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DESIGNING RC BUILDINGS-GF+3

 
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thirumalaichettiar
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PostPosted: Fri Jan 23, 2009 12:14 pm    Post subject: DESIGNING RC BUILDINGS-GF+3 Reply with quote

Hi Sefians
There are lots of discussions going on about the IS codal provisions of the load combinations. I have a doubt that is it necessaary to design a RC buildings with GF+3 floors for the Seismic forces since the combinations as per IS code involves  81 combinations and taking the worst BM and SF for all beams and columns are so tedious and very time consuming.

Is it not good enough to analysis the building for the combination of 1.5(DL+LL) and 1.2(DL+LL+WL) but detailing shall be done as per the IS 13932.
Hope that this will not be a problem for Zone II and Zone III. It becomes good enough even if the structure is symmetry is plan and elevation.
I need the experts like Dr.S.K.Jain and Dr.N.S opinion.
I know that it is a crude method.

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skjain.iitk
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PostPosted: Fri Jan 23, 2009 2:48 pm    Post subject: DESIGNING RC BUILDINGS-GF+3 Reply with quote

Dear Mr Rangarajan:

Basically, if any load case governs reinforcement at some locations in some members, that load case cannot be neglected. Many years ago, I tried a very simple one storey building in seismic zone I (in those days, zone I existed) and found to my surprise that I must not neglect the seismic loads or some of the load combinations !

I suggest that every structure should be checked for all the load combinations regardless of the seismic zone. For instance, in a large number of situations 1.5(DL+EQ) and 0.9DL+1.5EQ may come out more critical than 1.2(DL+LL+EQ).

Best regards,

Sudhir Jain


On 1/23/09, thirumalaichettiar <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
  Hi Sefians
There are lots of discussions going on about the IS codal provisions of the load combinations. I have a doubt that is it necessaary to design a RC buildings with GF+3 floors for the Seismic forces since the combinations as per IS code involves 81 combinations and taking the worst BM and SF for all beams and columns are so tedious and very time consuming.

Is it not good enough to analysis the building for the combination of 1.5(DL+LL) and 1.2(DL+LL+WL) but detailing shall be done as per the IS 13932.
Hope that this will not be a problem for Zone II and Zone III. It becomes good enough even if the structure is symmetry is plan and elevation.
I need the experts like Dr.S.K.Jain and Dr.N.S opinion.
I know that it is a crude method.



T.Rangarajan,B.E,M.Sc(Struc.Engg),FIE,CE,MACI.
Consulting Structural engineer,
Coimbatore,
E-mial ID: thirumalaichettiar@yahoo.com (thirumalaichettiar@yahoo.com)
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thirumalaichettiar
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PostPosted: Fri Jan 23, 2009 5:56 pm    Post subject: Re: DESIGNING RC BUILDINGS-GF+3 Reply with quote

skjain.iitk wrote:
Dear Mr Rangarajan:

Basically, if any load case governs reinforcement at some locations in some members, that load case cannot be neglected. Many years ago, I tried a very simple one storey building in seismic zone I (in those days, zone I existed) and found to my surprise that I must not neglect the seismic loads or some of the load combinations !

I suggest that every structure should be checked for all the load combinations regardless of the seismic zone. For instance, in a large number of situations 1.5(DL+EQ) and 0.9DL+1.5EQ may come out more critical than 1.2(DL+LL+EQ).

Best regards,

Sudhir Jain


On 1/23/09, thirumalaichettiar <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
  Hi Sefians
There are lots of discussions going on about the IS codal provisions of the load combinations. I have a doubt that is it necessaary to design a RC buildings with GF+3 floors for the Seismic forces since the combinations as per IS code involves 81 combinations and taking the worst BM and SF for all beams and columns are so tedious and very time consuming.

Is it not good enough to analysis the building for the combination of 1.5(DL+LL) and 1.2(DL+LL+WL) but detailing shall be done as per the IS 13932.
Hope that this will not be a problem for Zone II and Zone III. It becomes good enough even if the structure is symmetry is plan and elevation.
I need the experts like Dr.S.K.Jain and Dr.N.S opinion.
I know that it is a crude method.



T.Rangarajan,B.E,M.Sc(Struc.Engg),FIE,CE,MACI.
Consulting Structural engineer,
Coimbatore,
E-mial ID: thirumalaichettiar@yahoo.com (thirumalaichettiar@yahoo.com)
ALTERNATE E-MAIL ID
thirumalaichettiar@gmail.com (thirumalaichettiar@gmail.com)
Download My Tips : http://www.sefindia.org/?q=node/192







Dear Dr.Jain Sir,
Thanks for your kind response. It is true that most of the time when I check for the Seismic combination the critical load case comes as you said for 1.5(DL+LL) or .9DL+1.5EQ but in some software where it is difficult to input the datas fot the Seismic effect like entering the Joint weight, member weight etc so that the program automatically generates the EQ forces. In that case it is difficult to verify the input and out put.
So I am thinking of that idea if we follow the said simple means by  detailing w.r.t IS 13920 for small buildings will ease to some extent to save time and solve quickly. So I put this idea to get some experts like you to comment.
Thanks for your valuable guide once aagain.

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ykalamkar
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PostPosted: Sat Jan 24, 2009 8:06 am    Post subject: DESIGNING RC BUILDINGS-GF+3 Reply with quote

Sir,
To reduce the no of load combinations, can we neglect the wind load if.. Seismic load is greater than wind load? The distribution of wind load is nearly uniform against height where as seismic forces are high at upper level which will result in higher bending moment. Has any one come across such situation when wind forces are less than seismic but critical??
If we can do so, then no of load combination to be reduced to 13 only. This would not be very time consuming and difficult by using any software like STAAD or ETABS etc.  
Regards
Yogesh

From: skjain.iitk [mailto:forum@sefindia.org]
Sent: Friday, January 23, 2009 8:37 PM
To: general@sefindia.org
Subject: [SEFI] Re: DESIGNING RC BUILDINGS-GF+3



Dear Mr Rangarajan:

Basically, if any load case governs reinforcement at some locations in some members, that load case cannot be neglected. Many years ago, I tried a very simple one storey building in seismic zone I (in those days, zone I existed) and found to my surprise that I must not neglect the seismic loads or some of the load combinations !

I suggest that every structure should be checked for all the load combinations regardless of the seismic zone. For instance, in a large number of situations 1.5(DL+EQ) and 0.9DL+1.5EQ may come out more critical than 1.2(DL+LL+EQ).

Best regards,

Sudhir Jain


On 1/23/09, thirumalaichettiar forum@sefindia.org (forum@sefindia.org))> wrote:      
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uhvaryani
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PostPosted: Sat Jan 24, 2009 11:48 am    Post subject: Designing RC buildings-GF+3 Reply with quote

dear sefians,
this has reference to the advocacy of using load combinations
1.5(DL+EQ)
0.9DL+1.5EQ
for design of buildings.
SP:24 states that the first relation does not apply to buildings.
Rather it applies to steel transmission towers where wind
may be considered as primary load.
The second relation is to be applied to check overturning.
The correct form of this relation is
1.2(0.9 DL+ EQ )
Table 18 of IS:456-2000 is defective and needs to be corrected.
thanks and regards
uhvaryani

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thirumalaichettiar
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PostPosted: Sat Jan 24, 2009 11:48 am    Post subject: Re: DESIGNING RC BUILDINGS-GF+3 Reply with quote

ykalamkar wrote:
Sir,
To reduce the no of load combinations, can we neglect the wind load if.. Seismic load is greater than wind load? The distribution of wind load is nearly uniform against height where as seismic forces are high at upper level which will result in higher bending moment. Has any one come across such situation when wind forces are less than seismic but critical??
If we can do so, then no of load combination to be reduced to 13 only. This would not be very time consuming and difficult by using any software like STAAD or ETABS etc.  
Regards
Yogesh

From: skjain.iitk [mailto:forum@sefindia.org]
Sent: Friday, January 23, 2009 8:37 PM
To: general@sefindia.org
Subject: [SEFI] Re: DESIGNING RC BUILDINGS-GF+3



Dear Mr Rangarajan:

Basically, if any load case governs reinforcement at some locations in some members, that load case cannot be neglected. Many years ago, I tried a very simple one storey building in seismic zone I (in those days, zone I existed) and found to my surprise that I must not neglect the seismic loads or some of the load combinations !

I suggest that every structure should be checked for all the load combinations regardless of the seismic zone. For instance, in a large number of situations 1.5(DL+EQ) and 0.9DL+1.5EQ may come out more critical than 1.2(DL+LL+EQ).

Best regards,

Sudhir Jain


On 1/23/09, thirumalaichettiar forum@sefindia.org (forum@sefindia.org))> wrote:      
--auto removed--

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Mr.Y.Kalamkar,
In many cases while analysing the structure with EQ frces  in most cases as Dr.Jain informed the critical case comes with 1.5(DL+EQ) or 0.9(DL)+1.5EQ will be the critical load combinations. Having  a 13 combination for a symmetrical building but for non symmetrical the combinations may go from 81 down to 27 and avoiding the  load cases of WL is not a good idea against the code as the building is growing taller and taller the wind load dominates.
My idea is to make a simple solution for smaller buildings since after analysing picking up for the worst case for each and every member is time consuming and also we do not know how the inputs are correct and how much degree  the output of the software are true is a mystry since we trust the software. Hence I need a simple and straight forward correct method for building with G+3.

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T.Rangarajan,B.E,M.Sc(Struc.Engg),FIE,CE,MACI.Consulting Structural engineer,
Coimbatore,
Download My Tips : http://www.sefindia.org/rangarajan
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skjain.iitk
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PostPosted: Sat Jan 24, 2009 12:57 pm    Post subject: DESIGNING RC BUILDINGS-GF+3 Reply with quote

Dear Yogesh:

Indeed, if for a building you find that overall wind load in a direction is less than the overall seismic load in that direction, you need not analyze for the wind loads for that direction.

Best regards,

Sudhir Jain


On 1/24/09, ykalamkar <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
  Sir,
To reduce the no of load combinations, can we neglect the wind load if.. Seismic load is greater than wind load? The distribution of wind load is nearly uniform against height where as seismic forces are high at upper level which will result in higher bending moment. Has any one come across such situation when wind forces are less than seismic but critical??
If we can do so, then no of load combination to be reduced to 13 only. This would not be very time consuming and difficult by using any software like STAAD or ETABS etc.
Regards
Yogesh

From: skjain.iitk [mailto:forum@sefindia.org (forum@sefindia.org)]
Sent: Friday, January 23, 2009 8:37 PM
To: general@sefindia.org (general@sefindia.org)
Subject: [SEFI] Re: DESIGNING RC BUILDINGS-GF+3



Dear Mr Rangarajan:

Basically, if any load case governs reinforcement at some locations in some members, that load case cannot be neglected. Many years ago, I tried a very simple one storey building in seismic zone I (in those days, zone I existed) and found to my surprise that I must not neglect the seismic loads or some of the load combinations !

I suggest that every structure should be checked for all the load combinations regardless of the seismic zone. For instance, in a large number of situations 1.5(DL+EQ) and 0.9DL+1.5EQ may come out more critical than 1.2(DL+LL+EQ).

Best regards,

Sudhir Jain


On 1/23/09, thirumalaichettiar forum@sefindia.org (forum@sefindia.org) (forum@sefindia.org (forum@sefindia.org)))> wrote:
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ibarua
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PostPosted: Mon Jan 26, 2009 5:46 am    Post subject: DESIGNING RC BUILDINGS-GF+3 Reply with quote

26/1/2009

What I know for a fact is that in seismic zone V, seismic and not wind loads govern design of RCC framed buildings. This may be true for such buildings in zone IV also.

Indrajit Barua.


On Sat, 24 Jan 2009 ykalamkar wrote :
Quote:
Sir,
To reduce the no of load combinations, can we neglect
the wind load if.. Seismic load is greater than wind
load? The distribution of wind load is nearly uniform
against height where as seismic forces are high at
upper level which will result in higher bending moment.
Has any one come across such situation when wind forces
are less than seismic but critical??
If we can do so, then no of load combination to be
reduced to 13 only. This would not be very time
consuming and difficult by using any software like
STAAD or ETABS etc.  
Regards
Yogesh

From: skjain.iitk [mailto:forum@sefindia.org]
Sent: Friday, January 23, 2009 8:37 PM
To: general@sefindia.org
Subject: [SEFI] Re: DESIGNING RC BUILDINGS-GF+3



Dear Mr Rangarajan:

Basically, if any load case governs reinforcement at
some locations in some members, that load case cannot
be neglected. Many years ago, I tried a very simple one
storey building in seismic zone I (in those days, zone
I existed) and found to my surprise that I must not
neglect the seismic loads or some of the load
combinations !

I suggest that every structure should be checked for
all the load combinations regardless of the seismic
zone. For instance, in a large number of situations
1.5(DL+EQ) and 0.9DL+1.5EQ may come out more critical
than 1.2(DL+LL+EQ).

Best regards,

Sudhir Jain


On 1/23/09, thirumalaichettiar forum@sefindia.org
(forum@sefindia.org))> wrote:      
--auto removed--








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vikram.jeet
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PostPosted: Tue Jan 27, 2009 8:05 am    Post subject: DESIGNING RC BUILDINGS-GF+3 Reply with quote

I agree with Sh UH Varyani that case of 1.5(DL+EQ) is not  
relevant to buildings but to structures like chimneys/transmission
towers where EQ/Wind are the predominant loads and live loads
may not be very much relevant to such structures.SP-24 explains  
about it in its relevant clause
Logic also defies such combination for a BUILDING STRUCTURE
wherein Live load is not present ie A building w/o LL have comparatively
less importance than building accomodating LL during EQ and hence
load factor for such situation (No LL case ) cannot be more than 1.2

As regards case of 0.9DL +1.5EQ(or WL),IS 456-2000 footnote clearly state
'This value is to be considered when stability against Overturning or  
stress reversal is critical

regards

vikramjeet

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