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CHECKING OF THE STRUCTURE AGAINST OVERTURNING
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suresh_sharma
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PostPosted: Tue Sep 11, 2012 1:56 am    Post subject: CHECKING OF THE STRUCTURE AGAINST OVERTURNING Reply with quote

Checking of structure against overturning for earthquake loading and wind loading is undertaken. My question is that if the story drift due to EQ  and drift due to wind are within the permissible limit, then also whether it is desirable to undertake checking against overturning. Checking against overturning is a rigorous exercise fraught with the risk of committing omissions. THe software does not check the stability of the structure against overturning. Please advise some easy approach.
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rajancivildesign@gmail.co
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PostPosted: Wed Sep 12, 2012 10:16 am    Post subject: Check for overturning Reply with quote

In general for small buildings having h/b< 5.0 and natural frequency less than 1.0 , overturning will not  govern. For such building check for storey drift  for earthquake and wind are enough. Expert can  offer their views  for better clarity in this issue please.
V.M.RAJAN.
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Dr. N. Subramanian
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PostPosted: Wed Sep 12, 2012 1:44 pm    Post subject: Re: Check for overturning Reply with quote

Dear Er Rajan,

I agree with you on this.

But regarding Er Suresh Sharma question, I wish to state that if h/b is > 5.0, it may be necessary to check overturning, even  if the story drift due to EQ  and drift due to wind are within the permissible limit. However, in general such buildings will be supported by pile foundation and hence may not overturn.

Regards
NS
rajancivildesign wrote:
In general for small buildings having h/b< 5.0 and natural frequency less than 1.0 , overturning will not  govern. For such building check for storey drift  for earthquake and wind are enough. Expert can  offer their views  for better clarity in this issue please.
V.M.RAJAN.
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suresh_sharma
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PostPosted: Wed Sep 12, 2012 4:30 pm    Post subject: Reply with quote

Dear Dr. NS,

I vaguely remember to have read in some book that safety against overturning should be checked at each floor. If it is so then the matter is further further complicated. When I am asked to check, I check only at the foundation level. Please advise whether checking at founding level alone is not a foolproof checking?
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Dr. N. Subramanian
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PostPosted: Wed Sep 12, 2012 5:30 pm    Post subject: Reply with quote

Dear Er Suresh Sharma,

Overturning check is always done at the foundation level, that is overturning of the whole structure.

Best wishes,
NS
suresh_sharma wrote:
Dear Dr. NS,

I vaguely remember to have read in some book that safety against overturning should be checked at each floor. If it is so then the matter is further further complicated. When I am asked to check, I check only at the foundation level. Please advise whether checking at founding level alone is not a foolproof checking?
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vikram.jeet
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PostPosted: Sat Sep 22, 2012 5:24 am    Post subject: CHECKING OF THE STRUCTURE AGAINST OVERTURNING Reply with quote

Check for stability of Tall structure against overturning is must against Wind/EQ,,

In EQ zones , I feel this check need to be made considering absolute  
lateral forces based on MCE. Ductility provisions can protect the collapse
of structure ,but not overturning of structure under maximum anticipated EQ.

Generally pile foundations/ Raft sizing is such that such situation is not alarming.

Storey drift within permissible limits keep the second order effects in safer zone.

best regds

vikramjeet
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suresh_sharma
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PostPosted: Sat Sep 22, 2012 4:47 pm    Post subject: Reply with quote

Dear Vikram,
You are sure that overturning check need to be done against MCE which will be 10 times the DBE. please examine and advise. Is it not that it is to be checked for DBE unfactored EQ load or Wind load against the dead load i.e. LL is to be ignored for overturning check.
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prof.arc
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PostPosted: Sat Sep 22, 2012 11:10 pm    Post subject: CHECKING OF THE STRUCTURE AGAINST OVERTURNING Reply with quote

I would like to caution here that earthquake motion is dynamic in character and and treating it as a equivalent static force for overturning analysis would be in error. In other words, a smaller static lateral force may cause overturning as compared to a larger reversible dynamic load. We should permit "pounding" - it is done in raft analysis even for NPP's

There is an article [i do not remember the reference now - may be contacting IITR-DEQ may help] of the stability of the M17 block of Koyna Dam (at the level where small cracking was noticed during the event)


ARC

On Sat, Sep 22, 2012 at 2:56 PM, vikram.jeet <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Check for stability of Tall structure against overturning is must against Wind/EQ,,

In EQ zones , I feel this check need to be made considering absolute
lateral forces based on MCE. Ductility provisions can protect the collapse
of structure ,but not overturning of structure under maximum anticipated EQ.

Generally pile foundations/ Raft sizing is such that such situation is not alarming.

Storey drift within permissible limits keep the second order effects in safer zone.

best regds

vikramjeet
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suresh_sharma
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PostPosted: Sun Sep 23, 2012 2:56 pm    Post subject: Reply with quote

Prof. ARC,

I would request to provide some more information on the subject and/or reference.
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vikram.jeet
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PostPosted: Mon Sep 24, 2012 7:11 am    Post subject: CHECKING OF THE STRUCTURE AGAINST OVERTURNING Reply with quote

Dear Er Suresh ji

OT Check under MCE is not spelt out in code but the observation was based on
personal thinking.

Prof ARC sir's posting also indicate treating dynamic motion in form of equivalent static forces
for OT analysis would be on erroneous side.

Though equivalent static force under MCE will be(=2*5/3=3.33) i .e 3.33 times considering R
value for building structure OMRF=3.0 , still whether structure has chances of toppling or not  
can bother the designer of high rise development. Does Past EQ records indicate any overturning
failure of high rise , is to be seen.

Normally high rise buildings with pile foundations do have very high uplift resistance in addition
to the Dead weight of structure and probably OT check ,even considering , higher  
static forces may be safe. Medium rise Buildings with raft foundations also have adequate
raft sizing and Dead weight may generally be sufficient . However for high building with small
dimension on width side vis a vis height may need such check , in case raft size is also restricted.

Chimneys , OH tanks , are also high structures but these have adequate raft/ piling which  
provide high FOS against lateral forces under EQ ( be it even MCE, I think).

best regds

vikramjeet



Dear Vikram, You are sure that overturning check need to be done against MCE which will be 10 times the DBE. please examine and advise. Is it not that it is to be checked for DBE unfactored EQ load or Wind load against the dead load i.e. LL is to be ignored for overturning check.
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