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vegad ...

Joined: 25 Dec 2013 Posts: 138
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Posted: Thu Feb 20, 2014 5:35 am Post subject: Specific replies to Er. Gangadharan |
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Dear Er. Gangadharan,
I would like to reply you on each point you pointed out in a specific manner.
1) If the building is analyzed as a bare frame, neglecting the effect of infillsand the dynamic forces so determined in columns and beams of the soft (stilt)storey, are to be designed for 2.5 times storey shears and moments
Reply - We should be consistent in assumptions. If the building is analysed asbare frame, you are actually applying clause '7.10.3' which says in the lastpart of the sentence ' . . . neglecting the effect of infills in otherstories'. So your interpretation is correct. '7.10.3 - a' says 'columns andbeams of the soft storey are to be designed for 2.5 times the . . .', hence notthe whole structure.
2) OR if the Shear walls are introduced in the soft (stilt) storey, in bothdirections of the building, which should be designed for 1.5 times thecalculated storey shear forces.
Reply - As Clause '7.10.3 - b' columns designed and detailed for the"calculated storey shear and moments" and not enhanced. So you candesign columns as well as beams for the design forces resulting from your bareframe analysis, not enhanced for any factor related to this clause. But theshear wall shall be designed for the design forces resulting from 1.5 times ofthe lateral storey shear forces (calculated by bare frame-with-shear wall modelledfor analysis AND the respective storey still qualifying for soft storeydefinition).
3) Does this mean that the enhancement is applicable only to the soft storeybeams and columns alone or need be designed for the enhanced values of thewhole structure.
Reply - Enhancement to be applied as explained in reply to 1) and 2). Not thewhole structure.
4) The values of lateral forces can be increased by enhancing the scale factorin the earthquake analysis in ETABS .By doing so the lateral forces for thewhole building will increase; but how to limit this value for the soft storeyalone only, if applicable only to soft storey alone.
Reply - I am not sure if I have got your doubt correctly here. For enhancementof design forces on the respective soft storey elements, extract forces forthat element from original analysis and manually enhance these values andredesign. If design feature allows you to modify forces by factors, use thatfeature.
Scaling feature for earthquake analysis in ETABS is used to adjust dynamic baseshear 'https://wiki.csiamerica.com/display/etabs/Scale+factor+in+RSA'. Socorrect me if I have got you wrong.
5) Also in clause 7.12.2 of 1893 it is provided that for cantilever projectionsabove roof should be designed for 5 times the design horizontal seismiccoefficient ‘Ah’ specified in clause 6.4.2. Also anyhorizontal projection on the balconies or cantilever supporting floatingcolumns the cantilever need to be designed for 5 times the design verticalcoefficient as specified in clause 6.4.5 of the IS 1993-2000 part-1
Does this also apply to only the cantilever part of the building alone andother parts of the buildings can be designed for the normal loads obtained asper the dynamic analysis without enhancing the values. Is this applicable ifthe cantilever projections are below the roof.
Reply - Clause 17.12.2.3 - ' . . only for designing the projectingparts and their CONNECTIONS with the MAIN structure' if the cantileverprojections are below the roof and connecting the main structure, it means thatit is supporting critical loads, so yes. If you are talking of 'small chajjas',don't bother.
For all the above points, I have not revisited the manner of analysis, the timeperiods and all; I assume that all has been taken care already and we have onlydiscussed post analysis treatment.
If I have erred anywhere, please share the corrections, unless deviations.
Thank you.
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ibarua General Sponsor

Joined: 26 Jan 2003 Posts: 1039
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Posted: Mon Feb 24, 2014 9:00 am Post subject: Sub: Enhancement of force due to soft storey-provision in cl |
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24th Feb 2014
The comments of Prof ARC may please be heeded by BIS.
In Zone V (where I happen to be located), I always provide lateral stiffening systems like diagonal braces or shear walls for anything more than G+2 buildings, from top to bottom. I would advise my friends in Zones V and IV to do likewise.Thereby, the so-called 'soft story' can be avoided. However, half brick thick walls with plenty of door & window openings (as is common in apartment buildings) in the upper floors may not contribute anything to story stiffness. If I had been to be one of those fortunate ones who live in less seismically active zones, I would apply the 'soft story' concept only to the 'soft story' itself and not globally all over the entire frame. The rationale: the 'soft story' has to be stiffened to prevent it from damage /distress /collapse; elsewhere, the frame with infill walls (at least of full brick thickness and with small openings) which presumably (and hopefully) act as lateral stiffening systems may not need any further stiffening.
Indrajit Barua
From: prof.arc <forum@sefindia.org>
Sent: Thu, 20 Feb 2014 08:28:36
To: general@sefindia.org
Subject: [SEFI] Re: Sub: Enhancement of force due to soft storey-provision in cl
this topic [of enhancement] has been discussed in this forum so many
times. this response would have to be a repeat of what has been
mentioned by me in earlier postings
first comment is on projections which clause had been included since
the first version of 1893 in 1962 - followed in 1966/1970/1975/1984
and in 2002
this is purely a PUNITIVE clause in the hope that extra care would be
taken to construct these projections with care and these projections
do not fall down and hurt the pedestrians
it has to be realised that the effective forces given in all versions
of code is rather LOW
and that is why in the preamble all versions of the code states that
some degree of damage is inevitable
the stilt type of buildings became popular after eighties and damage
during earthquakes indicated distress to such type of building. data
on such buildings indicated that modelling of such building [hopefully
unintentionally] resulted in fictitious fundamental elongated period
which gave a low value of spectral acceleration. to compensate this
error, this clause of "2.5" was added to ensure a stiffer period more
near what is observed in actual experiments on such buildings to
determine fundamental period
it is a fact that the code has been generally mis-interpreted and the
fault lies in BIS not publishing authentic official handbook
explaining the background of all clauses
since commercial software is now invariably used unlike the pre 2002
era, modelling can be more detailed and hence BIS should specify
guidelines for analytical modelling
the arbitrary factors should be dispensed with in long overdue
revision of 1893-2002
ARC
On 2/18/14, k.gangadharan <forum@sefindia.org> wrote:
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ajeetkokil SEFI Regulars


Joined: 26 Jan 2003 Posts: 36
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Posted: Tue Feb 25, 2014 1:57 pm Post subject: Sub: Enhancement of force due to soft storey-provision in cl |
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One way could be -
+ copy the parent staad file
+ put fixed supports at the top of soft story and delete soft story members
+ get support reactions
+ amplify them by 2.5 times in excel
+ copy parent staad and delete all members above soft story
+ apply obtained support reactions, obtained as above as a load and then design the soft story members.
Hope this works.
Thanks
ajit On Feb 19, 2014 11:05 AM, "va" <forum@sefindia.org (forum@sefindia.org)> wrote: Quote: | Hello,
This is a very important issue to be discussed by experts from design and implementation point of view.
If we try to follow this, it is very difficult or not possible to design the columns for 2.5 times the forces - or what-ever design sizes of columns and beams are required are not acceptable by the architects / clients / municipal approval architects . Critical question is how to implement it in actual practice? I have never seen a building designed and constructed using these provisions. If someone has done it, kindly share the information.
Regards.
Hemant Vadalkar
Consulting Engineer , Mumbai.
From: k.gangadharan [mailto:forum@sefindia.org (forum@sefindia.org)]
Sent: Tuesday, February 18, 2014 3:14 PM
To: general@sefindia.org (general@sefindia.org)
Subject: [SEFI] Sub: Enhancement of force due to soft storey-provision in clause7.10.1, 7.10.2 &7.10.3 of IS 1893-2002 Part-1
To SEFI Experts
A Clarification is sought for the following points:
Attn: (Er.Arunkumar, Er.Sukantha Adhikari, Er.Kumar Abhishek Singh & Er.Rengarajan and other expert members of SEFI in Earthquake Design)
Sub: Enhancement of force due to soft storey-provision in clause7.10.1, 7.10.2 &7.10.3 of IS 1893-2002 Part-1
If the building is analysied as a bare frame, neglecting the effect of infils and the dynamic forces so determined in columns and beams of the soft (stilt) storey, are to be designed for 2.5 times storey shears and moments OR if the Shear walls are introduced in the soft (stilt) storey, in both directions of the building, which should be designed for 1.5 times the calculated storey shear forces.
Does this mean that the enhancement is applicable only to the soft storey beams and columns alone or need be designed for the enhanced values of the whole structure. The values of lateral forces can be increased by enhancing the scale factor in the earthquake analysis in ETABS .By doing so the lateral forces for the whole building will increase; but how to limit this value for the soft storey alone only, if applicable only to soft storey alone.
Can we take the resultant forces for the building above soft storey from the forces obtained from the result by doing the normal way (without increasing 1.5times scale factor for the soft storey) from a design Analysis without increasing the scale factor for the calculated storey shear force.
Also in clause 7.12.2 of 1893 it is provided that for cantilever projections above roof should be designed for 5 times the design horizontal seismic coefficient ‘Ah’ specified in clause 6.4.2. Also any horizontal projection on the balconies or cantilever supporting floating columns the cantilever need to be designed for 5 times the design vertical coefficient as specified in clause 6.4.5 of the IS 1993-2000 part-1
Does this also apply to only the cantilever part of the building alone and other parts of the buildings can be designed for the normal loads obtained as per the dynamic analysis without enhancing the values. Is this applicable if the cantilever projections are below the roof.
Thanking you
Regards
K.GANGADHARAN
General Sponsor
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binaya shahi SEFI Member

Joined: 03 Jun 2015 Posts: 19
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Posted: Sat Nov 26, 2016 7:39 pm Post subject: |
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I think as mass source in etabs is the earthquake load. multiplying mass source by 2.5 or multiplying earthquake load in load combination should give the same result.
The main problem is that multiplying mass source by 2.5 or earthquake load in load combination by 2.5 in not increasing the base shear much.
To get 2.5 times the base shear, I had to multiply the mass source by 6.
the reinforcement for one of the column for normal analysis was 1200 sq. mm. where as that for the 2.5 times base shear(or 6 times earthquake load) was 6200 sq. mm.
The reinforcement from soft story analysis is more than 5 times that of normal analysis. I want to know whether the difference in reinforcement between two analysis is plausible of not.
for normal case I used the mass source : 1 (dead load), 1(wall load), 1(floor finish), 0.25(live load)
whereas for soft story analysis, i used the mass source: 2.5(dead load), 2.5(wall load), 2.5(floor finish), 0.625(live load)
when I checked the m2 value for the same column in two analysis.
the m2 of the column for soft story analysis was about 3 times the m2 value for normal analysis.
My understanding is that according to IS code the base shear and the moment in beams and columns should be increased by 2.5 times for the soft story analysis.
My doubt is whether I should just use (2.5 times the earthquake load that resulted in not much increase in base shear) or (6 times the earthquake load that resulted in 2.5 times base shear).
Please help me out? |
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binaya shahi SEFI Member

Joined: 03 Jun 2015 Posts: 19
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Posted: Sat Nov 26, 2016 8:07 pm Post subject: |
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The simplest method to analyze soft story I think would be to make a normal model first and then copy the model and increase the base shear by 2.5 times And also the moments in beams and columns by 2.5 tiimes.
now we have two models one normal one and another for soft story analysis.
for normal storys, take the design values from first model(ie normal model)
and for the soft story structural elements take the design value from second model(lets call it softstory model).
But the problem is that how can we increase the base shear by 2.5 times and also the moments in beams and columns of soft story by 2.5 times.
I have seen many in this forum suggesting to multiply the Eq load in load combination by 2.5.
I multiplied EQ load in all load combinations by 2.5 but base shear didn't increased by 2.5.
I had to multiply EQ load by 6 to achieve 2.5 times base shear.
I also checked moment in one of the column which I found have increased by nearly 3 times.
So, I haven't found the easy method to control the base shear value and moment in beams and columns independently.
Although I can't control the bending moments in beams and columns sepatarely, increasing EQ load by 6 times seam to increase base shear by 2.5 times. And moment in beams and columnsincreased by 3 times which is more than 2.5 times.
So beams and columns are not weaker than the code requires them to be.
So this method shouldn't be bad to use I guess.
Guys, please tell be if my method is good or not. Or am I doing something
wrong?
And also keep in mind that by the way instead of increasing the EQ load in all load combinations by 2.5, I multiplied the mass source in etabs by 2.5.
I think multiplying mass source by 2.5 times should be equivalent to multiplying EQ loads in all load combinations by 2.5.
Guys please correct me if I am wrong or I have misunderstood something.
In etabs for normal analysis I used mass source: 1(dead load), 1(wall load), 1(floor finish), 0.25(live load)
and for soft story analysis I used mass source: 2.5(dead load), 2.5(wall load), 2.5(floor finish), 0.625(live load)
Guys, please post your views on my approach. |
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