2. Bar Stress Near Tension Face Equal to Zero, (ε_s = f_s = 0)

Figure 3 - Strains, Forces, and Moment Arms (ε_t = f_s = 0)

Strain ε_s is zero in the extreme layer of tension steel. This case is considered when calculating an interaction diagram because it marks the change from compression lap splices being allowed on all longitudinal bars, to the more severe requirement of tensile lap splices.     ACI 318-19 (10.7.5.2.1 and 2)

2.1. c, a, and strains in the reinforcement

c = d₁ = 13.5 in.

Where c is the distance from the fiber of maximum compressive strain to the neutral axis.    ACI 318-19 (22.2.2.4.2)

    ACI 318-19 (22.2.2.4.1)

Where:

a = Depth of equivalent rectangular stress block

    ACI 318-19 (Table 22.2.2.4.3)

    ACI 318-19 (Table 21.2.2)

    ACI 318-19 (22.2.2.1)

2.2. Forces in the concrete and steel

    ACI 318-19 (22.2.2.4.1)

The area of the reinforcement in this layer has been included in the area (ab) used to compute C_c. As a result, it is necessary to subtract 0.85f_c’ from f_s’ before computing C_s:

2.3. ϕP_n and ϕM_n