6. Pure Bending

Figure 7 - Strains, Forces, and Moment Arms (Pure Moment)

This corresponds to the case where the nominal axial load capacity, P_n, is equal to zero. Iterative procedure is used to determine the nominal moment capacity as follows:

6.1. c, a, and strains in the reinforcement

Try c = 3.25 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:

    ACI 318-19 (Table 22.2.2.4.3)

    ACI 318-19 (22.2.2.1)

    ACI 318-19 (Table 21.2.2)

6.2. 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:

6.3. ϕP_n and ϕM_n

The assumption that c = 3.25 in. is correct.