DIFFERENCE BETWEEN EARTHQUAKE LOAD & WIND LOAD & WHICH ONE IS MORE DANGEROUS.

“Here in this blog, we will try understand the difference between earthquake load & wind load & which one is more dangerous.

Author: Mohammad Sohel Akhtar(Sr. Structural Engineer,Specialist in design of building for earthquake & cyclone)

Before understanding the severity of wind and earthquake loads & wind load, we need to have a clarity between how earthquake and wind load acts.

Wind load is directly acting on the exposed potion of building or structure. As per the opening in the building,internal wind pressure is developed in the building. The net force on the wall or roof a building will be the relative difference of external wind wind pressure & internal wind pressure.

On the other hand,earthquake load is an induced load at the upper floors. However, as we all know, earthquake waves reach to the ground level of building in the form of transverse waves, compression waves or rotational waves. These wave create a heavy shear force or lateral force at the ground level but this load is distributed to the upper floor due to inertia of the buildings. However, no earthquake load directly act on the upper floor but occupant at upper floor feels jolting. Hence, we can say earthquake load is an induced load. As per the newtons first law,every object wants to be in the same state until and unless it is subjected with an external loads.We can say,earthquake load is an inertial load.

A person seating on the car at back seat feels backward push.However he /she was not directly pushed back by any external force. We can analogue this with force experienced at the upper floor however there is no direct load is acting on that floor.Either backward push felt by a person seating on the car when it start moving forward or jolting felt by a person in building during earthquake,both the forces are inertial forces.

Which one is more dangerous: Earthquake force or Wind Load:

Although both the forces are dynamic in nature. In case of cyclone, as people like fisherman or people working at the seashore are warned in advance.Also, people residing in any building can be moved to safer place if their buildings are not able to withstand the strong wind. But prediction for earthquake is not possible.It’s exact occurrence in terms of date, time & place is still under investigation among the Geologist & Scientist.No body can predict the exact date & time of dislocation of faults & relative moment of tectonic plates.Therefore, if earthquake occurs with Tsunami,many people are died at the seashore.People at the seashore cane be saved if it is possible to predict. But i am sorry to say, till date,we are not at that level to predict the exact date & timing of occurrence of earthquakes. Although Geologist can give a hint for it’s occurrence due the past seismic activity and the presence of fault planes or tectonic plate boundaries.

On the basis of intensity of wind and earthquake, any of the two may be more severe and life threatening but as earthquake can not be predicted with exact date and timing, we can say it is more dangerous.

Whichever is more dangerous, people are advised to construct building which can withstand safely for any earthquakes & strong winds which have been specified as per the earthquake zone & wind zone for a particular area.

Thanks!

ARE FREQUENT MINOR INTENSITY EARTHQUAKES HINTS A SEVERE EARTHQUAKE IN DELHI NCR REGIONS & IS IT A WAKE UP CALL FOR IGNORING SAFETY CODES?

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.

HOW EARTHQUAKE OCCURS?

Latest earthquake occurred dated on 28.01.2020 in Caribbean cities like Jamaica,Havana,Kingston, Miami indulged me to write on the subject of earthquake occurrence and it’s cause.

This blog will help many laymen to understand the cause behind the occurrence of earthquake. There are different myth behind the earthquake occurrence among different people. Therefore, as a structural engineer, it’s my duty to explain earthquake in a simpler way. There are two disturbance which are geologically proved are solely responsible for seismic activity. One is disturbance at tectonic plate boundaries and other is on the fault planes. Either of the two plays a vital role for any seismic activity which are explained below.

1. Disturbance on Tectonic Plates Boundaries:

Actually all natural or artificial structures are supported over the earth crust. It is the uppermost part of earth.Below this crust,there is layer of earth which is known as mantle. Mantle is further divided in two layer. Upper mantle & lower mantle.There is a layer above upper mantle known as Mohorovicic discontinuity or Moho. This layer “Moho” behaves like a viscous fluid having convective force due to high temperature. Earth crust being a rigid layer float over Mohorovicic discontinuity as Moho is a viscous fluid mass. How earth crust floats over Moho can be clearly understood when it can be correlated with a plate placed over utensil having water continuously boiling over it .Due to convective force of boiling water,plate starts floating it, in the same manner, earth crust floats over Moho being it a viscous fluid generating convective forces. Geological survey of earth has revealed that earth crust has been dived into twelve plates. All these plate are called as Tectonic Plates. All these plates are resting over Moho Discontinuity & moves with respect to each other due to convective force . As the convective forces are highly unsymmetrical and complex, sometime it exerts force to earth tectonic plate to make it move towards each other or sometime it exerts force on tectonic plate to make plates departs from each other. This process of colliding of one plate with other is convergence while departing of plates with respect to other is called divergence of plates. When two plates collides with each other, then huge amount of energy is released in the form of seismic waves. This release of high energy in the form of wave is called as Seismic Wave. When this wave strike to the structures, it exerts forces on the structure due inertia which is known as Seismic Forces. When two tectonic plates move away from each other, then volcanic eruption takes place in which molten Magma comes out on the earth crust. As this volcanic eruption is vigorous, it also produce vibration to the ground but is not much significant as compared to the earlier case when two plates collide with each other.

2. Disturbance at Faults Plane

Below the earth surface, there are many rocks which are in distorted or in fractured form. These fractures are formed as result of high pressure. These fractures are in unstable condition. When two parts of rocks have relative movement or deformation or dislocation then there is release of large amount of energy in the form of seismic waves. As there are many form of plate tectonic movement like convergence or divergence in vicinity of plate boundaries, in the same way different names of distortion in rock through fault boundaries are named based on the relative movement like Strike -Slip faults, Dip-Slip faults, normal faults or reverse faults. I invite you to learn more through these terms. Please read other literature for thorough knowledge of seismology.

Thanks & regards!

Mohammad Sohel Akhtar

(Structural Engineer)