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8.7 quake off Indonesia
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str.engr
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PostPosted: Thu Apr 12, 2012 8:09 am    Post subject: 8.7 quake off Indonesia Reply with quote

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bijay sarkar
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PostPosted: Thu Apr 12, 2012 3:01 pm    Post subject: Reply with quote

Dear Sefians,

Till date, due to earthquake around me , i was accustomed to some other kind of feelings with just vibrations of the earth for some seconds. But what happened on yesterday at about 2:11PM, i got the feelings of getting swayed along with the building.

I am telling about the shaking of 10th floor in our office building situated in Kolkata and it was not for seconds, but i can say that it may be near to one minute. Even after a while, when it did not stop, we started moving down by stairs and in stairs also, we felt that still the building is swaying like pendulum and we came down to earth.

When it stopped, after some time, we again went up. Second time around 4:15PM, swaying of the building again started. Though it was not as bad as earlier one, still we came down and went out.

Though there was no such damage in this building, many other buildings in Kolkata particularly in Saltlake Area got multiple cracks. I mention the name of Saltlake area in particular as because, this area was developed in 80's after filling up of a vast water logged area. The area was developed by putting sand bags to increase the speed of consolidation within a short period.

This was really panicky at such height under such to and fro motion !!
I have felt many other EQ vibrations, but this one gave me a new feelings like a bob in pendulum !!!

with regards,

bijay sarkar
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Dr. N. Subramanian
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PostPosted: Thu Apr 12, 2012 4:19 pm    Post subject: Reply with quote

Dear Er Bijay,

Thanks for sharing your experience. I could understand your feelings. It is really scary to be in a multi-storey building that is swaying during an EQ, especially when you are aware that your building was built on a swamp or built-up land.

Does anyone know the magnitude of EQ in Chennai or Kolkata?

Regards,
Subramanian
bijay sarkar wrote:
Dear Sefians,

Till date, due to earthquake around me , i was accustomed to some other kind of feelings with just vibrations of the earth for some seconds. But what happened on yesterday at about 2:11PM, i got the feelings of getting swayed along with the building.

I am telling about the shaking of 10th floor in our office building situated in Kolkata and it was not for seconds, but i can say that it may be near to one minute. Even after a while, when it did not stop, we started moving down by stairs and in stairs also, we felt that still the building is swaying like pendulum and we came down to earth.

When it stopped, after some time, we again went up. Second time around 4:15PM, swaying of the building again started. Though it was not as bad as earlier one, still we came down and went out.

Though there was no such damage in this building, many other buildings in Kolkata particularly in Saltlake Area got multiple cracks. I mention the name of Saltlake area in particular as because, this area was developed in 80's after filling up of a vast water logged area. The area was developed by putting sand bags to increase the speed of consolidation within a short period.

This was really panicky at such height under such to and fro motion !!
I have felt many other EQ vibrations, but this one gave me a new feelings like a bob in pendulum !!!

with regards,

bijay sarkar
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suresh_kumar
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PostPosted: Thu Apr 12, 2012 4:48 pm    Post subject: Reply with quote

Dear N.S sir

     In chennai, the magnitude is around 2.5 to 3 as per data  given govt agencies.

     My office is situated in 9th floor in teynampet area. As told by Mr.Bijay sarkar, i felt really the oscillation of the building.

     Immediately i have pressed the fire alarm and almost around 300 persons came out of the entire building.

     More over to avoid panic we are all advised to leave the office immediately.

Another information the building was built in 1984.

Regards

N. Suresh kumar
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Vishal
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PostPosted: Fri Apr 13, 2012 6:07 am    Post subject: Reply with quote

Hi,

Please see the building with cracks following the eqarthquake in Kolkata:

http://indiatoday.intoday.in/story/panic-in-eastern-india-after-sumatra-coast-earthquake/1/184081.html

Can we say that the reason for the cracks may be due to the discontinuities in the load transfer path as a result of the huge vent and the cantilevering portion on the first floor (walls not continuing to the ground floor)?

Vishal
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mohannaikleo
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PostPosted: Fri Apr 13, 2012 5:36 pm    Post subject: 8.7 quake off Indonesia Reply with quote

Dear Vishal,

  You r right, first of all the structural arrangements are clearly vulnerable especially for seismic forces. In such cases even critical structural detailing will not help under seismic forces, most of the cases the critical factors will not felt during the execution. Obviously earthquakes will clearly indicates the which part of the structure is most critical or which has been not taken care during construction. So please such critical issues shall be duly cared by both designing as well as execution side......

Thanks & Regards
Mohan Naik

On Fri, Apr 13, 2012 at 11:37 AM, Vishal <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Hi,

Please see the building with cracks following the eqarthquake in Kolkata:

http://indiatoday.intoday.in/story/panic-in-eastern-india-after-sumatra-coast-earthquake/1/184081.html

Can we say that the reason for the cracks may be due to the discontinuities in the load transfer path as a result of the huge vent and the cantilevering portion on the first floor (walls not continuing to the ground floor)?

Vishal
     



     


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prof.arc
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PostPosted: Fri Apr 13, 2012 11:19 pm    Post subject: 8.7 quake off Indonesia Reply with quote

Even if there are no strong motion accelerographs recordings in Kolkata [I doubt any SMA would have triggered as the motion would have been below the trigger level set], any plethora of empirical formulae relating Magnitude, distance from epicentre would give very low levels of acceleration.

Any structure in Kolkata, even assuming it has not been specifically designed for ground motion must have withstood such insignificant
ground motion. The ground motion intensity [say for predicting isoseismals] is to be never based on one stray incident.

i may be uncharitable with the following remark - please excuse me
we have noted that during site survey after a real damaging event, some house owners tried to say the cracks in the building are due to that event when it could be established that they pre-existed. in fact a number of owners would want the surveyors like us to state that the damages are due to the event in order to get more compensation

kolkata had been classified to be in earthquake zone [though moderate only] and any structure showing damage due to the recent indonesian event must have structural deficiencies unrelated to ground shaking due to that event

ARC

On Fri, Apr 13, 2012 at 11:37 AM, Vishal <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Hi,

Please see the building with cracks following the eqarthquake in Kolkata:

http://indiatoday.intoday.in/story/panic-in-eastern-india-after-sumatra-coast-earthquake/1/184081.html

Can we say that the reason for the cracks may be due to the discontinuities in the load transfer path as a result of the huge vent and the cantilevering portion on the first floor (walls not continuing to the ground floor)?

Vishal
           


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sakumar79
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PostPosted: Mon Apr 16, 2012 1:05 am    Post subject: Reply with quote

Dear Sirs,
    Is it even a crack in RCC? It appears to be on brick parapet wall only. It seems to me the column is not taken out as a "floating column" in which case the crack is on masonry only...

   Further, as Professor ARC pointed out, it is very likely that the crack existed before the quake itself. At the most, it may have widened a bit. On the one hand, owners like to act as if the crack was caused by the quake only, and on the other hand, media likes to lap it up without first getting expert opinion regarding the nature of the crack, seriousness, etc...

Arun
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rni
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PostPosted: Mon Apr 23, 2012 6:20 am    Post subject: 8.7 quake off Indonesia Reply with quote

I checked up from Chennai about records of ground motion. A broad band instrument (placed on rock) has responded near Chennai. By converting the counts to acceleration it is found the maximum ground acceleration was 0.0069g. This is a very small value to be of any consequence to safety of engineering structures. But I hasten to add human beings can easily feel and panic at as low as 1 cm/s/s that is nearly 0.001g. At Calcutta the ground motion might have been of the same order since the soil there is overlain with deep sediments.
At long distances ground motion will be at a very low frequency nearly equal to the local site frequency. My personal opinion is: if in a building pre-existing weaknesses respond to such a frequency there can be some visible distress. Probably the building in Kolkata had a long hidden crack or separation which opened out due to repeated cycling at a frequency conducive to itself.
 
Members perhaps recollect after the 2001 Bhuj earthquake, there was a minor shock felt in Bangalore on 28th January. I investigated this event in the field. Many peculiar behaviour of structures was observed. There was no major structural damage anywhere in Bangalore. But a long comopund wall in a factory collapsed in part. Some directional effect of ground motion could be sensed since the ground below showed a long crack which closed after a couple of days.
 
I had two interesting visits along with an insurance agency which liked to settle two claims.
 
The first was a m/c tool factory with a tall wall separating two halls. Large parts of the plaster of the wall fell down on the machines below stopping production for a couple of days. The risk claim was settled satisfactorily since the event was triggered by an act-of-God!  My personal feeling was the plaster layer had slightly separated from the structure and was holding due to some membrane type action. Perhaps it would have stayed in such a static position for several years if there were to be no dynamics coming into play. But on the fateful day the wall vibrated at a low frequency helping the plaster layer to give away! The high frequency machinery running on the floor for more than one year could not cause this action.
 
The second was in a modern Biology Lab where a sophisticated culturing facility showed severe signs of dampness indicating failure of water proofing in the roof. The accused was the earthquake as per the engineer-in-charge. But our inspection showed the roof top generator could have easily caused the roof cracks which were quite old and also showed a spatial pattern going towards the generator. These roof cracks were definitely not due to the earthquake!
 
RN Iyengar

 
On Sat, Apr 14, 2012 at 8:51 AM, prof.arc <forum@sefindia.org (forum@sefindia.org)> wrote:
[quote]  Even if there are no strong motion accelerographs recordings in Kolkata [I doubt any SMA would have triggered as the motion would have been below the trigger level set], any plethora of empirical formulae relating Magnitude, distance from epicentre would give very low levels of acceleration.

Any structure in Kolkata, even assuming it has not been specifically designed for ground motion must have withstood such insignificant
ground motion. The ground motion intensity [say for predicting isoseismals] is to be never based on one stray incident.

i may be uncharitable with the following remark - please excuse me
we have noted that during site survey after a real damaging event, some house owners tried to say the cracks in the building are due to that event when it could be established that they pre-existed. in fact a number of owners would want the surveyors like us to state that the damages are due to the event in order to get more compensation

kolkata had been classified to be in earthquake zone [though moderate only] and any structure showing damage due to the recent indonesian event must have structural deficiencies unrelated to ground shaking due to that event

ARC

On Fri, Apr 13, 2012 at 11:37 AM, Vishal forum@sefindia.org (forum@sefindia.org))> wrote:
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Dr. N. Subramanian
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PostPosted: Mon Apr 23, 2012 1:41 pm    Post subject: Re: 8.7 quake off Indonesia Reply with quote

Thank you Prof. Iyengar for your observations and comments!

Regards
Subramanian
[quote="rni"]I checked up from Chennai about records of ground motion. A broad band instrument (placed on rock) has responded near Chennai. By converting the counts to acceleration it is found the maximum ground acceleration was 0.0069g. This is a very small value to be of any consequence to safety of engineering structures. But I hasten to add human beings can easily feel and panic at as low as 1 cm/s/s that is nearly 0.001g. At Calcutta the ground motion might have been of the same order since the soil there is overlain with deep sediments.
At long distances ground motion will be at a very low frequency nearly equal to the local site frequency. My personal opinion is: if in a building pre-existing weaknesses respond to such a frequency there can be some visible distress. Probably the building in Kolkata had a long hidden crack or separation which opened out due to repeated cycling at a frequency conducive to itself.

Members perhaps recollect after the 2001 Bhuj earthquake, there was a minor shock felt in Bangalore on 28th January. I investigated this event in the field. Many peculiar behaviour of structures was observed. There was no major structural damage anywhere in Bangalore. But a long comopund wall in a factory collapsed in part. Some directional effect of ground motion could be sensed since the ground below showed a long crack which closed after a couple of days.

I had two interesting visits along with an insurance agency which liked to settle two claims.

The first was a m/c tool factory with a tall wall separating two halls. Large parts of the plaster of the wall fell down on the machines below stopping production for a couple of days. The risk claim was settled satisfactorily since the event was triggered by an act-of-God!  My personal feeling was the plaster layer had slightly separated from the structure and was holding due to some membrane type action. Perhaps it would have stayed in such a static position for several years if there were to be no dynamics coming into play. But on the fateful day the wall vibrated at a low frequency helping the plaster layer to give away! The high frequency machinery running on the floor for more than one year could not cause this action.

The second was in a modern Biology Lab where a sophisticated culturing facility showed severe signs of dampness indicating failure of water proofing in the roof. The accused was the earthquake as per the engineer-in-charge. But our inspection showed the roof top generator could have easily caused the roof cracks which were quite old and also showed a spatial pattern going towards the generator. These roof cracks were definitely not due to the earthquake!

RN Iyengar


On Sat, Apr 14, 2012 at 8:51 AM, prof.arc <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
  Even if there are no strong motion accelerographs recordings in Kolkata [I doubt any SMA would have triggered as the motion would have been below the trigger level set], any plethora of empirical formulae relating Magnitude, distance from epicentre would give very low levels of acceleration.

Any structure in Kolkata, even assuming it has not been specifically designed for ground motion must have withstood such insignificant
ground motion. The ground motion intensity [say for predicting isoseismals] is to be never based on one stray incident.

i may be uncharitable with the following remark - please excuse me
we have noted that during site survey after a real damaging event, some house owners tried to say the cracks in the building are due to that event when it could be established that they pre-existed. in fact a number of owners would want the surveyors like us to state that the damages are due to the event in order to get more compensation

kolkata had been classified to be in earthquake zone [though moderate only] and any structure showing damage due to the recent indonesian event must have structural deficiencies unrelated to ground shaking due to that event

ARC

On Fri, Apr 13, 2012 at 11:37 AM, Vishal forum@sefindia.org (forum@sefindia.org))> wrote:
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