Flat slab designing for shear- peak shear or average shear?
Flat slab designing for shear- peak shear or average shear?
(OP)
If you have a flat slab, and you are checking punching shear. you have made a FEA model of the slab, and you see there is a peak shear force. 8 times larger than the average along the perimeter. do you design for the average or the peak? will some sort of redistribution occur?
thanks sharp910sh
thanks sharp910sh






RE: Flat slab designing for shear- peak shear or average shear?
FEA programs will show a very high peak shear, which tapers off very quickly, around a support. But in order for a punching shear failure to occur, the entire shear perimeter must fail.
DaveAtkins
RE: Flat slab designing for shear- peak shear or average shear?
RE: Flat slab designing for shear- peak shear or average shear?
I have included the sketch of what im getting.
RE: Flat slab designing for shear- peak shear or average shear?
RE: Flat slab designing for shear- peak shear or average shear?
For example, for cases clearly defined in the codes (and should there be scarce ones that are not, but still there are, and many times quite intentionally) you can proceed as the code says and if in the end you see the thing correct, end of the issue. DaveAtkins correctly points that following the code normally may relieve you of the problem. In this case he comments as well on why, so at the same time is providing supporting engineering judgement for the code.
You may discover some conceptual error that leads to the problem, as hokie66 says, since at 50 kgf/cm2 shear stress ordinary concrete may be (at least conceptually) utterly cracked and just be rubble within cages (there have been failures where such kind of concentration of stresses have initiated locally a failure that from then the remaining reinforcement has not been able to restrain). Then, for the case, the scarce thickness leads to high (and quite likely unacceptable from a code standpoint) shear stresses, and the deformations are also high contributing to the concentration of stresses and your problem. Augmenting the thickness may reduce the problem to more acceptable averaging.
RE: Flat slab designing for shear- peak shear or average shear?
RE: Flat slab designing for shear- peak shear or average shear?
RE: Flat slab designing for shear- peak shear or average shear?
RE: Flat slab designing for shear- peak shear or average shear?
Similar if you have the support modeled as a point instead of a frame element
RE: Flat slab designing for shear- peak shear or average shear?
I don't totally agree with DaveAtkins about averaging over the the entire perimeter. I agree that you do not need to design for the peak, but averaging over the entire perimeter may be non conservative. ACI 421.1 says "When the maximum vu occurs at a single point on the critical section, rather than on a side, the peak value of vu does not govern the strength due to stress redistribution (Brown and Dilger 1994). In this case, vu may be investigated at a point located at a distance 0.4d from the peak point. This will give a reduced vu value compared with the peak value; the reduction should not be allowed to exceed 15%."
RE: Flat slab designing for shear- peak shear or average shear?
2) Not sure if I understand correctly but it is normal that you have a peak in one of the four corners of the critical perimeter because of the effect of the moment in both direction added to the averaged shear stress!.
RE: Flat slab designing for shear- peak shear or average shear?
I would never directly correlate the Finite Element analysis through-shear stresses from the mesh of shells to code allowable shear stresses.