Earthquake resistant design of building

By: Mohammad Sohel Akhtar(Sr. Structural Engineer)

New Delhi, India

“This blog is all about to wake up all residents of Delhi- NCR for their ignorance of building by-laws & structural safety codes. Through this, I have tried to create awareness among them regarding severe earthquakes”.

In recent past( within one & half month)many earthquake has found it’s epicenter in NCR & it’s nearby state. All these are minor earthquakes as the magnitude of these earthquakes are lesser than the permissible limit (for damage& loss of lives)categorized for earthquake in terms of magnitude in Ritcher Scale & Intensity in MSK1964 Intensity Scale, Annex-D of Indian Standard(IS-1893-2016) for general provisions for earthquake resistant design of buildings.

As these are minor earthquakes & has almost no significant damage to property and lives, people starts ignoring it. But frequent earthquakes hint a violent & severe earthquake in near future which can lead to massive loss to lives & properties.

How minor frequent earthquakes hint a major earthquake,Let us read here.

As it has been proved by many geologist, earthquake occurs due to disturbance inside the earth when crack starts developing in a rock. Due to crack widening in the stressed rocks, strain energy starts releasing. This initial release of lesser quantity of energy leads to minor earthquake. These minor earthquakes are sometimes unnoticeable. But if there is a large cracks developed in the rocks,which can lead to sliding of a rock over one another or one rock can laterally move with respect to another, then massive energy is released. These massive released energy can jolt a structure vigorously leading to the collapse of structure if structure is not designed as per earthquake resistant code.

Apart from this, earthquake occurs due to disturbance in plate boundaries. When there is relative movement between tectonic plates, earthquake originates.

Earth tries to reach at it’s stable position by continuously releasing strain energy once cracks are developed in the rock .When sudden and large amount of energy will be released, nobody can tell you the exact date & time of it’s occurrence.

Anyway, whatever be the reason, either disturbance in the existing fault planes or tectonic plates, earthquake in Delhi will trigger massive loss of lives & properties as safety code has ignored in previously constructed buildings in Delhi. Also, in previously constructed buildings,building bye laws were not followed giving sufficient escape passage to resident.Also, as buildings have been constructed very closely, it will lead to severe damage to building due to pounding or hammering action. It means, building will start colliding with each other in severe shaking.

Therefore, certainly, it’s a wake up call for Delhi for future prospectus for constructing any building.

In order to understand the severeness of earthquake,it is very essential to know certain terms for measuring quantity of earthquake in terms energy released & destructive effect.Energy released is measure in Ritcher Scale. While intensity is measured in the MSK Scale according to the loss of lives, vibration/jolting felt by an observer during earthquake inside the building and structural damage caused by earthquake.

“It is essential to know the difference between intensity & magnitude of earthquake so that people may understand it’s severity”

It is very interesting to note, energy released is constant because it is measured at the site of it’s occurrence by Seismogram whereas two buildings at the same site,far away from the epicenter, would have different damage. If a building is designed with earthquake resistant code,resident of building will feel lesser jolting with respect to the occupants of building which is not designed with any earthquake resistant design code. This part is very understood by structural engineer as only structural engineer can control the deflection of a building. Hence, designed building will have deflection & vibration under permissible limit whereas a building without consulting a structural engineer will have uncontrolled deflection.Building designed & constructed without following a structural safety codes & guidelines,people inside building will feel haunted when severe earthquake will occur. As in this case,building frame is not stiff enough to restore building at it’s initial state & building will be collapsed.

Intensity of earthquake can be understood with another analogy of reading a book at different distance from a bulb. If we read a book at a proper distance from a bulb, book can be read with greater ease and when we keep same book far away from the bulb, it will become difficult to read .However, bulb is releasing same quantity of heat energy every time, but intensity of bulb will be reduced as we keep distancing from the bulb. Therefore, here energy release by bulb is similar to the magnitude of earthquake because it is constant whereas reduced illumination of bulb with distance can be assumed identical with the intensity of earthquake.

I have stressed over the the concept of intensity & magnitude to explain our reader about the severity of earthquake on buildings based on the epicentral distance & building construction(Designed by following safety codes or not). It is a good news to the resident of Delhi -NCR that they are lucky when earthquake’s epicenter is at the hundreds of kilometre like in Afghanistan,Middle East etc. and may have magnitude more than 8 in Ritcher Scale but have mild effect on their structures.

But what if earthquake of same magnitude have epicenter in Delhi NCR regions. As i see, many buildings in Delhi have not followed the earthquake resistant code will have severe effect on this. If you believe in Structural Engineers, you need to show your buildings to structural engineers. Still you can improve response of building against earthquake by retrofitting.

“People will start to build an earthquake resistant building when they will start comparing their skeleton with building frame.Strong skeleton leads to very very efficient life & in the same way, building with stiff, ductile & stable frame will lead to a long lasting structure facing all natural calamity”

We consult doctors for our health but do not consult structural engineers, why? As we know,earthquake is always life threatening.

” You consult vendors for modular kitchen and interiors. But not structural consultant. Why such an ignorance.What if, your bone is not strong enough however you are a smart looking person.We should have a thought on it”.

I have tried to motivate people to build safer home & not to avoid building by-laws & safety codes.Indeed, they should consult structural engineers if they want to keep away from the haunting of that day when their building will jolt with no limits.I think, they will understand it.

Thanks for reading.

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