What is the Concept of Seismic Isolation?

Seismic isolation is a building design method  placed at various levels between the infrastructure and the superstructure  to ensure that the built structures are less affected during earthquakes and increase their resistance to earthquakes.

In classically built structures, during earthquakes, displacements are distributed over the height of the building and increase in the upper floors.

In structures with insulators at the base, the displacement is concentrated in the isolators during an earthquake and the relative acceleration is close to each other no matter how high the structure is. The isolation system separates the structure and the ground from the horizontal movement of the ground with its horizontal rigidity.

This system ensures that the frequency of the structure is much smaller than the built-in frequency and the dominant frequency of ground motion.

Seismic isolations do not absorb the energy of the earthquake, but reflect it to the dynamics of the system, thus increasing the durability of the building by ensuring that the structures are less affected by the earthquake energy.

The image below, the movements of the structures with and without seismic isolators during the earthquake. The first image represents the movement of the building with the insulator and the second image without the insulator.

What are the Advantages of Seismic Isolation?

1) Guaranteeing Earthquake Safety

Deviation of building behaviour from expected; ground conditions, ground movements, workmanship, material properties, and consistency of mathematical models.

Thanks to seismic isolation, the seismic behaviour of structures becomes much safer. Thus, structures are less sensitive to deviations from the ideal.

Performance targets in buildings where seismic isolation is used are as follows;

• It should be able to resist without damage to the structure’s content under mild or moderate earthquake conditions.
• Under severe earthquake conditions, the structure should be able to resist without heavy damage to structural elements, the collapse of the insulation system, and major damage to non-structural components.

2) Advantages of Immediate Use in Buildings After Earthquake

Although seismic requirements and all constructive rules have been complied with, according to the conventional design approach, damages in non-structural elements, large displacements and cracks in other elements are generally inevitable after an earthquake.

The seismic isolation method prevents relative storey drifts, bending deformations, plastic deformations and cracks. The damage level that may occur in structural and non-structural elements is reduced to the lowest level.

Thus, the use of the seismic isolation method allows the structures to be used immediately, even after high-intensity earthquakes.

3) Structural Advantages of Seismic Isolation

Seismic isolation increases the natural periods of the structures and removes the structure from the resonance effect.

With the effect of the earthquake, rigid structures are exposed to large accelerations with small displacements, while flexible structures are exposed to large displacements with small accelerations. Seismic isolation is a system that enables us to take advantage of both situations.

Both story relative translations and accelerations will be greatly reduced by seismic isolation. The reason for the decrease in the floor relative drifts is the rigid behaviour it will create in the structure due to the rigidity of the structure being much greater than the insulator. Since floor displacements will decrease, damage to structural and non-structural elements is prevented by minimizing damage.

Thanks to the correct placement of seismic isolators, it can bring the centre of rigidity closer to the centre of mass of the structure in structures that are horizontally irregular and may be exposed to torsion effects. In this way, architects have great convenience and flexibility in the design phase.

One of the advantages of seismic isolation is the increase in the damping ability of the structure. The energy absorption mechanism is a system based on inelastic deformations of the carrier elements in the traditional understanding.

In cases where seismic isolation is used in buildings, damping is collected at the levels where seismic isolators are located. In this way, it provides advantages both in terms of performance monitoring and design.

In the retrofit phase, thanks to seismic isolation, there is no need for loss of function in the structure and major changes. The seismic isolation method is the most permissive method for strengthening historical buildings and preserving the qualities of the building.

4) Economic Advantages of Seismic Isolation

With the desire to use seismic isolation in projects, the cost of this method is one of the first issues that come to mind.

It is unlikely to make an assessment of this question would apply to every project. In a project where seismic isolators are desired to be used, isolators will of course bring additional costs, but since the force of the earthquake will be greatly reduced in structures with isolators, the costs of the carrier system will also decrease.

5) Advantages of Convenience in Design and Detailing

The difficulty of the details causes the construction time to prolong and the costs to increase in the structures designed with a conventional understanding. This may result in serious restrictions on functionality.

In the buildings where the seismic isolation method is used, the structure will act as a rigid mass, thus providing comfort in terms of project and detailing. Even simple mathematical models will suffice to demonstrate this behaviour.

Can Seismic Isolators Be Added to the Building Later?

Since the feature of the seismic isolator method is that the structure moves to the right, left, forward and backwards during an earthquake, seismic isolators can be added to the buildings later, except for adjacent structures. However, there are some factors for this.

• The design of the building is made according to insulators.
• Electrical, architectural and static projects must be made in accordance with the insulator.
• It should be made in accordance with the system with insulators, including the sanitary installation materials.
• Since the part where the insulator will be attached will be cut off in the columns, the suitability of the floor and the carrier system should be checked.

If you want to add a seismic isolator to a structure already, the conditions written above must be met. Seismic isolators can be added to structures later, but they are quite variable.

Building Examples with Seismic Isolators in Turkey

As Bimgen, there are seismic isolators in Istanbul İkitelli City Hospital, Elazığ Fethi Sekin City Hospital and Bursa City Hospital, where our company is also located and where we do 3D BIM modelling and quantity studies.

With these seismic isolators, hospitals make it possible to continue all operational activities in the event of and after a possible earthquake without being affected by the earthquake.