolate it from ground movements in the event of an earthquake. There are different types on the market even if, as we will see, the operating principle is always the same. In this article we will deal with the basic theoretical concepts with some reflections on the types of insulators and the implementation construction techniques.
Why are buildings seismically isolated?
Modern seismic design has as its main objective the protection of human life. To achieve this objective, one is also willing to give up the functionality of the building to have substantial damage following a seismic event. However, strategic structures such as hospitals, fire brigade offices and similar places cannot be designed to the same level of performance as an ordinary building. Their operation must also be guaranteed in the face of violent earthquakes , which is why it may be appropriate (sometimes necessary) to have a seismic protection system that guarantees different performances from those guaranteed by ordinary structures. seismic isolationit can be the key element in the design of strategic structures.
The physical principle of seismic isolation
Let's take a quick look at the main physical principles on which seismic isolation is based.
The proper period of vibration
As a result of an earthquake, the structures in direct contact with the ground undergo the shaking of the subsoil. The kinetic energy accumulated by the masses of the construction is visually manifested through oscillations, which can lead to local or extensive damage and, in extreme cases, actual collapse . In practice, we can say that the horizontal forces exerted by an earthquake increase as the height of the building increases and depend on a parameter called the proper period of vibration . What is it about?
This is a measure of the time it takes for the structure to perform one complete swing (i.e. starting from a certain point and back to the same starting point). The values that this parameter assumes indicate a more or less important susceptibility of the structure to "attract" important seismic actions. The structures oscillate with their own periods of vibration ranging approximately from 0.1 second (low structures) up to 1 second (multi-storey structures). In this range of values, the amplifications assume the highest values. If it goes beyond 1 second , instead there is a progressive decrease of the seismic actions agents.
The disconnection between foundation and elevations
How do you increase the natural period of vibration of a structure? Let's imagine we have a construction resting on simple rollers. The structure is decoupled from the ground, so that if the ground moves, it remains practically stationary in an absolute sense. This leads to having a very high period of vibration and a consequent decrease in the stresses transferred from the foundations to the structures. The elements suitable for realizing this physical disconnection are called insulators. An isolated structure, unlike an ordinary structure, can get to have its own periods of vibration greater than 2 seconds .
Types of seismic isolators
There are different types of seismic isolators. Mainly, it is possible to identify them in two categories:
Elastomeric insulators :
these are usually cylindrical devices, made with layers of rubber with interposed horizontal metal plates which prevent the rubber from bulging under the weight of the building. The rubber provides an important lateral deformability , which involves a high increase in the natural period of vibration . Furthermore, this adds an additional level of damping to the system which further reduces the effects produced by the earthquake. The predominantly elastic characteristics of the rubber then allow an effective re-centering capacity of the system to the initial position;
Sliding isolators :
these are devices that allow the decoupling of the ground-structure motion through a relative sliding of two surfaces in contact . This sliding is activated when the friction force between the plates in contact is overcome. The contact surfaces can be flat or curved. In this second case we speak of pendulum insulators ( friction pendulum ). The advantage of using pendulum insulators is the self-recentering capability of the device, which is lost in the case of flat surfaces. An important advantage of this family of isolators is the high carrying capacity under vertical load when compared to the family of elastomeric isolators.
the most important parameter of the isolation system is its displacement capacity , i.e. the maximum permissible displacement before the triggering of the failure of the device. This value must be compared with what in technical jargon is called "seismic displacement demand", which mainly depends on the seismicity of the site. In essence, the seismic displacement demand represents the relative soil-structure displacement demand triggered by the earthquake.
Construction techniques
It is possible to seismically isolate both new and existing buildings. Let's see how it's done.
Insulation of ex-novo buildings
From a constructive point of view, the presence of an insulation system does not affect the design phases for the construction of a new structure, of any shape and type. The project will provide for their assembly in special housings, on which the coupling between the device itself and the building will then be carried out. To guarantee the inspection phase and possible replacement during the useful life of the work, adequate spaces must be ensured around the devices.
The adaptation of existing buildings
The seismic isolation technique is often used as a seismic retrofitting strategy for existing buildings. In this case, we mean a design procedure that guarantees seismic performance of an existing building similar to that of a new one. The construction technique is very invasive: in this case it is necessary to make a cutactual of the terminal part of the pillars on the floor to be isolated for the insertion of the device. Furthermore, a level connection must be made between the elements at the base, to guarantee monolithicity to the isolated body. This therefore requires sufficient operating space and a high degree of disturbance to the existing structure, which must be suitably stabilized during the assembly phases, in order to avoid partial collapses due to cutting the existing pillars.
