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Strong Assam earthquake rattles Bangladesh

Black_cats

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Strong Assam earthquake rattles Bangladesh

BANGLADESH
TBS Report
28 April, 2021, 08:25 am
Last modified: 28 April, 2021, 09:27 am


The quake was at a depth of 28.9 km

Representational Image. Photo: Collected

Representational Image. Photo: Collected

Representational Image. Photo: Collected

A strong earthquake has jolted Dhaka and other parts of Bangladesh at 8:24am today.
According to India's National Centre of Seismology, the earthquake measuring 6.4 on the Richter Scale originated in Sonitpur, Tezpur of Assam. The first earthquake was recorded at 7:51 am and (IST) according to the seismology centre, it was centred 43 km west of Tezpur.


There were three aftershocks following the first major earthquake, one around 7:55 am (IST) and other two were felt few minutes after that. The aftershocks measured 4.0, 3.6 and 3.1 on the Richter Scale.


Guwahati, Magalaya and northern districts of Bangladesh also felt the hit of the massive tremor.

Local lawmakers and citizens were seen tweeting pictures of damage caused by the tremors.
 

bluesky

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Assam was lucky this time I should say, 6.4 magnitude earthquake and not much of property/life loss
Note what the Internet sources say about this magnitude of earthquakes.

"Magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 is a moderate earthquake, and a 6.3 is a strong earthquake. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude as measured on a seismogram".

So, 6.3 is a strong magnitude (about 10 times larger than the magnitude of 5.3) that can be felt across a hundred kilometers. Assam falls in an earthquake zone that starts from the Himalayas and extends to Thailand. Assam is in between and is very near to the Himalayas.

Therefore, design engineers must take into account the horizontal sways of all high strictures and bridges during an earthquake. California and Japan have standardized the structural design analysis methods to adequately address the side swings.

Approximately speaking, the horizontal coefficient of an earthquake is taken as a maximum of 21% of the Dead Load of the structure. It roughly means that 21 tons of lateral/horizontal load will act to destabilize or break a structure if its Dead Load is 100 tons.

However, there are minute methods to compute the lateral component of the seismic load. No rules of the thumbs.
 
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Bilal9

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Note what the Internet sources say about this magnitude of earthquakes.

"Magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 is a moderate earthquake, and a 6.3 is a strong earthquake. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude as measured on a seismogram".

So, 6.3 is a strong magnitude (about 10 times larger than the magnitude of 5.3) that can be felt across a hundred kilometers. Assam falls in an earthquake zone that starts from the Himalayas and extends to Thailand. Assam is in between and is very near to the Himalayas.

Therefore, design engineers must take into account the horizontal sways of all high strictures and bridges during an earthquake. California and Japan have standardized the structural design analysis methods to adequately address the side swings.

Approximately speaking, the horizontal coefficient of an earthquake is taken as a maximum of 21% of the Dead Load of the structure. It roughly means that 21 tons of lateral/horizontal load will act to destabilize or break a structure if its Dead Load is 100 tons.

However, there are minute methods to compute the lateral component of the seismic load. No rules of the thumbs.

@bluesky bhai - sysmic isolator for bridges and building foundations have started being placed in Bangladesh. I do not know if it is required by govt. though using Bangladesh National Building Code (BNBC) or what is called "Imarat Nirman Bidhimala" in Bangladesh.






 
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bluesky

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@bluesky bhai - seismic isolator for bridges and building foundations have started being placed in Bangladesh. I do not know if it is required by govt. though using Bangladesh National Building Code (BNBC) or what is called "Imarat Nirman Bidhimala" in Bangladesh.

http://www.iabse-bd.org/2020/pdf/41.pdf
I have copied the link to read/study at a later time. Thanks for the input. Rubber isolators may be used between a bridge pier and a girder but I am not sure about placing them in multistoried buildings or in between a pile cap and a pier/column.

There are methods other than rubber isolators to offset the effects of earthquakes on the girders. Rubber isolators may still be of academic interest.

BNB is OK. But, we must follow international codes like the American ACI, ASTM, ACI, ASCE, AASHTO, etc., or BS codes to compile the rules for the BNB. It will be like re-discovering the shoes if we try to discover newer building rules when the ACI, BS, FN, DIN are all full of hundreds of formulas devised on the experiments conducted by bonafide engineers for more than a century.

We should just partially adopt and adapt any of these in the BNB. However, I am not sure if any of BNB rules are now followed by the concerned companies/people. Implementation is more important than formulating the rules on the papers.
 
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Bilal9

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I have copied the link to read/study at a later time. Thanks for the input. Rubber isolators may be used between a bridge pier and a girder but I am not sure about placing them in multistoried buildings or in between a pile cap and a pier/column.

There are methods other than rubber isolators to offset the effects of earthquakes on the girders. Rubber isolators may still be of academic interest.

BNB is OK. But, we must follow international codes like the American ACI, ASTM, ACI, ASCE, AASHTO, etc., or BS codes to compile the rules for the BNB. It will be like re-discovering the shoes if we try to discover newer building rules when the ACI, BS, FN, DIN are all full of hundreds of formulas devised on the experiments conducted by bonafide engineers for more than a century.

We should just partially adopt and adapt any of these in the BNB. However, I am not sure if any of BNB rules are now followed by the concerned companies/people. Implementation is more important than formulating the rules on the papers.

I believe BNBC was developed using ACI. ASTM, ASCE standards etc. as they should not re-invent the wheel, only adapt international building safety norms to local conditions/standards. I believe Japanese research in earthquake resistant structures was exemplary and have been adopted into BNBC as well. Latest revised version is BNBC 2020. Some discussion is here, where they discuss seismic standards per soil liquefaction rates for different zones in Bangladesh.


There is also some discussions from Professor Sheikh Hedaetullah's channel (see below, one that I found) on how to measure seismic load when designing tall structures. I believe a package called Etabs is used but I have also seen Finite Element Analysis used by other structural engineers to measure and compensate for different Seismic loads.

 
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bluesky

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I believe BNBC was developed using ACI. ASTM, ASCE standards etc. as they should not re-invent the wheel, only adapt international building safety norms to local conditions/standards. I believe Japanese research in earthquake resistant structures was exemplary and have been adopted into BNBC as well. Latest revised version is BNBC 2020. Some discussion is here, where they discuss seismic standards per soil liquefaction rates for different zones in Bangladesh.


There is also some discussions from Professor Sheikh Hedaetullah's channel (see below, one that I found) on how to measure seismic load when designing tall structures. I believe a package called Etabs is used but I have also seen Finite Element Analysis used by other structural engineers to measure and compensate for different Seismic loads.

Thanks, it is quite educational. About the uses of BNB Codes, I have seen these are not applied even in Dhaka. Codes and Standards are not really taught in Universities but engineers learn them when working in a design office.

I wonder if our Municipal engineers have the proper grasp on the codes. They are responsible to give permission for construction after checking the Design Analysis/Calculation Sheets and the detailed drawings.

But, are these followed in BD? I have seen a few structures whereby even the X-Y directions of the columns have been reversed. It surprised me. Anyway, I hope that the BNB codes will be strictly followed.

I have also noticed a proper soil investigation by the Standard Penetration Test (SPT) method is not followed. SPT is needed to calculate the soil or pile bearing capacity. People do not do this and arbitrarily decide 15 m long piles would be good enough. They call it Soil Testing - a wrong expression.

In the case of road construction, the CBR (California Bearing Ratio) tests are also not conducted to check the strength of the compacted soil bed under the road. Neither the soil materials are replaced with select soil consisting of broken stones/pebbles/sands. So, we soon get broken roads.

These are all symptoms of a Soft State. Bangladesh has a long way to go to enter a modern age. However, I am seeing recently quite a few good roads in the video clips, at least on the surface they look good.
 

Bilal9

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Thanks, it is quite educational. About the uses of BNB Codes, I have seen these are not applied even in Dhaka. Codes and Standards are not really taught in Universities but engineers learn them when working in a design office.

I wonder if our Municipal engineers have the proper grasp on the codes. They are responsible to give permission for construction after checking the Design Analysis/Calculation Sheets and the detailed drawings.

But, are these followed in BD? I have seen a few structures whereby even the X-Y directions of the columns have been reversed. It surprised me. Anyway, I hope that the BNB codes will be strictly followed.

I have also noticed a proper soil investigation by the Standard Penetration Test (SPT) method is not followed. SPT is needed to calculate the soil or pile bearing capacity. People do not do this and arbitrarily decide 15 m long piles would be good enough. They call it Soil Testing - a wrong expression.

In the case of road construction, the CBR (California Bearing Ratio) tests are also not conducted to check the strength of the compacted soil bed under the road. Neither the soil materials are replaced with select soil consisting of broken stones/pebbles/sands. So, we soon get broken roads.

These are all symptoms of a Soft State. Bangladesh has a long way to go to enter a modern age. However, I am seeing recently quite a few good roads in the video clips, at least on the surface they look good.

Well things do change.

I think road/rail right of way construction standards and easement limits have changed night and day compared to even two decades ago, this is in no small part due to overseas work experience/exposure (in middle east. Singapore and Malaysia) of our engineers and workers.

Back in the day- my Dad told me that few civil contractors even knew what a theodolite was. Nowadays, I see even diploma civil engineers using laser-levels and computerized gadgets in their work nowadays.
 
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bluesky

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Back in the day- my Dad told me that few civil contractors even knew what a theodolite was. Nowadays, I see even diploma civil engineers using laser-levels and computerized gadgets in their work.
Laser Levels have been in use for only a few decades that has reduced the workload of a surveyor while checking the levels of, say, a newly poured concrete floor.

There is another piece of equipment (it has been incorporated in the Transit Theodolite) used to measure distances. I do not know why it is called Total Stations in English, but the Japanese name KOHA is very appropriate because KO means light and HA means wave.

So the meaning is a lightwave machine that sends a ray of light to the prism at the other end that reflects back to the machine. Even a small 10/20 m distance can be accurately measured. Light travels at 299, 792. 458 km/s.

I am sure the Japanese Metrorail builders are using it. The Chinese must also be using it to measure distances between two sets of anchor bolt holes on the Padma Bridge piers.

BD should produce all these survey machines including tripods and their uses should spread.
 
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