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Sub: Enhancement of force due to soft storey-provision in cl
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k.gangadharan
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PostPosted: Tue Feb 18, 2014 9:13 am    Post subject: Sub: Enhancement of force due to soft storey-provision in cl Reply with quote

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
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Manoharbs_eq
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PostPosted: Tue Feb 18, 2014 10:04 am    Post subject: Reply with quote

Dear Sir,

Good question, actually there is no proper data available with me as of why to increase the forces to 2.5 and 1.5 times. What is the basis for this , i rather choose to provide more stiffness rather than designing the members for higher force.

However, it is possible to do this in Etabs as i have tried it before, the forces remain same for the structure, the additional force is applied on only supporting soft storey only (soft story members like beam , column and walls) and not for entire structure.

In Etabs create a load combination for 1.5 eq and 2.5 Eq and then design those elements for this combination. ie.

1.5 EQX

in combination

1.5DL+1.5(1.5EQX)


Similarly for all other combinations.


Rgds
Manohar
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va
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PostPosted: Tue Feb 18, 2014 12:51 pm    Post subject: Sub: Enhancement of force due to soft storey-provision in cl Reply with 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]
Sent: Tuesday, February 18, 2014 3:14 PM
To: 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
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sakumar79
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PostPosted: Tue Feb 18, 2014 12:53 pm    Post subject: Reply with quote

Dear Sir,
    The additional factors 1.5, 2.5, 5, etc to amplify the effect of earthquake for vulnerable portions like soft storeys and cantilevers are applicable only to those corresponding members. This is clearly stated in IS1893 part 1 under clauses 7.10.3 a and b, 7.12.2.1 and 7.12.2.2

   Please note that for the horizontal cantilevers like balcony (clause 7.12.2.2), we need to enhance the earthquake load in VERTICAL direction (ie, instead of enhancing EQX and EQZ, we need to create EQY even if it is not required otherwise and create load combinations for this and check only the cantilever slabs/beams for these special load combinations.)

   As pointed out by Er Manohar, you need to create additional load combinations with larger load factor for earthquake load and design the affected members for these special load combinations.

  One important point to remember is that generally, we will be modelling the building without taking the stiffness of the infill in the model (only wall load will be considered). Hence, STAAD or ETABS cannot identify by itself that a soft storey is present when you have a Stilt Floor. Similarly, if we have lot of 230 mm thick brick walls in some floors, but few walls in other floors (some floors are residences, others are offices, etc) here also there will be variation in stiffness which cannot be identified by the software. Hence, we should exercise our engineering judgment and identify those floors which would be soft storeys even if software doesnt identify it and enhance the design forces for such floors.

   One point I am not clear about is when the soft storey is an intermediate floor, whether we should consider the enhanced forces for the beams in the floor grid above or the floor grid below or for both. Hope other senior sefians can clarify this point...

Hope that helps
Arunkumar
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sukanta.adhikari
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PostPosted: Wed Feb 19, 2014 4:27 am    Post subject: Re: Sub: Enhancement of force due to soft storey-provision i Reply with quote

Dear sir,

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

My interpretation of the clause is that if soft storey is detected in a particular storey,only that storey needs to be designed for enhanced forces,ie, enhancing the forces  by  2.5 or 1.5.Other part of the building needs to be designed with out enhancing the forces.Similarly for vertical and horizontal projection enhancement is to be done for particular beam or column and not for entire structure.

Regards,
S Adhikari
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thirumalaichettiar
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PostPosted: Wed Feb 19, 2014 3:10 pm    Post subject: Reply with quote

Dear Sefians,
Why can not be designed the silt floor column with ENCASED COMPOSITE COLUMN -CONCRETE +STEEL REINFORCEMENT+STEEL SECTION. In this way the column size will be reduced with additional shear carrying capacity? Software like Etab version 2013 can handle such column design but only drawback is we have to design as per AISC L.F method.

Can be designed manually by taking the analysis results using IS code or Eurocode. Guidence can be taken from the INSDAG  chapter 25 -26 and IS CODE 111384.

T.Rangarajan.
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Dr. N. Subramanian
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PostPosted: Wed Feb 19, 2014 4:59 pm    Post subject: Reply with quote

Dear All,

I agree with the comments made by Er Adhikari and Er Arunkumar. If a soft sorey is there in the middle of the frame, the shear and BM at that level has to be enhanced.  


The idea given by Er Rangarajan is great.  With steel sections, it will satisfy the strength as well as stiffness.


Regards,
NS
thirumalaichettiar wrote:
Dear Sefians,
Why can not be designed the silt floor column with ENCASED COMPOSITE COLUMN -CONCRETE +STEEL REINFORCEMENT+STEEL SECTION. In this way the column size will be reduced with additional shear carrying capacity? Software like Etab version 2013 can handle such column design but only drawback is we have to design as per AISC L.F method.

Can be designed manually by taking the analysis results using IS code or Eurocode. Guidence can be taken from the INSDAG  chapter 25 -26 and IS CODE 111384.

T.Rangarajan.
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prof.arc
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PostPosted: Thu Feb 20, 2014 12:10 am    Post subject: Sub: Enhancement of force due to soft storey-provision in cl Reply with quote

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:
Quote:
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 &lsquo;Ah&rsquo; 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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Manoharbs_eq
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PostPosted: Thu Feb 20, 2014 3:05 am    Post subject: Sub: Enhancement of force due to soft storey-provision in cl Reply with quote

Dear sir,  
Should we increase the time period of structures with soft Storey or the elements to be designed for enhanced forces.  
Since in the trailing explanation it is stated as elongated period.

Another question to all sefins, why there is no different time period for base isolated buildings as base isolation also elongates the period  On Feb 20, 2014 8:25 AM, "prof.arc" <forum@sefindia.org (forum@sefindia.org)> wrote:[quote]            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 wrote:
      --auto removed--

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thirumalaichettiar
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PostPosted: Thu Feb 20, 2014 3:28 am    Post subject: Reply with quote

Dr. N. Subramanian wrote:
Dear All,

I agree with the comments made by Er Adhikari and Er Arunkumar. If a soft sorey is there in the middle of the frame, the shear and BM at that level has to be enhanced.  


The idea given by Er Rangarajan is great.  With steel sections, it will satisfy the strength as well as stiffness.


Regards,
NS
thirumalaichettiar wrote:
Dear Sefians,
Why can not be designed the silt floor column with ENCASED COMPOSITE COLUMN -CONCRETE +STEEL REINFORCEMENT+STEEL SECTION. In this way the column size will be reduced with additional shear carrying capacity? Software like Etab version 2013 can handle such column design but only drawback is we have to design as per AISC L.F method.

Can be designed manually by taking the analysis results using IS code or Eurocode. Guidence can be taken from the INSDAG  chapter 25 -26 and IS CODE 111384.

T.Rangarajan.


Dear Sir,
Now I have designed two GF columns which has span of 29'9" with this composite idea. As this is for a residential building and the G.F is for car parking . The long span is used for easy access for the cars. Also I designed the beam as a wide beam of size b=600,D=450mm so that there will be good head clearance.

T.RangaRajan
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