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Rudra Nevatia ...
Joined: 26 Jan 2003 Posts: 215

Posted: Mon Sep 20, 2004 6:17 am Post subject: Clause 7.9.2 of IS:1893 


It is proposed to change Clause 7.9.2 of IS:1893 which presently reads as follows:
The design eccentricity, edi to be used at floor i shall be taken as:
edi = 1.5esi + 0.05bi or esi  0.05bi
whichever of these gives the more severe effect in the shear of any frame
where:
esi = Static eccentricity at floor i defined as the distance between centre of mass and centre of rigidity, and
bi = Floor plan dimension of floor i, perpendicular to the direction of force
NOTE  The factor 1.5 represents dynamic amplification factor, while the factor 0.05 represents the extent of accidental eccentricity
The proposed change envisages changing factor 1.5 to 1.0 and factor 0.05 to 0.1.
My understanding of the clause in its existing form is as follows. Please correct me if I am wrong.
In its general form the expresion for edi should read:
edi = A*esi + B*bi
Factor A accounts for possible coupling of torsional and lateral modes of vibration and depends on the ratio of frequencies in the two modes. As this ratio approaches 1, resonance will set in. Hence a multiplier of 1.5 on esi. FEMA relates factor A to the extent of torsion irregularity and in extreme case, this factor can reach up to 3!
Factor B accounts for possible variation of ground motion along the width of building. Obviously, the factor can take a positive or negative value.
Two extreme cases are possible:
1. Torsional coupling occurs and accidental eccentricity is in the same direction as the static eccentricty which is reflected by the first equation of the present clause
2. Torsional coupling does not occur and the static eccentricty is in the direction opposite to the static eccentricty which is what the second part of the equation implies.
The proposed change would wrongly
 ignore the effect of coupling of lateral and torsional modes
 give benefit of reduction in torsional shear
May I therefore suggest that the clause be retained as it is.
Rudra Nevatia
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alpa_sheth at vsnl.com Guest

Posted: Mon Sep 20, 2004 7:27 am Post subject: Clause 7.9.2 of IS:1893 


Dear All, I too have concerns about the proposed revision to Clause 7.9.2 of IS:1893 but for different reasons: I feel that the revision from 0.05 to 0.1 bi unnecessarily and perhaps unconsciously punishes perfectly symmetrical structures. ASCE 702 also has min torsional design mom based on actual torsion (Mt) + that caused due to displacement of mass by distance of 5% of dimension of structure perpendicular to direction of seismic force (Mta). In higher seismic zones, Mta is to be multiplied by a factor of ((max displacement/(1.2* avg displacement)) ^2. Thus for a perfectly symmetrical structure, where max displacement = avg displacement, Mta would be based on eccentricity of 5% * (1/1.2)^2= 0.69 or very roughly 3.5% eccentricity. On the other hand, the proposed revision to IS 1893 will increase the accidental torsion from 5% bi to 10% bi for perfectly symmetrical structures
Regards, Alpa  Original Message  From: <rudra_nevatia@yahoo.com> To: <alpa_sheth@vsnl.com> Sent: Monday, September 20, 2004 11:33 AM Subject: [Econf] Clause 7.9.2 of IS:1893
Quote:  ************************************************* SEFI eConference on Proposed Revisions to IS:1893 & IS:13920 from
 September 20 to 25, 2004
Quote:  *************************************************
It is proposed to change Clause 7.9.2 of IS:1893 which presently reads as follows:
The design eccentricity, edi to be used at floor i shall be taken as:
edi = 1.5esi + 0.05bi or esi  0.05bi
whichever of these gives the more severe effect in the shear of any frame
where:
esi = Static eccentricity at floor i defined as the distance between
 centre of
Quote:  mass and centre of rigidity, and
bi = Floor plan dimension of floor i, perpendicular to the direction of
 force
Quote: 
NOTE  The factor 1.5 represents dynamic amplification factor, while the factor 0.05 represents the extent of accidental eccentricity
The proposed change envisages changing factor 1.5 to 1.0 and factor 0.05
 to
Quote:  0.1.
My understanding of the clause in its existing form is as follows. Please correct me if I am wrong.
In its general form the expresion for edi should read:
edi = A*esi + B*bi
Factor A accounts for possible coupling of torsional and lateral modes of vibration and depends on the ratio of frequencies in the two modes. As
 this
Quote:  ratio approaches 1, resonance will set in. Hence a multiplier of 1.5 on
 esi.
Quote:  FEMA relates factor A to the extent of torsion irregularity and in extreme case, this factor can reach up to 3!
Factor B accounts for possible variation of ground motion along the width of building. Obviously, the factor can take a positive or negative value.
Two extreme cases are possible:
1. Torsional coupling occurs and accidental eccentricity is in the same direction as the static eccentricty which is reflected by the first equation of the present clause
2. Torsional coupling does not occur and the static eccentricty is in the direction opposite to the static eccentricty which is what the second part of the equation implies.
The proposed change would wrongly
 ignore the effect of coupling of lateral and torsional modes
 give benefit of reduction in torsional shear
May I therefore suggest that the clause be retained as it is.
Rudra Nevatia
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******************************************* The views, opinions, analyses and assessments contained herein do not
 necessarily reflect the views of SEFI. Also SEFI does not guarantee the accuracy of the data included in the proceedings of this econference and accepts no responsibility for any consequences of their use.
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 (http://www.reach1to1.com)======
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mc.upadhyay1 ...
Joined: 26 Jan 2003 Posts: 134

Posted: Mon Sep 20, 2004 7:58 am Post subject: Clause 7.9.2 of IS:1893 


õ
Hello Madam Alpa
you yourself (being administrator of SEFI) did not follow the following dont's
* Please delete the message you are responding to from the body of your email so as to limit the size of the email.
Regards
Mukesh Upadhyay
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alpa_sheth at vsnl.com Guest

Posted: Mon Sep 20, 2004 8:11 am Post subject: Clause 7.9.2 of IS:1893 


Yes, I agree and plead guilty ))) But I think what we really mean is that if there is a loooong thread of discussion on a topic we should not carry all of the thread in the reply so as to limit email size. Retaining the most recent email on the topic in the reply is desirable so that people know the reference of the reply. Cheers, Alpa
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