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LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load
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vikram.jeet
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PostPosted: Fri Jan 23, 2009 10:16 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

To : Prof ARC, Madam Sangeeta, Mr Jiwaji Desai &Mr Arun

With due respect to all , I agree that clause 7.3.3 of IS 1893 definately allows  
designers to use uniform LL % in Load combination but discussion is only
based on logic and aimed at structural safety aspect

Taking arbitrary example of frames in two different buildings

Building-I(High Rise)
DL BM---------------------16tm
100%LLBM -------------- 16tm
EQ BM(DL+50%LL)----50tm

factored moments in different combinations are:
1.5(DL+LL)---------------48tm
1.2(DL+100%LL+EQ)--98.4tm
1.2(DL+50%LL+EQ)----88.8tm



Building-II(Low height)
DL BM---------------------16tm
100%LLBM -------------- 16tm
EQ BM(DL+50%LL)----10tm

factored moments in different combinations are:
1.5(DL+LL)---------------48tm
1.2(DL+100%LL+EQ)--50.4tm
1.2(DL+50%LL+EQ)----40.8tm

the increase of actual factored moment in a particular floor 100% loaded during
during EQ is more in high rise and less in low height

regards to all  

vikramjeet

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ishacon
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PostPosted: Sat Jan 24, 2009 1:44 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

As a follow up of my previous mail, the following corrections to
Mr. Vikramjeet's mail make an interesting
comparison for design of beams with 100% LL combination. !!

I certainly hope that no one thinks that I am advocating
design of columns on this basis !!!
Would it not be prudent for those who have access to latest IBC,
give relevant clause applicable for such situation. To the best of my
knowledge, UBC 1997 & IBC 2003 were advocating use of Live load %
as appropriate for the particular situation.
Thanks to all for participating in this discussion.

VP AGARWAL

ISHA CONSULTANTS (P) LTD
NEW DELHI

PH : 011- 2630 1158
(M) 93 1345 2180
(M) 98 6826 2759
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----- Original Message ----- From: vikram.jeet (forum@sefindia.org)
To: general@sefindia.org (general@sefindia.org)
Sent: Friday, January 23, 2009 4:53 PM
Subject: [SEFI] Re: LOAD COMBINATION


To : Prof ARC, Madam Sangeeta, Mr Jiwaji Desai & Mr Arun

With due respect to all , I agree that clause 7.3.3 of IS 1893 definately allows
designers to use uniform LL % in Load combination but discussion is only
based on logic and aimed at structural safety aspect

Taking arbitrary example of two different buildings for floor beam design :

Building-I(High Rise)
DL BM---------------------16tm
100%LLBM -------------- 16tm
EQ BM(DL+50%LL)----50tm

factored moments in different combinations are:
1.5(DL+LL)---------------48tm
1.2(DL+100%LL+EQ)--98.4tm
1.2(DL+50%LL+EQ)----88.8tm
1.5(DL+EQ) --------99 t-m ------Governs !!!


Building-II(Low height)

DL BM---------------------16tm
100%LLBM -------------- 16tm
EQ BM(DL+50%LL)----10tm

factored moments in different combinations are:
1.5(DL+LL)---------------48tm
1.2(DL+100%LL+EQ)--50.4tm --------Governs !!!
1.2(DL+50%LL+EQ)----40.8tm
1.5(DL+EQ) --------39 t-m

the increase of actual factored moment in a particular floor 100% loaded during
during EQ is more in high rise and less in low height .

regards to all

vikramjeet

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ishacon
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PostPosted: Sat Jan 24, 2009 5:10 am    Post subject: LOAD COMBINATIONS Reply with quote

Dear Dr. Jain and all other members,

I am not trying to rake up the issue of Load Combinations again,
but the Load Combinations as set in IS 456
and IS 1893 bothered me again and I investigated the provisions of
ACI 318-05 & of IBC 2003.

A close look at these will show how conservative we in India are
in our load combinations, specially for Live load and earthquake load inclusions.
Our IS 1893 provisions of Importance factor also need to be relooked at to be in line with
International / Euro Codes

Is it not the right time to force this issue on to the respective committees
of IS 456 & IS 1893, after taking consensus from various renowned Professors & experts,
who are also members of these committees.

Thanks + regards,

V.P. Agarwal

ISHA CONSULTANTS (P) LTD
NEW DELHI

PH : 011- 2630 1158
(M) 93 1345 2180
(M) 98 6826 2759
ishacon@bol.net.in (ishacon@bol.net.in)
ishacon@rediffmail.com (ishacon@rediffmail.com)

Quote:
----- Original Message -----
From: jiwajidesai (forum@sefindia.org)
To: general@sefindia.org (general@sefindia.org)
Sent: Friday, January 23, 2009 1:19 PM
Subject: [SEFI] Re: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load


Dear Vikram Jeet

As I said earlier I agree that some members will be subjected to 100% gravity loads, but isnít this condition taken care by the normal gravity load combination, when no EQ is involved? That is, if the gravity combination BM / shear is higher it will be designed for. Besides since majority of floors will be subjected to less than design LL, some none at all, in EQ condition, the base shear will be on the conservative side.
If we give 100% gravity load with EQ, we have to live with highly conservative design for all the frame members for full gravity loads under EQ condition. Besides if we know that some floors have a likelihood of high gravity loads, we can create a separate EQ combination with full LL for these members, instead of penalizing the whole frame.

Regards



Jiwaji Desai




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skjain.iitk
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PostPosted: Sat Jan 24, 2009 6:13 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

Dear Mr Agarwal:

Thank you for raising a very important issue. I think we need to see safety in the overall context and not in isolation of an individual codal clause.

Are the buildings (or bridges) being designed as per Indian codes for a given town more conservative (or less conservative), as compared to similar structures if designed for towns of similar seismic hazard as per US / European / New Zealand / Japan codes? We need to adjust our codes (up or down), depending on answer to this question. The adjustment could be made at various levels: in terms of specifying Z value, I value, R value, load combinations, partial safety factor on materials, detailing rules, etc.

Some years back, a very interesting international study was conducted wherein four bridge piers were considered with a fixed weight of superstructure: 7m and 30m height, for PGA of 0.4g and 0.8g. Acknowledged experts designed these four RC piers as per codes of different countries and compared the pier sizes and the reinforcement (longitudinal and transverse) obtained.

It will be good to do a somewhat similar study in India to caliberate our code with international practice. We could assume a simple building situated in Chandigarh. One design office could volunteer to carry out complete design and detailing as per IS codes, while the other design office could do this as per US practices (ASCE7 + IBC + ACI). A third design office could review the design and calculations of both to ensure that the are no major inconsistencies in the design process. Thereafter, we could compare the response of the two structures by a push over analysis and compare the bill of quantities / cost. Any volunteers?

Indian structural engineering profession needs to take more interest in these issues and invest time, money and resources on issues of professional interest. Waiting for others (government, academic institutes, BIS committees, ...) to act will take us nowhere. Hard data based on thorough professional work is the best way to move forward.

Thank you and regards,

Sudhir Jain


On 1/24/09, ishacon <forum@sefindia.org (forum@sefindia.org)> wrote: [quote]  Dear Dr. Jain and all other members,

I am not trying to rake up the issue of Load Combinations again,
but the Load Combinations as set in IS 456
and IS 1893 bothered me again and I investigated the provisions of
ACI 318-05 & of IBC 2003.

A close look at these will show how conservative we in India are
in our load combinations, specially for Live load and earthquake load inclusions.
Our IS 1893 provisions of Importance factor also need to be relooked at to be in line with
International / Euro Codes

Is it not the right time to force this issue on to the respective committees
of IS 456 & IS 1893, after taking consensus from various renowned Professors & experts,
who are also members of these committees.

Thanks + regards,

V.P. Agarwal

ISHA CONSULTANTS (P) LTD
NEW DELHI

PH : 011- 2630 1158
(M) 93 1345 2180
(M) 98 6826 2759
ishacon@bol.net.in (ishacon@bol.net.in) (ishacon@bol.net.in (ishacon@bol.net.in))
ishacon@rediffmail.com (ishacon@rediffmail.com) (ishacon@rediffmail.com (ishacon@rediffmail.com))

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vikram.jeet
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PostPosted: Sat Jan 24, 2009 12:13 pm    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

Dear Mr VP Aggarwal

Thanks for further discussion on the subject.Please kindly elaborate
your views regarding conservative clauses in our codes when compared
to other Countries Codes especially in r/o load combination.wether our
codes are more conservative or it is other way round since ,definately
a designer's endeavour is to be as economical as possible, but for
structural safety

may kindly enlighten us ,if possible

thanks and regards

vikramjeet

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ishacon
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PostPosted: Mon Jan 26, 2009 4:59 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

We had an opportunity to check design of a building for seismic Zone III ,
based on ACI 318 and BS codes, with UBC / IBC provisions and we found
that with load combinations as spelled out in ACI and IBC 2003,
the sections selected for column and beams would be adequate
and the reinforcements would be less than if designed as per IS codes.

Load Combination with earthquake loads, was as under :
1.05DL+1.275LL+1.4025EL
0.9DL+1.43EL ;
V.P. Agawral
ISHA CONSULTANTS (P) LTD
NEW DELHI

PH : 011- 2630 1158
(M) 93 1345 2180
(M) 98 6826 2759
ishacon@bol.net.in (ishacon@bol.net.in)
ishacon@rediffmail.com (ishacon@rediffmail.com)
Quote:
----- Original Message -----
From: vikram.jeet (forum@sefindia.org)
To: general@sefindia.org (general@sefindia.org)
Sent: Saturday, January 24, 2009 7:02 PM
Subject: [SEFI] Re: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load


Dear Mr VP Aggarwal

Thanks for further discussion on the subject.Please kindly elaborate
your views regarding conservative clauses in our codes when compared
to other Countries Codes especially in r/o load combination.wether our
codes are more conservative or it is other way round since ,definately
a designer's endeavour is to be as economical as possible, but for
structural safety

may kindly enlighten us ,if possible

thanks and regards

vikramjeet











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ibarua
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PostPosted: Mon Jan 26, 2009 6:55 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + Reply with quote

26/1/2009

Dear Mr Agarwal,

I suggest we should leave the load combination in the BIS Codes as they are. We should not compare Indian conditions with those in countries like the USA, the UK & elsewhere in Europe, as well as in Japan, given the standards of construction in India. Poor control over w.c ratio, poor placement and compaction, poor curing et al have all to be taken into account.

Comments from Sefians are most welcome.

Regards,
Indrajit Barua.

On Sat, 24 Jan 2009 ishacon wrote :
Quote:
Dear Dr. Jain and all other members,

I am not trying to rake up the issue of Load
Combinations again,
but the Load Combinations as set in IS 456
and IS 1893 bothered me again and I investigated the
provisions of
ACI 318-05 & of IBC 2003.

A close look at these will show how conservative we in
India are
in our load combinations, specially for Live load and
earthquake load inclusions.
Our IS 1893 provisions of Importance factor also need
to be relooked at to be in line with
International / Euro Codes

Is it not the right time to force this issue on to the
respective committees
of IS 456 & IS 1893, after taking consensus from
various renowned Professors & experts,
who are also members of these committees.

Thanks + regards,

V.P. Agarwal

ISHA CONSULTANTS (P) LTD
NEW DELHI

PH : 011- 2630 1158
(M) 93 1345 2180
(M) 98 6826 2759
ishacon@bol.net.in (ishacon@bol.net.in)
ishacon@rediffmail.com (ishacon@rediffmail.com)


Quote:
----- Original Message -----
From: jiwajidesai (forum@sefindia.org)
To: general@sefindia.org (general@sefindia.org)
Sent: Friday, January 23, 2009 1:19 PM
Subject: [SEFI] Re: LOAD COMBINATION : (0.9)Dead Load
+ (1.5)Seismic Load
Quote:


Dear Vikram Jeet

As I said earlier I agree that some members will be
subjected to 100% gravity loads, but isnít this
condition taken care by the normal gravity load
combination, when no EQ is involved? That is, if the
gravity combination BM / shear is higher it will be
designed for. Besides since majority of floors will be
subjected to less than design LL, some none at all, in
EQ condition, the base shear will be on the
conservative side.
Quote:
If we give 100% gravity load with EQ, we have to live
with highly conservative design for all the frame
members for full gravity loads under EQ condition.
Besides if we know that some floors have a likelihood
of high gravity loads, we can create a separate EQ
combination with full LL for these members, instead of
penalizing the whole frame.
Quote:

Regards



Jiwaji Desai








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ibarua
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Joined: 26 Jan 2003
Posts: 1039

PostPosted: Mon Jan 26, 2009 7:02 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + Reply with quote

26/1/2009

Dear Sudhir,

Please take a close look at our standards of construction before thinking about relaxing our codal provisions to compete with those of other countries!

Regards,
Indrajit Barua.


On Sat, 24 Jan 2009 skjain.iitk wrote :
Quote:
Dear Mr Agarwal:

Thank you for raising a very important issue. I think
we need to see safety in the overall context and not in
isolation of an individual codal clause.

Are the buildings (or bridges) being designed as per
Indian codes for a given town more conservative (or
less conservative), as compared to similar structures
if designed for towns of similar seismic hazard as per
US / European / New Zealand / Japan codes? We need to
adjust our codes (up or down), depending on answer to
this question. The adjustment could be made at various
levels: in terms of specifying Z value, I value, R
value, load combinations, partial safety factor on
materials, detailing rules, etc.

Some years back, a very interesting international study
was conducted wherein four bridge piers were considered
with a fixed weight of superstructure: 7m and 30m
height, for PGA of 0.4g and 0.8g. Acknowledged experts
designed these four RC piers as per codes of different
countries and compared the pier sizes and the
reinforcement (longitudinal and transverse) obtained.

It will be good to do a somewhat similar study in India
to caliberate our code with international practice. We
could assume a simple building situated in Chandigarh.
One design office could volunteer to carry out complete
design and detailing as per IS codes, while the other
design office could do this as per US practices (ASCE7
+ IBC + ACI). A third design office could review the
design and calculations of both to ensure that the are
no major inconsistencies in the design process.
Thereafter, we could compare the response of the two
structures by a push over analysis and compare the bill
of quantities / cost. Any volunteers?

Indian structural engineering profession needs to take
more interest in these issues and invest time, money
and resources on issues of professional interest.
Waiting for others (government, academic institutes,
BIS committees, ...) to act will take us nowhere. Hard
data based on thorough professional work is the best
way to move forward.

Thank you and regards,

Sudhir Jain


On 1/24/09, ishacon <forum@sefindia.org
(forum@sefindia.org)> wrote:
Quote:
  Dear Dr. Jain and
all other members,

I am not trying to rake up the issue of Load
Combinations again,
but the Load Combinations as set in IS 456
and IS 1893 bothered me again and I investigated the
provisions of
ACI 318-05 & of IBC 2003.

A close look at these will show how conservative we in
India are
in our load combinations, specially for Live load and
earthquake load inclusions.
Our IS 1893 provisions of Importance factor also need
to be relooked at to be in line with
International / Euro Codes

Is it not the right time to force this issue on to the
respective committees
of IS 456 & IS 1893, after taking consensus from
various renowned Professors & experts,
who are also members of these committees.

Thanks + regards,

V.P. Agarwal

ISHA CONSULTANTS (P) LTD
NEW DELHI

PH : 011- 2630 1158
(M) 93 1345 2180
(M) 98 6826 2759
ishacon@bol.net.in (ishacon@bol.net.in)
(ishacon@bol.net.in (ishacon@bol.net.in))
ishacon@rediffmail.com (ishacon@rediffmail.com)
(ishacon@rediffmail.com (ishacon@rediffmail.com))

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Dr. N. Subramanian
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PostPosted: Mon Jan 26, 2009 6:24 pm    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

Dear Mr. Indrajit Barua,

I totally agree with you. In this connection I just want to mention about the construction loads, imposed on slabs, which are not considered in many designs. I am also not sure they are properly accounted for in construction.

I am quoting a few lines from the article by Prof. W.F.Chen and associates, published in Concrete International, ACI(June 1988, pp.21-30) "Many investigations from USA, Soviet Union and Japan have shown that a significant portion (about 70% in Soviet Union) of disasters involving RC buildings is due to construction problems such as excessive loads imposed into the supporting system, which include shores, reshores, and supporting slab. The problem is troublesome in high-rise RC buildings in which the live load is small compared to dead load. Moreover, another primary cause is the premature removal of shores or reshores"

During construction, the dead load of the higher floor+ load of props and shuttering is transmitted to the slab below, for which the shores or props might have been removed after 7 days. Thus this slab which has a lower strength (7-day strength as against the 28-day strength, as per design) is overloaded. Hence it is necessary to prop at least two to three floors below the floor that is being concreted. Prof. Chen describes a method to evaluate the safety of the slab. ASCE has issued a code for considering the construction load(ASCE 37-2002). I doubt whether such a code is available in India.

Regards
Subramanian

Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Consulting Structural Engineer
Maryland, USA

See my books at: www.multi-science.co.uk/subramanian-book.htm
www.oup.co.in/search_detail.php?id=144559





--- On Mon, 1/26/09, ibarua <forum@sefindia.org> wrote:
[quote]From: ibarua <forum@sefindia.org>
Subject: [SEFI] Re: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load
To: general@sefindia.org
Date: Monday, January 26, 2009, 1:02 PM

26/1/2009

Dear Mr Agarwal,

I suggest we should leave the load combination in the BIS Codes as they are. We should not compare Indian conditions with those in countries like the USA, the UK & elsewhere in Europe, as well as in Japan, given the standards of construction in India. Poor control over w.c ratio, poor placement and compaction, poor curing et al have all to be taken into account.

Comments from Sefians are most welcome.

Regards,
Indrajit Barua.

On Sat, 24 Jan 2009 ishacon wrote :
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PostPosted: Tue Jan 27, 2009 4:35 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

Dear Mr. Barua,

While agreeing with you about deficiencies in construction practices
here in India, that should be no excuse to have unrealistic load combinations.

Let us face the reality that the uncouth unprincipled contractors will do
whatever they can to make extra profit, but failures do not occur on account
of higher factor for earthquake loads or live loads !!

Certainly Govt departments have more to be concerned about what goes on
with the nexus of unholy trinities, but why project this to penalise others ?

VP Agarwal

ISHA CONSULTANTS (P) LTD
NEW DELHI

PH : 011- 2630 1158
(M) 93 1345 2180
(M) 98 6826 2759
ishacon@bol.net.in (ishacon@bol.net.in)

ishacon@rediffmail.com (ishacon@rediffmail.com)
[quote] ----- Original Message -----
From: ibarua (forum@sefindia.org)
To: general@sefindia.org (general@sefindia.org)
Sent: Monday, January 26, 2009 1:02 PM
Subject: [SEFI] Re: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load


26/1/2009

Dear Mr Agarwal,

I suggest we should leave the load combination in the BIS Codes as they are. We should not compare Indian conditions with those in countries like the USA, the UK & elsewhere in Europe, as well as in Japan, given the standards of construction in India. Poor control over w.c ratio, poor placement and compaction, poor curing et al have all to be taken into account.

Comments from Sefians are most welcome.

Regards,
Indrajit Barua.

On Sat, 24 Jan 2009 ishacon wrote :
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