Lever Arm (Internal Moment Arm)

The lever arm (z, sometimes jd) is the distance between the line of action of the resultant compressive force in the concrete and the resultant tensile force in the reinforcement of a flexural member. The section resists the applied moment as an internal couple M = T·z = C·z, so the lever arm directly converts moment into the required steel: Ast = M / (0.87 fy z) at limit state.

For a singly-reinforced rectangular section at limit state, z = d − 0.42 xu (d = effective depth, xu = neutral-axis depth), and is capped at z = d − 0.42 xu,max for a balanced section. A useful preliminary value is z ≈ 0.9d. The deeper the section (larger d) the larger the lever arm and the less steel needed for the same moment — the core reason depth is structurally more efficient than reinforcement for resisting bending.