Two-way shear, or punching shear, is calculated for
a section that is a distance of
d/2 from the edge
of the concrete column or modified base plate.
The shear capacity is calculated using the smaller of the
values calculated according to ACI 318-14 Table 22.6.5.2:
where
βc
| = | Ratio of the long side to the short side of
the concrete column or modified base plate. |
αs
| = | 40 for interior columns, 30 for edge
columns, and 20 for corner columns.
Note: Currently
all checks are made assuming interior columns.
|
λ | = | Modification
factor to reflect the reduced mechanical properties of lightweight concrete
relative to normal weight concrete of the same compressive strength.
Note: The program uses the value provided by ACI 318-14 section
19.2.4.2 of λ = 1 for normal weight concrete.
|
bo
| = | Perimeter of critical section for two-way
shear in footing. |
In this case,
d is calculated
from the average reinforcement depth for the reinforcement parallel to the
major and minor axes of the footing.
Two-Way Shear
Critical Section
The ultimate shear,
Vu, on
the critical section is calculated as follows:
| Equation 4-13 |
| Equation 4-14 |
Note: Equation 4‑14 is
derived from the commentary for ACI 318-95 Section 11.12.6.2. Also see Figures
below.
| Equation ACI 318-14 8.4.4.2.2 |
| Equation ACI 318-14 8.4.2.3.2 |
| Equation ACI 318-14 8.4.2.3.2 |
| Equation 4-15
|
| Equation 4-16 |
where
Pu_Soil
| = | Total soil force underneath critical area for
two-way shear. |
CL
| = | Column dimension parallel to the footing local
major axis. |
CW
| = | Column dimension parallel to the footing local
minor axis. |
b1
| = | Width of critical section measured in the
direction of the span for which moments are determined (in units). |
b2
| = | Width of critical section defined in the
direction perpendicular to . |
Note: Equation 4‑14 and
Equation 4‑13 are derived from the commentary for ACI 318-14 Section 8.4.4.2.3.
The footing is taken to be acceptable in two-way shear if
ACI 318-14 14.5.1.1(c) is satisfied.
Note: Currently only the
two-way shear using
αs = 40 is
used. If the supported column is close enough to the footing edge or corner,
the user will need to do supplemental checks to confirm that the footing
capacity is acceptable if the assumption for
αs is
not valid.
Two-Way Shear
Critical Section Dimensions
Stress Distribution
Around Critical Section