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Clarification on the values of Stiffness Modifiers for DL+LL Combination

 
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aditya
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PostPosted: Sun Apr 04, 2021 6:45 am    Post subject: Clarification on the values of Stiffness Modifiers for DL+LL Combination Reply with quote

Dear N. S. Subramanian Sir and fellow Sefians,
IS 1893 Part 1: 2016 introduced the idea of using cracked section properties of RC section and clause 6.4.3.1 of the code has given stiffness modifiers applicable for beams & columns. However, there is some confusion on how and where to apply them. Would you please clarify on the following points:
1. Since IS 1893 Part 1: 2016 is a code related to seismic or lateral load analysis, whether a) the stiffness modifiers are applicable only to those cases where lateral SEISMIC load analysis is being carried out OR b) stiffness modifiers are to be used even when only dead load (DL) and live load (LL) are acting. This will be applicable for the load combination 1.5*(DL+LL) at the time of RC design. It is understood that section modifiers are used in the frame analysis and the design will be done based on the results from frame analysis including all load combinations, i.e. including 1.5*(DL+LL) combination. This confusion arose because IS 456: 2000 has not been amended yet to include specific values of stiffness modifiers for different load combinations. If stiffness modifier is not be used for DL+LL combinations, then separate analysis may need to be done for (DL+LL) combination and (DL+LL+EL) or (DL+EL) combinations with different sets of section modifiers or uncracked/cracked section properties.
2. Whether the stiffness modifiers will be applicable for wind load cases also. For example, 1.2*(DL+LL+WL) combinations. However, IS 16700: 2017 has given separate stiffness modifiers for "unfactored loads" and "factored loads". It was explained during e-conference discussion that "unfactored loads" means wind loads but my question is whether the stiffness modifier for "unfactored loads" will be applicable for analysis in the case when DL and LL only are present. Moreover, IS 16700 has given 0.7*Ig for beams and 0.9*Ig for columns instead of using full section moment of inertia, Ig. Why full section moment of inertai not taken for "unfactored loads"?
3. IS 1893 Part 1: 2016 in clause 7.11 tells to use the same cracked section properties as given in clause 6.4.3.1 to get the deformation of a RC building from structural analysis. What is the reason behind using the same section modifiers or cracked section properties in serviceability check in which load factor is 1 meaning the section may be uncracked? ACI 318-19 allows to use increased section properties for serviceability checks in RC building.

with regards
Aditya
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vikram.jeet
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PostPosted: Sun Apr 04, 2021 9:02 am    Post subject: Reply with quote

Since seismic event is resisted by a rcc structure through ductility  ,the multipliers given indicate provision of strong column concept required to save total collapse of structure  .But adopting same values for gravity loadings , will be drifting away from realistic structural behaviour . How to manage the two sets of stiffness in one model is a matter of discussion.
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vikram.jeet
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PostPosted: Sun Apr 04, 2021 9:15 am    Post subject: Reply with quote

Forgot regarding wind loads in previous post
Wind load stiffness multipliers would be same as gravity load ing.

Probably the software developers may  revise the version incorporating   facility to have two sets of multipliers  in a single model, one to be used under gravity/ wind loading  , and others under EQ. How
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aditya
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PostPosted: Fri Apr 16, 2021 9:34 am    Post subject: Reply with quote

Dear Fellow Sefians,
I observed that there is a lot of confusion on interpretation of Clause 6.4.3.1 and Clause 7.11 of IS 1893 Part 1:2016 and being discussed in other forums as well. It would be really useful if somebody could put forward the correct interpretation of the clauses. I also noticed that in the illustrative example on effective moment of inertia, IITGN has aslo used reduced moment of inertia even for drift check.
Please kindly elaborate on uses and meanings of clauses on effective stiffnesses.
with regards,
Aditya
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vikram.jeet
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PostPosted: Sun Apr 18, 2021 2:58 am    Post subject: Reply with quote

Dear Er

Clause 6.4.3.1 provides  the values of MOI to be used for structural analysis as 70% of Ig for columns and 35%Ig for beams in rcc structures. In steel structures it is Ig  to be used for columns as well as beams.
Clause 7.11 clearly states that Deformation of RC building shall be obtained from structural analysis using structural model based on section properties given in 6.4.3.

Hence IS 1893 is very clear  and there seems no ambiguity.
I think this model with properties stated above is to be exclusively used for checking  str deformations in EQ cases. The model for gravity loads cases & wind cases for analysis will be using section properties as provided in IS 456.

As we all know that actual forces that appear on structures during EQ are much higher than the design forces and it the ductility of structures which provides reserve strength and deformations needs to be checked under this scenario .
Thus deformations needs to be checked for actual EQ intensity OR as an alternative method Code restricted the section properties for checking the deformations under Design seismic forces.

May be other eminent members throw more light on this issue.
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vikram.jeet
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PostPosted: Mon Apr 19, 2021 2:12 am    Post subject: Reply with quote

In continuation to previous post on this subject,  

It can be said that for  analysis &design of structural members  the load combinations for Gravity loads , wind loads AND seismic loads be attempted with model using section properties as per IS 456 as usually being done.

Only for deformations check under EQ cases, the structural model with MOI given in 6.4.3.1 of IS 1893 to be presented thru staad. The load factor for this check is 1.0.
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aditya
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PostPosted: Wed Apr 21, 2021 3:37 am    Post subject: Reply with quote

Dear Respected Vikram Jeet Sir,
In your post, you have written that "It can be said that for  analysis &design of structural members  the load combinations for Gravity loads , wind loads AND seismic loads be attempted with model using section properties as per IS 456 as usually being done"


But IS 1893 Part 1: 2016 clearly says in clause 6.4.3.1 that FOR STRUCTURAL ANALYSIS, THE MOMENT OF INERTIA SHALL BE TAKEN AS:
a.     In RC and masonry structures: 70 percent of Igross of columns, and 35 percent of Igross of beams; and
b.     In steel structures:     Igross of both beams & columns.


Thus it means that cracked section properties shall be used for structural analysis while doing seismic analysis and design and NOT ONLY for Drift Calculation as per clause 7.11 of IS 1893 Part 1:2016.
Moreover, the draft of proposed revision on IS 1893 Part 1: 2016 prepared by IITGN and available in NICEE website (IITGN-World Bank Codes: EQ01.pdf) for download has kept the clause unchanged but added the cracked section properties for walls and slabs.
May I request further views in this regard?
with regards,
Aditya



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vikram.jeet
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PostPosted: Wed Apr 21, 2021 9:56 am    Post subject: Reply with quote

Dear Er Aditya,
I think you are right. Code vide cl 6.4.3.1 clearly say about the structural analysis.

This complicates the job of designers unless IS456 also follow the same.

As regards design, due to provision of ductility we are designing the structure for design EQ force only and not for actual EQ which is much higher , probably I felt that usual way of structural analysis for design  with IS 456 stiffness parameters (for various load combinations incl EQ) may be ok, and for deformation check 7 .11 is very clear ,and there is no ambiguity.

But I agree with you , the need for  clarity on this issue persists.


.Best wishes & rgds
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PostPosted: Wed Apr 21, 2021 10:15 am    Post subject: Reply with quote

In continuation

Also   it  seems IS 1893 cl 6.4.3.1  intent to consider   0.70*Ig for columns and 0.35*Ig for beam members , is  to  make the design on STRONG column WEAK beam concept . This theory is essential in structures ,especially high rise, to safeguard against failure of columns prior to failure of beams during the severe EQ event. If this is the intent, then EQ cases need to be analysed on structural model with above stated MOI' s.
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