# Design of RC Structures Questions and Answers – Composite Sections Analysis by Elastic Theory

This set of Design of RC Structures Multiple Choice Questions & Answers (MCQs) focuses on “Composite Sections Analysis by Elastic Theory”.

1. What is the formula for equivalent area of concrete?
a) Ac+mAs
b) As+mAc
c) As+Ac
d) Ac-mAs

Explanation: Equivalent area of concrete = Ao = Actual Concrete area + m * Steel area
Since m is the modular ratio between Steel and Concrete.

2. Steel Member is subjected to greater load than concrete.
a) True
b) False

Explanation: We know that
Ws= m/{(m+1)W}
Wc= 1/{(m+1)W}
If m=18
Then, Ws= 18.W/19
Wc= 1.W/19

3. Why is Steel used as reinforced material in RCC?
a) Steel provides tensile strength to the structure
b) Steel provides compressive strength to the structure
c) Thermal coefficient of steel is less than that of concrete
d) Steel is not reactive to water

Explanation: As per the nature of steel and concrete, steel has tensile strength and concrete has compressive strength so they are joined together to form a perfect bond.

4. What is the modular ratio of M35 grade concrete?
a) 8.1
b) 24.34
c) 9.3
d) 10.98

Explanation: m= 280/3σcbc
And σcbc for M35 is 11.5

5. What is the meaning of M25 in M25 grade concrete?
a) The strength of concrete is 25N/mm2 after 28 days of casting
b) The strength of concrete is at least 25N/mm2 after 28 days of casting
c) The strength of concrete is at least 25N/mm2 after 7 days of casting
d) The strength of concrete is 25N/mm2 after 7 days of casting

Explanation: M stands for mixed concrete and mix design is done in such a way that it gains at least the given or specified strength after 28 days of casting.

6. What is the nature of 20mm diameter bars in the following figure?

a) Compression
b) Tension
c) Same as of 25mm steel
d) It can resist all types of forces

Explanation: It is known that the steel which is placed on the top of the structure is known as compression steel.

7. Write the formula for stress in steel.
a) Fc= W/(Ac+mAc)
b) Fc= W/(Ac+mAs)
c) Fc= W/(As+mAc)
d) Fc= W/(As+mAs)

Ac = Area of concrete
m=Modular Ratio

8. If M20 grade concrete and Fe 415 grade steel is used in a rectangular beam of 560mm effective depth, then what will be its depth of critical neutral axis?
a) 161.62
b) 140.35
c) 230.25
d) 120.42

Explanation: σcbc / (σst/m) = xc / (d-xc)
7/ (230/13.33) = xc / (560-xc)
Xc = 161.62

9. Which of the following dimension is not used in steel reinforcement?
a) 16mm diameter
b) 25mm diameter
c) 20mm diameter
d) 24mm diameter

Explanation: 24mm diameter steel bars are not generally available in market.

10. What is the factor of safety for concrete?
a) 3
b) 1.78
c) 6
d) 2.3

Explanation: As per IS 456, factor of safety is 3 for concrete and 1.78 for yield strength of steel.

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