If d and n are the effective depth and depth of the neutral axis respectively of a singly reinforced beam, the lever arm of the beam, is A. d B. n C. $${\text{d}} + \frac{{\text{n}}}{3}$$ D. $${\text{d}} – \frac{{\text{n}}}{3}$$

The correct answer is C. $${\text{d}} + \frac{{\text{n}}}{3}$$

The lever arm of a beam is the distance from the neutral axis to the tension or compression force. The effective depth of a beam is the distance from the top of the beam to the neutral axis. The depth of the neutral axis is the distance from the top of the beam to the centroid of the cross-section.

The lever arm of a singly reinforced beam can be calculated using the following formula:

$$L = d + \frac{n}{3}$$

where:

• $L$ is the lever arm
• $d$ is the effective depth
• $n$ is the depth of the neutral axis

The lever arm is important because it determines the bending moment capacity of the beam. The greater the lever arm, the greater the bending moment capacity.

Option A is incorrect because it is the effective depth of the beam, not the lever arm.

Option B is incorrect because it is the depth of the neutral axis, not the lever arm.

Option D is incorrect because it is the negative of the lever arm.