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Stiffness modifiers (effective stiffness) for analysis_IS:1893-1-2016 vs IS:16700-2017 vs upcoming IS:456

 
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hemalmistry
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Joined: 01 Jul 2009
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PostPosted: Sun May 24, 2020 11:07 am    Post subject: Stiffness modifiers (effective stiffness) for analysis_IS:1893-1-2016 vs IS:16700-2017 vs upcoming IS:456 Reply with quote

Dear Members,

In upcoming 456, cracked section properties (effective stiffness) are specified for various structural elements based on type of loads (EQ and Non-EQ), however level of load (service or factored) is not specified, like in 16700. NON EQ loads includes wind load, so, it will indicate to use same modifiers for wind drift/lateral deflection (SLS) and ULS/strength design for wind. This will reflect same level of cracking for SLS and ULS for wind. Generally, SLS checks are required for wind having 10 to 25 year return period (20 year RP as per IS 16700) while ULS/strength design with 50 year RP wind. If 50 year RP wind is to be used for SLS, either load factor of wind shall be reduced (below 1) or deflection/drift limit shall be revised. Present IS 456 have specified lateral deflection limit for wind but not the drift limit.

IS:16700-2017 requires actions for ULS/strength design for wind and EQ to be extracted from analysis having factored level (load) modifiers. These are similar to EQ load modifiers in upcoming 456. However, IS:16700-2017 requires SLS checks for wind to be performed using service level modifiers with 20 year RP wind, which are similar to NON EQ modifiers as per upcoming 456. I believe cracking (effective stiffness) at ULS/strength will not be similar for EQ and wind but at the same time cracking (effective stiffness) will not be same for wind under SLS and ULS.

For EQ, requirement of upcoming 456, IS 16700 and that of IS 1893-1 are similar and as EQ loads are factored, drift checks and strength design needs to be performed with factored level (EQ) modifiers. However, IS 1893-1 have not specified stiffness modifiers for slab while same are mentioned in upcoming 456 and in IS 16700. One interpretation may be that, IS 1893-1 requires to neglect bending stiffness of slab to exclude them from LLRS. However, neglecting stiffness of slab may result in higher drift and also it may deviate from actual behavior of structures. So, for seismic drift check effective flat slab stiffness may be considered. However, for strength design either bending stiffness of CS & MS (flat slab) shall be neglected or load factor in load combinations shall be modified to assign code specified lateral load to LLRS (frames & walls).

Also, In upcoming 456, separate cracked section properties are specified for column strip (CS) and middle strip (MS) of flat slab with effective stiffness of CS similar to that of beam and effective stiffness of MS somewhat less and similar to that of "slab" as per IS 16700.
This may result in considerable contribution of slab-column frame in lateral load. This model may be suitable to check seismic/wind drift. However, for strength design either bending stiffness of CS & MS shall be neglected or load factor in load combinations shall be modified to assign code specified lateral load to LLRS (frames & walls).

Separate modifiers for column and middle strip in 456 and similar modifiers for CS & MS (i.e. of slab) in IS 16700 may need revision to unify them or we need to assume that for building below 50 m separate modifiers for CS and MS as per 456 and for building above 50 m same modifiers for CS and MS of flat slab.

Stiffness modifiers in 16700, 1893 and upcoming 456 are more or less similar to that of ACI318. ACI have specified modifiers for "flat slab/plate" and have not used word "slab". So, they require stiffness of one way or two way slab supported on beam to be neglected. However, beam may be considered as flanged beam. Also, according to ACI, specified flat slab modifiers are applicable when it is not part of LLRS and mentioned that if flat slab is part of LLRS, effective stiffness shall be based on specific study/testing. ACI have specified unique (not separate for CS & MS) modifier for flat slab/plate similar to "slab" in IS 16700. Seismic drift as per upcoming 456 will be less compared to ACI, as 456 have specified larger effective stiffness of column strip of flat slab.

I believe different terminology, purpose  and vales of stiffness modifiers in 456, 16700 and 1893 will create confusion and may result in different interpretation. Also, these codes are revised at different times, so any revision in one code may create further mismatch with other codes. So, being a mother code unified terminology and values of effective stiffness shall be specified in upcoming 456 and effective stiffness requirements in 1893-1 and in 16700 shall be removed.  

Regards,
Hemal Mistry
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