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LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load
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skjain.iitk
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PostPosted: Thu Jan 22, 2009 7:15 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

Dear Sri Agarwal:

You make an interesting point and I entirely agree with you.

I had discussed this issue in the
e-Course on IS1893 that I conducted in January 2003. HOwever, I will mention it here to complete the discussion. CD containing the e-Course can

be obtained from NICEE by visiting http://www.nicee.org/Publications.php.

Clause 7.3.3 of IS1893 actually allows one to use only 25% or 50% (as the case may be) of
live load when considering the load combination 1.2 (DL+LL+EQ). However, I do
not agree with this provision and have proposed that this clause be dropped from the next revision
of the code. The version with the BIS for next revision of IS1893 does not contain this clause but one cannot
say when this will happen.

Thank you for bringing it up. Best regards,

Sudhir Jain


On 1/22/09, ishacon <forum@sefindia.org (forum@sefindia.org)> wrote: [quote]  Thanks Dr. Jain, for the detailed explanation
which should put a lot of minds to rest.

Unfortunately in several design cases we find that for 1.2(DL+EQL+LL)
case also, there are misconceptios. I see no merit in using only 25%
of LL in above formulation as for design of beams on any particular floor
of a multi storeyed housing building tower,
the full live load should be applicable and not 25% as several designers
take as per IS 1893, since only 25% LL is used for seismic weight calculations.

V.P. Agarwal
ISHA CONSULTANTS (P) LTD
NEW DELHI

PH : 011- 2630 1158
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ishacon@bol.net.in (ishacon@bol.net.in) (ishacon@bol.net.in (ishacon@bol.net.in))
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Jalil A. Sheikh
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PostPosted: Thu Jan 22, 2009 8:38 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

Respected Subramnian sir,

Now a days integrated structural design programes are available in market (e.g. STRUDS, BUILDMASTER, PLANMENU, ETC.) in which you have to give wind load data and building dimentions for considering wind effect on building. Programe will not consider wall at periphery is there or not for calculating wind load on particular side of building. By using such programmes how can we rely that programe has considered only effective area (of members of structure in way of wind) is considerd? Please help me in this regard.

JALIL SHEIKH

--- On Wed, 21/1/09, drnsmani <forum@sefindia.org> wrote:

[quote]From: drnsmani <forum@sefindia.org>
Subject: [SEFI] Re: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load
To: general@sefindia.org
Date: Wednesday, 21 January, 2009, 4:04 PM

Dear Mr. Jalil,

From your description of the building being designed by you, I find that there are not many elements on which wind will be impinging; Hence even if you calculate the wind load it may not be high enough to affect your design.

Regards,
NS

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 Wed, 1/21/09, Jalil A. Sheikh wrote:
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vikram.jeet
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PostPosted: Thu Jan 22, 2009 9:39 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

Dear Mr Jiwaji Desai

I do agree with you that designers differ on this issue.

I think , for working out seismic forces , IS 1893 allows 25% or 50% LL,
thus providing reduction for overall mass of LL Present in a building
during EQ

But during EQ, floor beams can experience 100%LL on a particular floor
whereas at floor next to it LL may not be even 10% .Threfore even though
the LL % for working EQ forces is reduced (25% or 50%)keeping in view
the overall mass present at time of EQ as stated above,
the bending moments / shears due to gravity loads could be 100%
on a particular floor and logically reducing the same by 25% or 50%
looks to be very much on unsafer side

Taking an example of a frame beam  

DL BM = 16 tm
100%LL BM =16 tm
EQ BM =10tm(worked based on IS 1893)

Suppose 100% LL is present during EQ on a particular floor

Actual Design BM = (16+16+10) =42 tm  

Design BM as per your contention is =[16+( 50% of 16)+10] =34 tm which is lesser
than 42tm actual BM on beam and even the Load Factor of 1.2 would not
cover it.
So logically a judgement can be made.  

regards

vikramjeet

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sdec.in
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PostPosted: Thu Jan 22, 2009 11:03 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

Dear Dr Sudhir Jain
Do you remember that the interpretation now presented as "wrong" was given to us as right, at one of the discussions in the EQ seminars? and was later seperately confirmed by Prof Arya in a CEAI seminar when the revised 1893 came out.

Pl let us know what is the Final correct interpretation.
Regards
Sangeeta Wij

Quote:
----- Original Message -----
From: vikram.jeet (forum@sefindia.org)
To: general@sefindia.org (general@sefindia.org)
Sent: 22 January, 2009 11:21 AM
Subject: [SEFI] Re: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load


Dear Dr SK Jain & Mr VP Agarwaal

Thanks for the clarrifications on the forum

consensus on this aspect was due because lot many times
the designers differ in interpretation of load combination
to be adopted during EQ conditions and tend to err on
unsafer side

thus for load combination of 1.2(DL+LL)+1.2EQ

The right interpretation is:
1.2(DL +LL as per IS 875with reduction in vertival load for foundations & columns only)
+1.2EQ [computed on(DL+ 25%or 50%LL)]


WRONG Interpretation:
1.2[DL + (25%or 50%)LL] +1.2EQ [computed on(DL+ 25%or 50%LL)]

with regards

vikramjeet







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jiwajidesai
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PostPosted: Thu Jan 22, 2009 11:28 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

Dear Vikram Jeet
I understand your point for a particular floor or beam-group, however what about the frame as a whole? In case of your example, reduction of LL even in EQ case-calculations – how is it logical and safe?

Continuing with your example, it is possible that 100% (or near about) LL is present on one or more floors in a tall building. What is the reason to consider mass due to reduced LL(50 % or 25%) on all floors calculated at 10 tm in your example, for seismic force? Would not the seismic forces be reduced accordingly in that case? May be the increase is small in certain cases, but is it correct and safe to generalize?

In such a case I would think it would be preferred to avoid any reduction in LL at all whether gravity or EQ load calculation.  

However the Code in Clause 7.3.3 clearly stipulates that the same % as used in 7.3.1 be used for EQ load combinations, albeit whole-frame, which we create in our analysis/design. And the Code does not make any exceptions to cater to such cases.
Regards

Jiwaji Desai
Sr. GM (C & S)

WSP Engineering Services Ltd.



From: vikram.jeet [mailto:forum@sefindia.org]
Sent: Thursday, January 22, 2009 3:56 PM
To: general@sefindia.org
Subject: [SEFI] Re: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load


Dear Mr Jiwaji Desai

I do agree with you that designers differ on this issue.

I think , for working out seismic forces , IS 1893 allows 25% or 50% LL,
thus providing reduction for overall mass of LL Present in a building
during EQ

But during EQ, floor beams can experience 100%LL on a particular floor
whereas at floor next to it LL may not be even 10% .Threfore even though
the LL % for working EQ forces is reduced (25% or 50%)keeping in view
the overall mass present at time of EQ as stated above,
the bending moments / shears due to gravity loads could be 100%
on a particular floor and logically reducing the same by 25% or 50%
looks to be very much on unsafer side

Taking an example of a frame beam

DL BM = 16 tm
100%LL BM =16 tm
EQ BM =10tm(worked based on IS 1893)

Suppose 100% LL is present during EQ on a particular floor

Actual Design BM = (16+16+10) =42 tm

Design BM as per your contention is =[16+( 50% of 16)+10] =34 tm which is lesser
than 42tm actual BM on beam and even the Load Factor of 1.2 would not
cover it.
So logically a judgement can be made.

regards

vikramjeet

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sakumar79
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PostPosted: Thu Jan 22, 2009 12:57 pm    Post subject: Reply with quote

Dear Mr. Vikram jeet
   While it may be theoretically correct that the full live load on a particular floor may be present, the code provision does not seem to indicate different values of Live Load to be considered in the 1.2 (DL+LL+EL) combination between design for beams and design for columns... It simply states that for the above mentioned combination, it is enough to consider the reduced live load (25% or 50% of floor live loads and no contribution from roof live load) that was used in calculating the seismic weight.

Arun
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Dr. N. Subramanian
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PostPosted: Thu Jan 22, 2009 3:57 pm    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

Dear Mr. Jalil,

It may not be possible for me to give a suggestion based on a program. The best way to find how the program works is to give a small example and check the results. You may even work out this example by hand and compare the results. After you are satisfied, you can feed in the data for the project at hand.

Best wishes
NS

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 Thu, 1/22/09, Jalil A. Sheikh <forum@sefindia.org> wrote:
Quote:
From: Jalil A. Sheikh <forum@sefindia.org>
Subject: [SEFI] Re: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load
To: general@sefindia.org
Date: Thursday, January 22, 2009, 2:27 PM

Respected Subramnian sir,

Now a days integrated structural design programes are available in market (e.g. STRUDS, BUILDMASTER, PLANMENU, ETC.) in which you have to give wind load data and building dimentions for considering wind effect on building. Programe will not consider wall at periphery is there or not for calculating wind load on particular side of building. By using such programmes how can we rely that programe has considered only effective area (of members of structure in way of wind) is considerd? Please help me in this regard.

JALIL SHEIKH

--- On Wed, 21/1/09, drnsmani  wrote:

Quote:
From: drnsmani
Subject: [SEFI] Re: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load
To: general@sefindia.org (general@sefindia.org)
Date: Wednesday, 21 January, 2009, 4:04 PM

Dear Mr. Jalil,

From your description of the building being designed by you, I find that there are not many elements on which wind will be impinging; Hence even if you calculate the wind load it may not be high enough to affect your design.

Regards,
NS

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 Wed, 1/21/09, Jalil A. Sheikh wrote:
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prof.arc
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PostPosted: Fri Jan 23, 2009 12:44 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

For seismic conditions only, 25% live load is more than adequate or alternately in earlier versions of the code the designer was permitted to estimate his own based on misuse of his building such as using residential buildings for godown storage of heavy items.
Please remember that live loads are not generally rigidly attached to the floor but rest due to friction. The effective live load is generally less than actual value in such cases.
Further, the live load likely to be incident at the time of earthquake is far less than that estimated for static conditions.
IIT-R has done analytical and experimental work on shake tables regarding effective live load.
ARC

On Thu, Jan 22, 2009 at 7:56 AM, ishacon <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Thanks Dr. Jain, for the detailed explanation
which should put a lot of minds to rest.

Unfortunately in several design cases we find that for 1.2(DL+EQL+LL)
case also, there are misconceptios. I see no merit in using only 25%
of LL in above formulation as for design of beams on any particular floor
of a multi storeyed housing building tower,
the full live load should be applicable and not 25% as several designers
take as per IS 1893, since only 25% LL is used for seismic weight calculations.

V.P. Agarwal



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vikram.jeet
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PostPosted: Fri Jan 23, 2009 6:16 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

Dear Mr Jiwaji Desai,

As I see:

During EQ, Frame as whole will have varying LL on various floors  
and for working out seismic mass/Forces , IS 1893 stipulates LL % to be
taken as 25% or 50%(I think as an overall average) .Since Probability of
all the floors having 100% LL simultaneously during EQ is very remote ,
calculating EQ forces based on 100% LL is not justified by any reason.

However probaibilty of one particular floor with 100% LL is quite
substantial and therefore safeguard is needed for beams supporting
the frame/columns( though have reduced vertical loads) but are subject
to full gravity BM from particular floor beams

I still feel that designers differ on this issue unless adressed by
experts

regards

vikramjeet

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jiwajidesai
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PostPosted: Fri Jan 23, 2009 7:39 am    Post subject: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load Reply with quote

Dear Vikram Jeet

As I said earlier I agree that some members will be subjected to 100% gravity loads, but isn’t 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


From: vikram.jeet [mailto:forum@sefindia.org]
Sent: Friday, January 23, 2009 11:58 AM
To: general@sefindia.org
Subject: [SEFI] Re: LOAD COMBINATION : (0.9)Dead Load + (1.5)Seismic Load


Dear Mr Jiwaji Desai,

As I see:

During EQ, Frame as whole will have varying LL on various floors
and for working out seismic mass/Forces , IS 1893 stipulates LL % to be
taken as 25% or 50%(I think as an overall average) .Since Probability of
all the floors having 100% LL simultaneously during EQ is very remote ,
calculating EQ forces based on 100% LL is not justified by any reason.

However probaibilty of one particular floor with 100% LL is quite
substantial and therefore safeguard is needed for beams supporting
the frame/columns( though have reduced vertical loads) but are subject
to full gravity BM from particular floor beams

I still feel that designers differ on this issue unless adressed by
experts

regards

vikramjeet

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