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Stiffness modifiers in IS 1893-2016-1 and draft tall building code.

 
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hemal
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PostPosted: Sun Oct 01, 2017 7:43 am    Post subject: Stiffness modifiers in IS 1893-2016-1 and draft tall building code. Reply with quote

Dear all,

New IS 1893 have same stiffness modifiers for SLS (unfactored loads) and ULS (factored loads) as per clause 6.4.3.1. There are different stiffness modifiers for SLS and ULS in draft tall building code as per clause 7.2 (table 7).

Clause 1.3 of draft tall building code states that buildings below 45 m can also be designed using tall building code to add value to the design.

Stiffness modifiers for ULS are almost same in both codes. However, that for SLS are more in tall building code.

So, if a 45 m tall building is desined using both codes, drifts and deflections as per IS 1893 will be more compared to tall building code and building designed using I 1893 will be more stiff compared to building designed with tall building code.

We can also compare design of two simillar buildings, one with 45 m height designed using IS 1893 and other with 45.1 m height designed with tall building code.

Waiting for your comments.

Regards
Hemal Mistry
Surat
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Yogesh.Pisal
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PostPosted: Sun Oct 01, 2017 8:34 am    Post subject: Reply with quote

Dear Sir,

I think IS1893 Part 1 has missed the provisions of separate stiffness modifiers for SLS.

If we compare these provisions with ACI318-11 (ACI318-14 is having major changes about the provision of Cracked MI), then provisions in draft tall building code are near to the provisions in ACI318-11.

Best Regards,
Yogesh Pisal
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hemal
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PostPosted: Sun Oct 08, 2017 8:31 am    Post subject: Reply with quote

I would like to share my views on current topic. Respected sefi members may correct me where necessary.

ACI, EURO, NEWZEALAND and other foreign codes required to consider appropriate stiffness reduction factors (modifiers) at appropriate load level (factored or unfactored) for elastic seismic analysis to account for cracking at inelastic state. Such modifiers are not required if cracked nonlinear analysis is performed.

Seismic/wind loads as per ASCE/ACI are factored (strength level). Seismic/wind loads have load factor of 1.0 when combined with other loads with appropriate load factor for strength design (ULS). DBE for ACI is0.66 (2/3rd) of MCE compared to 0.5 of MCE in Indian code.

ACI have three options for Stiffness modifiers at factored load elastic analysis.
Option 1 is similar to Indian code (Ie=0.35Ig for beams,I=0.25Ig for flat slab/slab, I=0.7Ig for column and I=0.35 or 0.7Ig for shear walls),
Option 2 requires calculating stiffness modifiers based on Pu, Mu, Ast etc calculated from load combination that produces least stiffness,
Option 3 requires to consider Ie=0.5Ig for all members or Ie from detailed cracked section analysis for factored load.

According to ACI Stiffness modifiers for unfactored  (service) load elastic analysis shall be 1.4 times that for factored load analysis, subjected to maximum of 1. Here factor1.4 is load factor for seismic load which is near to 0.66/0.5 = 1.32 (ratio of DBE/MCE as per ACI to that for Indian code). However, service load elastic analysis with proposed stiffness modifiers is generally used to check vertical deflections and vibration under unfactored gravity load. Many softwares available today can perform non linear cracked section analysis to access vertical deflections and vibration under unfactored gravity load, in such case use of stiffness modifiers is not required. IS:1893 shall mention that stiffness modifiers given shall not be used for to access vertical deflections and vibration under unfactored gravity load and IS:456-2000 shall be referred for the same.

IS:1893 stiffness modifiers are similar to Factored load analysis stiffness modifiers in ACI. Seismic/wind loads as per ASCE/ACI are factored (strength level). Seismic/wind loads have load factor of 1.0 when combined with other loads with appropriate load factor for strength design(ULS). IS:1893 requires to use ACIís  Factored load analysis stiffness modifiers for service load analysis. So, appropriate stiffness modifiers are not considered at appropriate level of load. However,analysis results of seismic loads are combined with other load with load factor of 1.2 or 1.5. Though not significant, effect of improper stiffness modifiers on design combinations of strength design shall be investigated.


ACI required calculating drift from factored load analysis with factored load stiffness modifiers. This is appropriate as structure is in postelastic state under seismic event. Indian code requires calculating drift from service load analysis with load factor of 1.0 with ACIís Factored load analysis stiffness modifiers for service load analysis. This looks fine as cracking in members under seismic event considered (stiffness modifiers) is appropriate.However, drift limit in ACI is 2% compared to 0.4% in Indian code, as ACI driftis factored load inelastic drift. ACI requires to check drift with factored load even for allowable stress design.

In short, using same stiffness modifiers for strength design and drift checks as per IS:1893-2016 with service load analysis looks fine but vertical deflection and vibration shall be checked as per IS:456-2000.

Draft Tall building code has given separate stiffness modifiers for factored and unfactored load analysis. However, seismic load asper IS:1893-2016 is unfactored and how it can be converted to factored for factored load analysis is not mentioned.

Regards,
Hemal Mistry
Surat, Gujarat
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alpa_sheth
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PostPosted: Tue Oct 10, 2017 6:31 am    Post subject: Stiffness modifiers in IS 1893-2016-1 and draft tall building code. Reply with quote

Hemal:

Greetings and thanks for your detailed mail on stiffness modifiers. I agree with you that the stiffness modifiers in the Tall building draft code (and IS 1893) were intended for lateral load deformations and may  not per se be used to calculating vertical deformations or vibrations arising out of gravity loads.

a) I think the IS 1893 has done away with DBE and MCE as those are very fraught with interpretations. People would often ask- Is the factor of 2 in denominator of the formula for calculating seismic coefficient for reducing MCE to DBE or is it to reflect the correct R- since the R in IS 1893 is almost half of that in other international codes. I will not go into more detials of this so as to avoid opening a Pandora's box.

b) The Tall Building Code has two sets of stiffness factors as it deals with two sets of Lateral loads- earthquake and wind.  It says use one set of values for factored loads (Typically for Earthquake) and another set of stiffness modifiers for unfactored loads (service condition- usually to be used for wind loads).  

I hope this helps.

best regards,
Alpa

hemal wrote:
I would like to share my views on current topic. Respected sefi members may correct me where necessary.

ACI, EURO, NEWZEALAND and other foreign codes required to consider appropriate stiffness reduction factors (modifiers) at appropriate load level (factored or unfactored) for elastic seismic analysis to account for cracking at inelastic state. Such modifiers are not required if cracked nonlinear analysis is performed.

Seismic/wind loads as per ASCE/ACI are factored (strength level). Seismic/wind loads have load factor of 1.0 when combined with other loads with appropriate load factor for strength design (ULS). DBE for ACI is0.66 (2/3rd) of MCE compared to 0.5 of MCE in Indian code.

ACI have three options for Stiffness modifiers at factored load elastic analysis.
Option 1 is similar to Indian code (Ie=0.35Ig for beams,I=0.25Ig for flat slab/slab, I=0.7Ig for column and I=0.35 or 0.7Ig for shear walls),
Option 2 requires calculating stiffness modifiers based on Pu, Mu, Ast etc calculated from load combination that produces least stiffness,
Option 3 requires to consider Ie=0.5Ig for all members or Ie from detailed cracked section analysis for factored load.

According to ACI Stiffness modifiers for unfactored  (service) load elastic analysis shall be 1.4 times that for factored load analysis, subjected to maximum of 1. Here factor1.4 is load factor for seismic load which is near to 0.66/0.5 = 1.32 (ratio of DBE/MCE as per ACI to that for Indian code). However, service load elastic analysis with proposed stiffness modifiers is generally used to check vertical deflections and vibration under unfactored gravity load. Many softwares available today can perform non linear cracked section analysis to access vertical deflections and vibration under unfactored gravity load, in such case use of stiffness modifiers is not required. IS:1893 shall mention that stiffness modifiers given shall not be used for to access vertical deflections and vibration under unfactored gravity load and IS:456-2000 shall be referred for the same.

IS:1893 stiffness modifiers are similar to Factored load analysis stiffness modifiers in ACI. Seismic/wind loads as per ASCE/ACI are factored (strength level). Seismic/wind loads have load factor of 1.0 when combined with other loads with appropriate load factor for strength design(ULS). IS:1893 requires to use ACIís  Factored load analysis stiffness modifiers for service load analysis. So, appropriate stiffness modifiers are not considered at appropriate level of load. However,analysis results of seismic loads are combined with other load with load factor of 1.2 or 1.5. Though not significant, effect of improper stiffness modifiers on design combinations of strength design shall be investigated.


ACI required calculating drift from factored load analysis with factored load stiffness modifiers. This is appropriate as structure is in postelastic state under seismic event. Indian code requires calculating drift from service load analysis with load factor of 1.0 with ACIís Factored load analysis stiffness modifiers for service load analysis. This looks fine as cracking in members under seismic event considered (stiffness modifiers) is appropriate.However, drift limit in ACI is 2% compared to 0.4% in Indian code, as ACI driftis factored load inelastic drift. ACI requires to check drift with factored load even for allowable stress design.

In short, using same stiffness modifiers for strength design and drift checks as per IS:1893-2016 with service load analysis looks fine but vertical deflection and vibration shall be checked as per IS:456-2000.

Draft Tall building code has given separate stiffness modifiers for factored and unfactored load analysis. However, seismic load asper IS:1893-2016 is unfactored and how it can be converted to factored for factored load analysis is not mentioned.

Regards,
Hemal Mistry
Surat, Gujarat
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hemal
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Joined: 01 Apr 2008
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PostPosted: Tue Oct 10, 2017 6:33 pm    Post subject: Reply with quote

Respected alpa madam,

Thank u for the valuable reply and clarifications.

I still got some queries / doubts :

(1) U mention that factored load stiffness modifiers in draft tall building code are for seismic loads. Seismic loads as per IS:1893-2016 & draft tall building codes are unfactored. We factor it after analysis for design.

Does it mean that analysis results (design forces & drifts) from below two cases are same:

(a) Unfactored seismic load as per code analysed with proposed factored level stiffness modifiers and multiplied with load factor 1.2 or 1.5 after analysis.

(b) Factored seismic load (derived by multiplying unfactored seismic load as per code with load factor of 1.2 or 1.5 ) analysed with proposed factored level stiffness modifiers.

(2) Does the drift limit of IS:1893-2016 takes care of overstrength, response reduction factor (inelastic state) and load factor?

Assuming

overstrength factor (Ω) =2,
response reduction factor (R) = (5+3)/2 = 4 (average)
Load factor (PSF) = (1.2+1.5)/2 = 1.35 (average)

If i dividede ACI inelastic drift limit by  
Ω*R*PSF to get elastic drift under unfactored load then it comes out as 2%/(2*4*1.35) = 0.185% ≈ 0.2 %, which is half the limit in IS:1893-2016

(3) U mention that service load stiffness modifiers can be used for wind as per draft tall building code. Seismic and wind both are factored load as per ACI/ASCE. Though behaviour (cracking etc.) of seismic and wind load at factored load is different, ACI reccomends same stiffness modifiers for factored load analysis irrespective of type of load (wind or seismic).

(4) For seismic loads, factored load stiffness modifiers are same for IS:1893-2016 and draft tall building code. However, for wind load there are service load stiffness modifiers in draft tall building code, but no
stiffness modifiers in IS:875-part 3 for buildings below 45 m height. It means that for wind analysis of buildings below 45 m height, we can take gross moment of inertia of all members for analysis.

Regards,
Hemal mistry,
Surat

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