Design for Shear in ACI 318-19 |
  
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Design for Shear in ACI 318-19 |
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ACI 318-19 introduced some new requirements for shear design.
One item is the concept of maximum permissible spacing of stirrup legs in the transverse direction (across the width of the beam). These requirements are presented in ACI 318-19 Table 9.7.6.2.2, and ENERCALC SEL reports the maximum permissible transverse spacing on Calculations > MVD Summary > Design Shear.
Another item relates to the calculation of shear strength of concrete.
ACI 318-14 says that a minimum area of shear reinforcing is required whenever Vu > phiVc/2.
ACI 318-19 says essentially the same thing by saying Av,min, shall be provided in all regions where Vu > ϕλ sqrt(f'c)bwd.
Both of these imply that Av,min is NOT required when Vu is below the corresponding strength limit from each code.
Here's where things diverge:
In ACI 318-14, the shear strength of concrete is specified in 22.5.5.1, and it has nothing to do with the relationship between Av,provided and Av,min.
However in ACI 318-19, there is a lot of discussion about the shear strength of concrete as specified in 22.5.5.1. The discussion points out that the shear strength of concrete depends on the relationship between Av,provided and Av,min.
To go a little deeper, Section 9.6.3.3 states:
9.6.3.3 If shown by testing that the required Mn and Vn
can be developed, 9.6.3.1 and 9.6.3.2 need not be satisfied.
Such tests shall simulate effects of differential settlement,
creep, shrinkage, and temperature change, based on a realistic
assessment of these effects occurring in service.
(For reference, 9.6.3.1 is Minimum shear reinforcement for nonprestressed beams.)
So now we know where it may be possible within the parameters of ACI to have a condition where Av,provided < Av,min.
And section 9.6.3.4 about Av,min says, "If shear reinforcement is required and torsional effects can be neglected according to 9.5.4.1, Av,min shall be in accordance with Table 9.6.3.4." This supports the notion that Av,min is only relevant in situations where shear reinforcing is actually required.
Putting all of this together, the Concrete Beam module never gets into the condition of line (c) in Table 22.5.5.1, where Av,provided < Av,min. And along similar lines, when the Av,min is not required, ENERCALC will call for no stirrups at all.
