Question

Two rectangular under-reinforced concrete beam sections X and Y are similar in all aspects except that the longitudinal compression reinforcement in section Y is 10% more. Which one of the following is the correct statement?

• A. Section X has less flexural strength and is less ductile than section Y
• B. Section X has less flexural strength but is more ductile than section Y
• C. Section X and Y have equal flexural strength but different ductility
• D. Section X and Y have equal flexural strength and ductility

Answer 1

The correct option is A Section X has less flexural strength and is less ductile than section Y

Both the sections i.e. x and y are under reinforced. So, in both the cases the flexural strength (MOR) can be calculated from tension side.

When additional compression reinforcement is placed i.e. in cases of cross-section y, neutral axis will shift upward.
(Strain Diagram)

MOR = Tension $\times$ Lever arm

In case of cross section y, lever arm is more as depth of neutral axis is less. However, tensile force is constant in both cases as the steel would have yielded in both cases. Hence flexural capacity of 'y' is more.

Also, as section y is more strong in compression, yielding in steel will be more at limit state of collapse.

Hence option (a) is correct.