Earthquakes are one of the most destructive and unpredictable natural phenomena. They not only cause damage to the property but are responsible for a large number of casualties. Although the 2005 Kashmir earthquake has served to renew people’s awareness of this phenomenon, but a lot needs to be done on ground to adopt technical measures as outlined in various earthquakes Indian Standard Codal provisions for constructing earthquake resistant structures. This earthquake is considered to be “the worst natural disaster in Kashmir” over the past 100 years. The seismologists have brought out that the geology of the Northern Himalayas starting right from Kashmir to Assam is a very unstable seismic zone viz-a-viz a long thrust fault known as the Great Boundary Fault exists along the foothills of the Himalayan region.
As a result, this region had witnessed the high magnitude earthquakes and is considered to be a well known high vulnerable earthquake zone. Therefore, in order to have advanced information of a particular area with regard to its vulnerability to the earthquakes, a country or region is divided into different zones on the basis of their seismicity or vulnerability termed as Seismic zoning. Listed below are the varied zones, according to the Seismic zone map of India:
4 Zone - II: Least active seismic zone.
4 Zone - III: Moderate seismic zone.
4 Zone - IV: High-damage risk zone.
4 Zone - V: Highest risk damage zone.
From the perusal of the above, it is clear that zones of high vulnerability can be easily identified by way of using the seismic zone map that can assist us in taking the measures as laid down in the various earthquake Codal guidelines for improving the earthquake resistance of the structures. It may be noted here that it is not possible to prevent an earthquake, but it is viable to construct the structures that do not collapse, thereby saving the lives of the people, and therefore the main aim behind the earthquake resistant construction is fulfilled. It has been rightly said that “Earthquakes do not kill people but it is the structures built by them that do so”.
Pertinently, Kashmir valley falls in Zone V which makes it more susceptible to earthquakes and big and small earthquakes will continue to occur in this region. In this article, only the special methods /techniques have been briefly discussed, which enables a structure to combat high magnitude earthquakes, particularly those structures where the likelihood of causalities during earthquake is high, as there is going to be lot of gathering of people and are also highly susceptible because they attract more lateral forces on account of their rigidity. The examples of the important structures wherein the special techniques need to be adopted are hospitals, schools, railways, malls etc and as such these buildings need to be designed to resist greater forces enough to prevent large displacement and acceleration so that they can maintain their functions even when excited by a severe ground motion. In order to save lives of people, the basic principle taken on board while designing is to make the structures that do not collapse during earthquake vibrations but may get damaged. However, this may result in rendering the building non-functional and thus the buildings like hospitals, fire stations etc., can suffer badly, as remaining functional of these buildings after an earthquake event is pivotal. For that purpose, the two special techniques that are implemented in the important buildings now-a-days, so that they remain practically undamaged during a severe earthquake are listed as under:-
1. Base Isolation Devices
2. Seismic Dampers
1. Seismic Base Isolation: Base isolation system for buildings is a mechanism that provides earthquake resistance to the structure. In this method, the structure is essentially detached from the earthquake ground vibrations by way of introducing some type of support mainly rubber bearings between the building (superstructure) and the foundation Fig 2. Cylindrical bearings with alternating layers of steel plates and hard rubber pads between the base of the superstructure and the foundations are frequently used base isolators. The main objective of the base isolation system is to diminish the horizontal acceleration transmitted to the superstructure and subsequently the earthquake vibrations are not allowed to get transferred to the superstructure of the building so that the same does not experience any earthquake. Although this is a costly approach than normal buildings, but the same is justified on account of numerous benefits inter-alia saving precious human lives. In India, soon after 2001 Gujarat earthquake disaster, the base isolation technology was first introduced at the Bhuj Hospital in Gujarat. It may be noted here that base isolation system is not effective for all buildings but can be beneficial in low to medium rise buildings resting beneath on a soil of hard strata.
Fig 2: Base isolation system.
2. Seismic Dampers: This method of improving the seismic performance of structures is analogous to hydraulic shock absorbers in cars which absorb shocks or sudden jerks, so that the vibrations are being prevented from moving above to the frame of the car. In other words, the dampers perform the function of energy dissipaters by transforming the earthquake induced energy into heat and other forms of energy which ensures protection of human lives and the property. A damper may consist of piston head inside a cylinder filled with silicone oil and when an earthquake occurs, the building leads the vibration energy to strike into the piston, which pushes against the oil, as a result the energy is transformed into heat, dissipating earthquake vibrations.
During an earthquake event, the damages which the structure has undergone need to be repaired, which is a very expensive method and in certain cases requires evacuation of the building, while as by installing seismic dampers/ energy dissipation device to the new and already existing structures in place of structural elements like diagonal bracings, earthquake-induced energy can dissipate efficiently and hence the potential of the building to combat the earthquakes without any major damage shall be improved to substantial degree. Listed below are the commonly used seismic dampers:
A. Viscous Dampers: In this case, the energy is absorbed by silicone-based fluid passing between piston-cylinder arrangements. Also discussed above. (Source : C.V.R Murty, Learning earthquake Design and Construction)
B. Friction Dampers. In this case, the energy is absorbed by surfaces with friction between them rubbing against each other.
C. Yielding Dampers. In this case, energy is absorbed by metallic components that yield.
Fig 3: Seismic Energy Dissipation Devices
Source: C.V.R Murty, Earthquake tips learning Earthquake Design and Construction
However, the aforesaid methods used to protect the life and property from the adverse effects of the earthquakes is yet to be fostered in this region on larger scale. Since, the earthquakes will continue to occur in this region because of high seismicity as discussed above, hence if these techniques are not widely followed in the important buildings, this could result in a colossal loss of life and property.
Er M. A. Dar is a Civil Engineer.
Disclaimer: The views and opinions expressed in this article are the personal opinions of the author. The facts, analysis, assumptions and perspective appearing in the article do not reflect the views of GK.