Punching Shear / Pad Footings / Shear Rails

By intuition, I would say this (not validated by any textbook or codes !)

"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 ???

Just make it thick enough to satisfy all code required checks. Reinforcement for shear in pad footings is not a good idea. And cog those bottom bars.

I would use shear ties. It's not high strength rock. Normally the geotech would give you 1000-2000kpa so you can strut the load straight into the rock instead of relying on such wide outstands. I have found in Australia the preference is to use deformed bar for shear reo than studrails.

Your assumption of disregarding the load component that is beneath the punching cone is standard and should be fine. The proportions scream strut and tie design. One nice feature of that: no punching shear. You may still need a thick footing with hooked bars however.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

That's one thing I've always wondered KootK - when you go down the STM road, do you still need to consider punching shear...

Would you look at a kind of 4-pronged STM model for a single column coming down to a pad footing?

I would. My office has a spreadsheet that tries out a bunch of alternatives regarding the number of struts and then designs based on the optimum. It's a bit crazy for just a pad footing design really. The procedure is suggested in the "Design Aids" section of the Canadian concrete code. They'll let you do the Park and Paulay lever arm method too.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

yeah, but the greater than strut angle is the less efficient it becomes, and then it drives you to provide tie reinforcement which get's you back to providing shear reo.

The way it works in my neck of the woods is as follows:

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.

as KootK mentioned at the end of his last post. My general design method for footings is to choose a depth that is equal to the distance from edge of column to edge of pad. i.e. depth=(b-t)/2 where b is footing width and t is column width. That is the 1:1 slope he mentions and then I don't need any reinforcing. If constrained in depth (or realism of footing depth) then I look harder at STM or sectional method with punching and one way shear.