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.