5. Bar Strain Near Tension Face Equal to εy + 0.003
Figure 6 - Strains, Forces, and Moment Arms (εs = 0.00507 in./in.)
In ACI 318-19 provisions, this control point corresponds to the tension-controlled strain limit of ɛy + 0.003 (used to be 0.005 in ACI 318-14). It is the strain at the tensile limit of the transition zone for ϕ, used to define a tension-controlled section.
Additional resources concerning code provision changes in ACI 318-19 can be found in “ACI 318-19 Code Revisions Impact on StructurePoint Software” technical article.
The following show the general procedure to calculate the axial and moment capacities of the irregular wall section at this control point, all the calculated values are shown in the next Table.
5.1. c, a, and strains in the reinforcement
Where c is the distance from the fiber of maximum compressive strain to the neutral axis. ACI 318-19 (22.2.2.4.2)
a = Depth of equivalent rectangular stress block
5.2. Forces in the concrete and steel
Since a = 125 in. > h1 = 14 in., the area and centroid of the concrete equivalent block (see the previous Figure) can be found as follows:
If the reinforcement layer is located within the depth of the equivalent rectangular stress block (a), it is necessary to subtract 0.85fc’ from fs,i before computing Fs,i since the area of the reinforcement in this layer has been included in the area used to compute Cc.
The force developed in the reinforcement layer (Fs,i) is considered as compression force (Cs,i) if the effective depth of this steel layer (di) is less than c (the distance from the fiber of maximum compressive strain to the neutral axis), otherwise it is considered as tension force (Ts,i).
