Question

Consider the following statements:
In an under-reinforced concrete beam,

1. actual depth of neutral axis is less than the critical depth of neutral axis.
2. concrete reaches ultimate stress prior to steel reaching the ultimate stress.
3. moment of resistance is less than that of balanced sections.
4. lever arm of resisting couple is less than of balanced sections.

Which of these statements is/are correct?

• A. 1 and 2
• B. 1 and 3
• C. 2, 3 and 4
• D. 1 and 4

Answer 1

The correct option is B 1 and 3

For under-reinforced beam

(i) $x_u<x_{u,lim}or{x}_c$

(ii) Steel reaches ultimates stress before concrete reahing the ultimate stress

(iii) $M_u<M_{u,lim}$

(iv) Lever arm $=d-0.42x_u$ will be more in the case of under-reinforced section.