19-11-2020 | Posted by Joaquín Martí
Engineers build things that are meant to last, perhaps not forever, but at least for the time indicated in the specifications. This implies that our structures must be capable of sustaining the loads that will be felt during their useful life. To achieve this, one may increase the strength that the structure can mobilise or somehow limit the loads that it will experience. Or, indeed, a combination of both.
Earthquakes are responsible for some of the loads that structures must face and there is very little we can do about their occurrence or magnitude. Furthermore, there is something very awkward about those events, which is that we can only characterise them statistically: thus, we design for an earthquake with a given return period, say 500 or 5000 years, but there is no guarantee that the structure will not experience a larger event.
Seismic isolation attempts both to decrease the loads that the earthquake transmits to the structure and, to some extent, also to establish some limits on them. The concept is simple: if we remove or soften drastically the connections of the structure to the ground, the ground will not transmit its motions to the structure.
In the extreme, a structure freely floating on a lake will not notice any earthquakes. This solution is effective, but not always practical: our structures will generally need to be tied to the ground and, at least in the vertical direction, the connection to the ground cannot be softened much, lest the gravity loads cause unacceptable settlements.
All structures are inherently strong in the vertical direction because they need to resist gravity loads. But this does not happen in the horizontal direction, which is precisely that in which the seismic effects tend to be more demanding. Hence, in that direction, we can introduce systems or elements between the structure and the ground to reduce the seismic forces generated in the structure, at the cost of increasing its relative displacements with respect to the ground. There are many types of seismic isolation, based on springs, dashpots, frictional sliding, deforming neoprene, etc., even some that are smart and can adapt to the changing circumstances.
At Principia we have worked with seismic isolation in petrochemical facilities, nuclear power plants and, particularly, storage tanks for liquefied natural gas. We have also provided isolation for vibrations of non-seismic origin, such as machinery or pedestrian action. And we were a Partner in Project Indepth, financed by the European Community and dedicated to the study of various types of seismic isolation devices for the protection of petrochemical plants. One of the most useful results of this project was, for a given structure and function, to determine the levels of seismic input at which seismic isolation is still unnecessary, starts to be progressively useful and economically more attractive and, finally, the point after which it becomes essential, the structure cannot be built without it.
Next time you have a structure in a particularly seismic setting or one that is especially sensitive to earthquake effects, come to the experts.
