Impact factors are introduced to amplify the effect of dynamic loading such as vehicular load to enhance the magnitude of the load.

Generally, the impact factors are applied as a fraction of dead load. For the case of India, the value and

## calculation of impact factor are described below as per IRC: 6-2000

### For the IRC Class A loading

I=A/(B+L)

Where,

- I = Impact factor fraction
- A= A constant having the value of 4.5 for reinforced concrete bridges and 9.0 for steel bridges
- B = A Constant having a value of 6.0 for reinforced concrete bridges and 9.0 for steel bridges
- L = Span in Meters

### For the IRC Class AA or 70 R Loading

#### For spans less than 9 m

For tracked vehicle – 25% of the span up to 5 m linearly reduced to 10% for span of 9 m

For wheeled vehicle – 25%

#### For spans of 9 m and above

For tracked vehicles on RC Bridges, the value of impact factor is 10% up to a span of 40 m and for bridge built up of steel truss 25% for spans up to 23 meters.

### Values of Impact Factor allowance for the British standard loading could be calculated as follows.

**For HA loading, the British standards are giving an allowance of 25% of the heaviest loading axle** of the loading train from which we calculate the loading

Surprisingly the British standards do not stipulate the **impact allowance for HB loading** as it is considered as abnormal loading.

**Read Also-**

**IRC:6-2017 [Loads and Load Combinations] PDF Download**

**External Resources-**