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For RCC bridges, the smallest span beyond which the impact factor is same for class A or B or AA and 70R loading(wheeled vehicles) is
1. 9 m
2. 12 m
3. 36 m
4. 45 m

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Best answer
Correct Answer - Option 2 : 12 m

Explanation:

IRC Class A Loading

(i) This type of loading is used in the design of all permanent bridges. It is considered as standard live load of bridge. When we design a bridge using class AA type loading, then it must be checked for class A loading also.

IRC Class B Loading

(i) This type of loading is used to design temporary bridges like Timber Bridge etc. It is considered as light loading.

For IRC class A and class B loadings

Impact factor (I) = A/(B+L)

Where L = span in meters and A and B are constants

Bridge type

A

B

RCC

4.5

6.0

Steel

9.0

13.50

 

So for span=12 m Impact factor =4.5 / (6+12)=25%

For IRC Class AA Loading and 70R Loading

Span

Vehicle type

Impact factor

Less than 9 meters

Tracked vehicle

25% up to 5 m and linearly reducing to 10% from 5 m to 9 m.

Wheeled vehicle

25% up to 9 m

Greater than 9 meters

Tracked vehicle (RCC bridge)

10% up to 40 m

Wheeled vehicle (RCC bridge)

25% up to 12 m

Tracked vehicle (steel bridge)

10% for all spans

Wheeled vehicle (steel bridge)

25% up to 23 m

So from the above table and calculation we can conclude that For RCC bridges, the smallest span beyond which the impact factor is same for class A or B or AA and 70R loading(wheeled vehicles) is 12 m.

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