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Posted: Tue Nov 27, 2012 7:28 pm Post subject: Secondary elements as part of strucutral system for high ris
I remember from some paper I was reading that there is a push in ACI (based on lessons learnt from the Chile EQ) to include the ground floor slab as a part of the lateral force resisting system. This is an offen ignored elemnts. In India especially engineers take the easy way out (read coeff method of IS456), being totally oblivioud to the complexities involved in proper design.
For example at the ground floor of a high rise that is connected to the extended basement, the base shear transfer happens thru the flat slab. Further the retainiing walls finally take the lateral loads. In many cases there are liberal punctures/ discontiniuities in these.
People are going 120 m plus without an expansion joint on the ground loor. How the load transfers fron the shear walls to the peripheral retaining walls is "ram bharose" Till the load gets to the walls there are slab folds, large openings & of course missing wall segments. This should be properly accounted for.
Slabs at higher levels sometimes act like coupling beams. However these are rarely designed
Axial shortening of columns being discussed in other strings is also an example of this. Slabs get advrsely affected. Top steel is no longer required over supports due to axial shortening, instead top steel is provided (conventionally).
I have seen consultants design extended basement slabs with secondary beams. When the analysis is seen in a slab design program based on FE there is tension in the slab at the bottom on either side of a mid span beam. However as per coefficient method of a code tension is to be provided at top of a secondary beam. Besides the bottom bars of the slabs were discontinuous at the point of max, moment!
These ideas could be brougt into the IS codes in due course after proper deliberation.
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