enter image description here Might there a difference in design approach for punching shear for the two scenarios? Or something to pay particular attention to?

Is the slope of the shear plane (normally 1:1) the same for both scenarios? I would assume that it is, since the two design codes I usually use (Eurocode and SABS) do not differentiate between the two scenarios.

  • $\begingroup$ Just as a comment because I have no authoritative answer to give, but it seems reasonable to me that the behavior in both cases would be at least similar. $\endgroup$ – Wasabi Aug 31 '16 at 21:32
  • $\begingroup$ And for what it's worth, the Brazilian code also does not seem to differentiate between these cases. $\endgroup$ – Wasabi Aug 31 '16 at 21:46
  • $\begingroup$ I agree, since the forces have to be in equilibrium, it does not matter which way is up (or down). On the other hand, I attended a seminar on durability of concrete and it was mentioned there that the modulus of elasticity of concrete is different at the top of a slab, compared to the bottom. The magnitude of the discrepancy depends on aggregate properties as well as mixing, placing and compaction. Apparently this is being researched in more detail in South Africa to more accurately predict deflection,especially for slabs. $\endgroup$ – NamSandStorm Aug 31 '16 at 21:52
  • $\begingroup$ I take it that there is no difference for the two scenarios. Wasabi, would you like to add the authoritative answer you spoke about? $\endgroup$ – NamSandStorm Sep 21 '16 at 6:51
  • $\begingroup$ I have no such answer. What I said about the Brazilian code is simply that it doesn't differentiate between the cases, but simply explains what to do in cases of punching shear (just as the Eurocode and SABS). $\endgroup$ – Wasabi Sep 21 '16 at 10:04

As an honours civil engineering student I have been taught by three different doctors of civil and structural engineering that there is no difference between the two scenarios in design practice. What matters is how you prevent actual punching of slabs using collars, rebar design and other methods.

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