wide beam punching shear checks

huangyhg

wide beam & punching shear checks
to my understanding there are 4 main checks on  concrete design.(not including overturning,sliding, or anchorage)
1. required steel for bending
2. bearing/crushing strength
3. wide beam/one-way shear
4. punching/ two-way shear
my confusion comes with the 1 & 2 way shear checks. i could see if you have an elevated slab supported by columns or a pile cap having 1&2 way shear failures, but if you have a slab on grade i just don't see how this shearing could happen. the way i see it is if a slab is sitting on compacted soil the only checks would be bending and crushing. the vertical load on the column will distribute through the slab and the soil will have reactions which would be a crushing failure. just wondering what i am missing here? greatly appreciate any direction on the issue. i have been assigned the task of checking a backlog of calculations and wondering if i am wasting my time checking the slabs on grade for shearing failures.
the biggest check that you have missed on the list is a check of deflections under service loading, this can be a governing check in the design.
any concentrated load on a concrete section can cause punching shear. for the slab on grade, if your soil does not have adequate bearing capacity and is required to distribute out over an area of soil to achieve the required bearing capacity, then the soil reaction on the slab can punch through.
i see it this way. the reaction offered by the compacted soil is similar to the uniformly applied load on an elevated slab. if an elevated slab is designed for punching shear then an upside down situation is no different.
if you have a large pedestal reaction (i'm thinking of a tall skinny vessel sitting on a round or octagon pedestal), then that load will likely be distributed over a much larger foundation area because of allowable soil bearing and / or settlement issues.
if the load in the pedestal gets too high, then it will tend to punch through in a classic two-way shear failure.  this is definitely a non-ductile failure.  will it result in total collapse like for an elevated slab?  perhaps not....if you exceed the allowable soil pressures underneath the punching area, that soil partially supports the pedestal.   that doesn't mean that you didn't experience a two-way failure... it just means that the consequences of the failure were slightly less severe than for an elevated slab.  
i'm not aware of any published case studies or such that show this type of failure for slabs, but it has always been a major part of my design for these types of foundations.

the checks need to be done because there are such kind of failures, only the load needs be big enough on a flexibe soil and you have them, or combined with flexure. were the thing a slab on unyielding rock, still the stresses in the 3d solid concrete would need be checked, then without the help of the standard checks allowed for plates. all is a matter of level of solicitation for the failures to appear.