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RE: Shear Failure of shallow foundation

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Yes got 4 copies.

Also posted my guess in answer to the question asked.

My guess was based on design of punches for sheet metal work, not quite the
same: as involves punch and die, along with clamp, and dependent on
clearance between die and punch. If clearance equals thickness of sheet
metal then have potential for bending and deep drawing, slightly less then
get ironing which helps sets the form and reduce spring back. There has to
be very fine clearance to actually punch a hole through the sheet metal and
relatively high clamp force on the sheet. For deep drawing a low clamp force
is required to prevent buckling and wrinkling of the sheet, yet still allow
material flow into the die.

If the conditions are right for punching, then a crack propagates ahead of
the punch. Typically the punch penetrates about 2/3 rds of the depth
producing a smooth surface, and the crack propagates the last 1/3 rd
producing a rough crystalline facture surface, at 45 degrees, which
depending on application may or may not then require reaming to remove.
Since the fracture plane is undesirable some effort is taken to limit to the
1/3rd depth. But getting it right is a lot more art than science.

In any case my rough answer is that a 45 degree failure plane less than the
depth of the concrete slab away from face of column, would partly cut the
column, thus the shear plane is not entirely in the slab. For punching,
expect some penetration of the column into the slab, and a facture plane to
start to propagate at some depth into the slab, say half the depth of the
slab and still 45 degrees.

But without the die and clamp, likely to get a lot more bending and
stretching of the slab, than pure shear.  Also foundation suggests support
by the ground below, so to punch column through slab have to displace soil
below.

But as I say, just a guess since not all that familiar with concrete design.
So may be someone more familiar with concrete can provide a more suitable
explanation.



Regards
Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
mailto:sch.tectonic(--nospam--at)bigpond.com
Adelaide
South Australia





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