Punching Shear Capacity 1

[CorporalToe]

Hello, I have a question regarding the punching shear capacity for a foundation footing. I am using ACI 318-14 Section 22.6.5.2 to determine vc (nominal shear strength), to get the shear force (Vc) from this I use Vc = vc * b0 * d, but I can't seem to find this equation in ACI 318. Could someone reference me this.

Also I have a follow-up question to this, why is it that you multiple vc by the b0 and d? b0 is the perimeter of the critical section and d in the effective depth but why does multiplying these by the nominal shear strength given you the shear force?

 

[hokie66]

Stress x area = force.

 

[lexpatrie]

See the FAQ:

ACI 318-14 (archive.org).

(please hold more detailed answer to come)

See 22.6.4.1, page 362.

 

[CorporalToe]

How does perimeter x depth = F? I am confused because b0 is the perimeter of the critical section.

 

[hokie66]

Perimeter x depth = area

 

[Nick6781]

22.6.4.1 tells you how to locate the critical perimeter for punching shear

 

[BulbTheBuilder]

You can compute your shear in stresses(ksi) or lumped into force (kips).

ACI 22.6.5.2 gives you stress (ksi). Force(kips) = stress(ksi) *area(sq-in), thus multiply the stress capacity(ksi) per the area(sq-in) gives you the shear force capacity (kips).

Example, if your shear capacity is 0.25ksi (ACI 22.6.5.2) and area is 3024sq-in (168"*18") your capacity is 756kips. You can reference figure R8.4.4.2.3 for the critical area.
As Hokie66 mentioned the [critical section perimeter * depth = Area]. Note, depth here is average slab effective depth

 

[EngDM]

Force(kips) = stress(ksi) *area(sq-in)

In my personal experience, I've found that breaking down and writing all components of the equation with their units makes it so much easier to interpret when trying to understand the equation and what components act with each.

 

[JoshPlumSE]

How does perimeter x depth = F? I am confused because b0 is the perimeter of the critical section.

vc is a limiting shear stress, right?

The punching shear perimeter is a linear value. Multiply that by depth and you have a shear area.

Therefore, punching shear stress * punching shear area = Punching Shear Force


Note, I'm not trying to be glib or make fun. Decades ago (man I'm old) I was confused by the calculation of vc. Especially when moment is included. So, my confusion then isn't much different than the confusion you talked about in your initial post. FWIW, it helps me to first think of punching shear capacity without ANY consideration of moment. Then I can see that the moment just serves to locally amplify the shear stress (vu) that you need to compare against vc.