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[aditya]

Dear Sefians,
I am designing a 12 storeyed RCC framed building in seismic zone V. I observed that IS 1893 Part 1:2016 has given cracked section properties for beams and columns only. However, tall building code IS 16700 has given cracked section properties for shear walls but my building is less than 50 m in height. Will it be prudent to analyze and design the building without considering cracked section properties for shear wall? Obviously, there wall be large difference in design when considering and not considering cracked section properties for walls. If it is a omission on part of code committee, then why no amendment on this clause even after releasing two amendments for IS 1893 Part 1:2016? Please kindly advise.
with regards,
Aditya

[vikram.jeet]

Dear Er

Shear walls primary function is to bear seismic forces  . Shear walls strength is in its longitudinal direction . It is to be seen whether section is cracked or ( uncracked as per recommendation of IS 456).
In width( thickness) direction , this section needs to be taken as cracked. In this direction it mostly takes gravity loading .

Thus , I think considering properties of uncraked section in length direction and properties of cracked section in thickness direction may yield tolerable results. But discuss this issue beforehand to the proof checker of designs . My interpretation is just a thought.

[pvgraju]

Dear Sir,

IS 16700 says 0.7Ig for both columns and walls for factored loads and 0.9Ig for unfactored loads.

IS 1893 says 0.7Ig for columns only. By interpretation this clause applies to walls also which carry compression loada, bending moments.

Regards,

P.V.Gavarraju.

[vikram.jeet]

Probable reasons for considering cracked properties in IS 1893 latest :-

I think 0.7Ig for columns and 0.35 Ig for beams  matters in gravity losdings where unbalanced moments are distributed among various members meeting at joint. Further during seismic event , it ensures strong column- weak beam concept to safeguard structure from total collapse.
Also considering cracked properties, the drifts in stories are near to realistic when structure under severe seismic event is in ductile state to resist .

However as regard distribution of seismic forces among lateral load resisting elements, (columns , shear walls ),  the distribution remains same wether Ig for all / Or  0.70 Ig for all as I forsee as designer from manual working domain .

For a shear wall in its longitudinal direction , considering 0.7 Ig  will definitely amount to higher  drift.



But IS 1893 do not include shear walls for considering cracked properties , the clause speak about columns and beams . However as enlightened by Er PV Gavarraju , Code on Tall buildings  do includeshearr walls  too for cracked properties. This will  definitely affect the results .
Shear walls are compression members but primarily these take care of horizontal forces .

[vikram.jeet]

Just to add in previous post, considering cracked properties for shear wall in longitudinal direction , the second order moments will add further.

As said in earlier postings, shear wall is much stronger than column entities , in its longitudinal direction, I feel some higher value of cracked factor could be assigned to it in strong direction. i.e.it could be around 0.90*Ig or so, as it's cracking is not the extent of column entities.