Every civil engineer in the country probably knows that there are 5 seismic zone in India.
However before discussing the 5 seismic zone in India, let us discuss earthquakes in general.
Earthquake engineering has progressed over the last century. All the seismological research and development in the structural analysis tools gives us both the understanding and the ability to incorporate this understanding into the designs. It has become a necessity in the current world to ensure that all the building structures are designed for seismic resistance.
We all know that the entire Indian subcontinent is seismically active. We also have experienced devastating earthquakes in the recent past.
Therefore all the buildings in India have to be designed for earthquake forces.
However many engineers do not do this needed seismic design and many buildings, especially small buildings are designed for only gravity loads.
One of the reasons for this compromise in structural design is a lack of understanding by end clients on what seismic forces are and the significance to consider seismic force in structural analysis and structural design. Of course, another reason is compromises made by every stakeholder including structural engineers by ignoring Earthquake forces.
In this blog, I will explain what an earthquake is in a very simple manner. I will also explain how earthquakes are becoming a lateral force in the building.
We will also briefly discuss, why India is divided into 5 seismic zones as per IS 1893 : 2016
What is an earthquake?
It is important to understand what an earthquake is and how an earthquake is becoming a force in a building. Many structural engineers understand that earthquakes are waves. Of course it is a wave. However, there is more to understand on the earthquake behavior of buildings. In fact, a ground movement is an earthquake. It is a kind of acceleration of the ground. It could be shaky in nature but then basically it's a ground movement.
How does ground movement become a lateral force in the building?
This is the most important aspect a beginner needs to understand about Earthquake.
Most engineers know that wind and earthquakes are a lateral load. However there is lack of understanding on how a ground movement is becoming a lateral force.
Many entry level engineers think that an earthquake in the form of a wave comes and hits the building and pushes it laterally. In fact, this is not a clear understanding.
The sudden movement or shaking of the ground induces inertia in the building.
The self weight of the building ,superimposed load in the building and a part of the live load in the building acts as a horizontal load. In fact, the same gravity load in the building acts as lateral load because of the inertia. This is what we define as a mass source in a software like ETABS. The source of seismic mass is gravity load by itself.
To understand inertia better, take the example of a moving bus or a stationary bus that suddenly takes off. When the bus takes off, the people in the bus experience inertia.
Inertia is nothing but the resistance to that sudden movement because the self weight is acting down. The gravitational pull is to the Centre of the earth and when the bus takes off, the people in the bus continue to be pulled down by the gravity and people try to remain where they are. However the bus is moving, and this induces horizontal force in the people and they fall down or injure their joints if they are holding on to a bar or seat.
The same phenomenon occurs during earthquakes and the building experiences lateral forces. So basically, an earthquake is a deflection type of loading induced by the Inertia due to the ground movement. You can see the video above to know more.
Why 5 seismic zone in India ?
India is divided into 5 seismic zones. Considering Zone 1 is not having any place included in it, effectively we have 4 seismic zones in India.
To know the reasons for this division into seismic zones, we will have to discuss what is the design philosophy for seismic resistance.
Earthquake force Vb = Ah x W as per IS code 1893-2016.
As we discussed before in the previous section of this blog, it means that earthquake force is related to the gravity load W as well as acceleration of the ground. It is in fact not possible to know exact expected acceleration of ground as seismic activity is a natural disaster and not in our control. The only possible way to establish acceleration of ground is to rely on historical data of occurrence of earthquakes in any place. Also studying the seismological characteristics of the ground will give you the predictability of an earthquake and the probable intensity.
Even if this data is available it's not quite possible to design and build earthquake proof buildings. This is due to 2 reasons. If we try to meet the seismic demand to make a building earthquake proof, the building will be really bulky structurally and the functional use of the building will be restricted due to large structural member sizes. Also doing earthquake proof design will be uneconomic. So the seismic design philosophy is that we design for a part of the actual acceleration expected in the place and make the building earthquake resistant.
The design philosophy for seismic resistance is that for minor seismic tremors, there shall be no cracking in the building structure and for moderate there shall only be repairable cracking , for severe earthquakes, even irreparable damage is acceptable and on very severe earthquakes, there shall be no sudden and brittle collapse. The reason to adopt ductile detailing and design is also as part of avoiding this brittle failure.
Through various seismic studies and research and simulations undertaken all over the world over decades, we have realized that what is needed and possible is earthquake resistance and not earthquake proof buildings. It has also been established that by considering a fraction of the expected seismic force, it is possible to attain seismic resistance.
However, rather than giving a flat acceleration value for each place for design, IS I893 code is relating the acceleration value to a lot of parameters. These parameters are related to the place, the soil type, seismic characteristics of the place and so on.
Since the history of earthquakes in a place and the earthquake characteristic of the substrata plays a role in deciding the acceleration, the country is divided into different seismic zones. The earthquake forces to be considered in these zones are different based on the possibility of earthquake and the possible intensity of the earthquake.
Summary
Earthquake, a natural disaster that can cause damage to property and loss of life, is a phenomenon that has always fascinated mankind. This blog post gives you an idea on earthquake forces, their intensity and magnitude, and how India is divided into different zones based on the seismological activity they experience. We hope you enjoyed this post. What I mentioned here is just the very basics and there is quite a lot to master if you are a Civil or Structural engineer.
For young civil & structural engineers it is very important to understand the concepts of seismic engineering. There is no point in learning software like ETABS without understanding these structural concepts and structural principles.
Unfortunately, even now there are huge gaps in the college curriculum. It becomes essential for the engineers to cover these gaps in order to be able to design buildings on their own or be job ready.
If you wish to master structural concepts, seismic analysis and design and be an independent structural engineer who can take structural decisions, consider my mentoring programs. You can see the video by clicking the button below and then book a call with me to discuss.