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Transfer Slab - Shear Check 2

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BOCH

BOCH

Structural
Aug 28, 2013
32
Hello guys,
I have a concern regarding transfer slabs design; I have a 360m height tower which include a 4000mm thick transfer slab at intermediate level,
- While checking shear, shall I check it with normal shear check (φVc = φ x 0.17 x SQRT (f’c) x b x d) or it can be considered like a deep beam and check the corresponding shear (φVc = φ x 0.83 x SQRT (f’c) x b x d)?
- Does it make sense to check the punching shear at this kind of slabs (transfer which includes supported columns-walls and above transferred columns-walls)? Is there any risk for punching in this is case?
Thank you.
 
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Boch,

How would we know if you do not tell us the spans of the transfer slab.

Normally a transfer slab will be sized based on shear requirements, and yes punching shear could be critical.

I would not be trying to take short cuts on something like this. If it is close to a Deep Beam, I would be designing it both ways and adopting the worst case solution!
 
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BOCH,
Rapt has given a concise and perfect answer to your question.
 
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Agree with RAPT, Also agree shear requirements typically dictate the thickness.


"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976
 
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Hello guys,
Thank you for your reply.
The transfer slab span is 12000mm (span over thick is less than 4, that's why i am asking this question), this is not a matter of short cuts, i am in front of a case on which we can apply shear requirements for deep beams which is more than normal shear by 0.83/0.17 = 4.88 times! (just trying to be consistent with the code).
 
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The use of Strut and Tie analysis is, I believe, standard practice for design of transfer slabs with a 3/1 span/depth ratio. Beam theory is not appropriate.

Where do those 0.83 and 0.17 numbers come from?
 
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These are from the metric version of ACI code, when using f'c in MPA the factor is 0.17 same like when using f'c in PSI the factor is 2.
 
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OK, but I still don't know where the 0.83 comes from. Is that a specific clause in the ACI code, or is it what you have derived?
 
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It is stated in ACI 11.7.3 that "Vn for deep beams shall not exceed 0.83xSQRT(f'c)xbxd"
 
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You are mixing/confusing Vc with the upper limit for Vn imposed for deep beams. Vc is beam shear taken by concrete only, Vn is limit on total shear.
 
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