Seismic isolation is a proven technology that allows a structure to safely “dance” with the Earth, rather than fight against it. Isolating any structure from seismic motion is the most effective way to protect its contents, functionality, and the lives of its occupants, thereby minimizing losses associated with suspended operations and liability-generating events. 

* This article was published in Geociencias SURA Journal | Issue 2 | September 2017.

Imagine your house sitting gently on a frozen lake, not attached to it, when a large earthquake occurs. Other than noticing a minor vibration, how would you know that an earthquake occurred? The absence of a connection to the Earth would allow the ice to slide horizontally without affecting the house. 

Neither you, nor your house, nor even a cup of coffee on the table would experience horizontal ground movements. With this idealized scenario, Mason Walters, M.Sc., structural engineer at the California firm Forell/Elsesser, explains the concept of “seismic isolation.” 

It is clear that the reality of seismic isolation is not so ideal, if we take into account the legal limits of the property that each building or structure can occupy, which implies both restrictions on how much the isolated structure can move during the seismic event, as well as the design of a mechanism for it to recover its original location after the earthquake. 

Engineer Ivan Skinner was a pioneer in seismic isolation. This eminent engineer said: “We want to give the structure a smooth ride.”He proposed the idea of ​​seismic isolation in the late 1970s, and everything that was required to initiate the project became a reality in the following decade. The first design project with seismic isolation was the William Clayton Building in Wellington, New Zealand. 

How does seismic isolation work?

It consists of replacing the direct and rigid connection between the structure and the supporting ground, with a set of flexible supports in the horizontal sense, which allow it to remain without major disturbance, even if the supporting ground moves violently. 

The supports responsible for separating the structure from the ground are called “isolators”, which are designed for the specific strength, flexibility and energy dissipation requirements of each project. 

In this way, as explained by engineer Mario Lafontaine, from the Chilean firm René Lago Engineers, In every work with a seismic isolation solution there is the so-called isolation plane, which is defined as the boundary between what is above the insulators (protected structure) and what is below the insulators (which moves with the ground). 

This isolation plane enables a relative horizontal displacement between the structure and the ground, which changes the horizontal dynamic conditions of the structure with respect to those it would have if it were built in a conventional manner, and allows: 

• That the structure above the insulators is protected from large relative displacements between floors, which are the main cause of damage in conventional structures. 
• Significantly reduce the horizontal force at the base and the overturning moment on the isolated structure, compared to what it would experience if it were built conventionally, which results in a lower design horizontal force at the base. 
• Reduce accelerations on floors, which are the primary cause of damage to contents and electromechanical elements that perform vital functions in certain buildings. 

Isolated structures require some details in the connections of structural and non-structural elements., such as: 

• Pipes and wiring must have flexible connections between the protected structure and what is below the insulators, so that they can easily handle the displacement in the isolation plane during an earthquake. 
• Entrances, connections between bridges, stairs and elevators must have clearances to prevent them from colliding during the seismic event.

“Any structure that requires operational continuity is a candidate for using seismic isolation systems. For example, in industrial structures the cost of interrupting production due to damage after an earthquake is very high, which motivates the use of this technology.” 

Engineer Mario Lafontaine, René Lagos Engineers, Chile. 

What are seismic isolators like?

The development of the types of insulators that make this gold technology viable has evolved dramatically since its initial idea. Today, their mechanical properties, practical application, and real-world performance characteristics are much better understood. Additionally, the sizes, displacement capacities, and energy dissipation of insulators have increased considerably. 

The basic requirements that insulators must meet are the following::

• Isolate the structure from the ground.
• Support the weight of the structure. 
• Cushion the seismic response of the structure. 
• Restore the structure to its original position after the earthquake. 

Seismic isolation systems proved their effectiveness during the March 11, 2011 Tohoku-oki earthquake with a magnitude of 9.0 (Mw), considered the fifth largest recorded worldwide and the longest-lasting recorded in the history of Japan. 

During this event, buildings constructed with seismic isolation performed excellently, as is the case of a nine-story reinforced concrete office building located in Sendai, built in 1981 and renovated in 2009 by implementing seismic isolators. 

Regarding the applications of seismic isolation in buildings, engineer René Lagos explains, “when we talk about rigid floors and soft floors, rigid buildings or flexible buildings, they are all relative terms, that is, Seismic isolation seeks to concentrate all the deformation on the isolators and increase the vibration period of the structure compared to what it would have if it were not isolated., so that it is not located in the range where the maximum seismic response of the soil is located.” 

Fonts

• Jose Ignacio Restrepo. Ph.D. in Earthquake Engineering from the University of Canterbury, New Zealand. Professor at the University of California, San Diego and Professor at the Rose School of Earthquake Risk Reduction in Pavia, Italy.
• Mario Lafontaine. Civil engineer from the University of Chile, linked to the company René Lagos Engineers since 2008.
• Rene Lagos. Civil engineer from the University of Chile, partner and general manager – CEO of René Lagos Engineers.