Question

The following two statements pertain to a simply supported under reinforced concrete beam. Select the correct option with regard to these statements.

1: The failure of the beam takes place due to the yielding of steel.

2: The moment of resistance of the beam is more than that of the balanced section of the beam.

1. Statement 1 is true but Statement 2 is false
2. Statement 1 is true but Statement 2 is true
3. Both statements 1 and 2 are true
4. Both statements 1 and 2 are false

Answer 1

Correct Answer - Option 1 : Statement 1 is true but Statement 2 is false

Explanation:

Under Reinforced Beam:

• Reinforced concrete beam sections in which the steel reaches yield strain at loads lower than the load at which the concrete reaches failure strain are called under-reinforced sections.
• Every singly reinforced beam should be designed as under-reinforced sections because this section gives enough warning before failure.
• Yielding of steel in the under-reinforced beam section does not mean the structure has failed, as when steel yields, excessive deflection and cracking in beam will occur before failure which gives enough time to occupants to escape before the section fails.
• The failure in the under-reinforced beam section is due to the concrete reaching its ultimate failure strain of 0.0035 before the steel reaches its failure strain which is much higher 0.20 to 0.25.

The characteristics of under-reinforced section and over reinforced section when compared with the balanced section are given below in the tabulated form.

Under-reinforced section when compared with Balance section	Over-reinforced section when compared with balance section
Steel yields first than concrete	Concrete will reach to maximum stress first
Reinforcement is less than a balanced section	Reinforcement is more than a balanced section
Primary tension failure	Primary compression failure
Ductile failure	Brittle failure
Depth of neutral axis shift upward i.e. it is less than that of balanced section	Depth of neutral axis shifts downward i.e. it is more than that of balanced section
Strain in steel is greater than that of balanced section	strain in steel is less than that of in balanced section
More deflection because it has less steel	Less deflection because it has more steel
Moment of resistance is less than that of balanced section	Moment of resistance is more than that of in balanced section