Punching Shear / Pad Footings / Shear Rails
Punching Shear / Pad Footings / Shear Rails
(OP)
Hi all,
What's everyone opinion on dealing with punching shear on a pad footing? Increase depth to make the concrete do all the work? Provide closed ligs? Provide shear rails?
Current example: 1000 x 300 RC column coming down on a 3.5m x 3.5m x 1m pad footing with a working load of 6000 kN and ultimate load of 7500 kN. XW Sandstone with allowable BC = 500 kPa.
Now, I don't really want to call up a 1m deep footing as I find it a bit tricky to visualize how a pad footing will actually punch through, given the circumstances are very different to that of a slab.
I saw that I can reduce my load slightly by the amount of load that goes straight into the ground... shown below.

What's everyone opinion on dealing with punching shear on a pad footing? Increase depth to make the concrete do all the work? Provide closed ligs? Provide shear rails?
Current example: 1000 x 300 RC column coming down on a 3.5m x 3.5m x 1m pad footing with a working load of 6000 kN and ultimate load of 7500 kN. XW Sandstone with allowable BC = 500 kPa.
Now, I don't really want to call up a 1m deep footing as I find it a bit tricky to visualize how a pad footing will actually punch through, given the circumstances are very different to that of a slab.
I saw that I can reduce my load slightly by the amount of load that goes straight into the ground... shown below.







RE: Punching Shear / Pad Footings / Shear Rails
"it might depend on soil stiffness."
on rock, it might be acceptable as the compression force might go directly to rock without big footing 'flanges' deformation.
On soil, you get punching failure if the footing is not thick enough because soil will deform and distribute force all along the footing.
Still, I always check for punching on rock anyways just to be safe.
Anyone agree ???
RE: Punching Shear / Pad Footings / Shear Rails
RE: Punching Shear / Pad Footings / Shear Rails
RE: Punching Shear / Pad Footings / Shear Rails
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Punching Shear / Pad Footings / Shear Rails
RE: Punching Shear / Pad Footings / Shear Rails
RE: Punching Shear / Pad Footings / Shear Rails
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Punching Shear / Pad Footings / Shear Rails
RE: Punching Shear / Pad Footings / Shear Rails
1) If shear span to depth < 2, strut and tie with no punching shear.
2) If shear span to depth > 2, sectional method with punching shear.
By the book, we're allowed to use strut an tie for anything. However, using it for situations where the proportions don't necessitate it leads to a bunch of unnecessary work. And stirrups.
If you follow these guidelines, you won't ever get stirrups in a footing designed via STM. Certainly, I've done dozens of them without ever including stirrups. Trenno's shear span to depth ratio is 1.60/1.25.
Are punching shear failures in AU assumed to occur on 2:1 failure planes as suggested in the sketch at the top? In North america, the assumed surface is 1:1. At 1:1, the punching cone is darn near the entire footing which kind of makes the whole thing moot.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Punching Shear / Pad Footings / Shear Rails