Impact Factors Applied to Bridge Loading

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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

Calculations of Impact Factor are Described Below as per IRC: 6-2000

For the IRC Class A loading



  • 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

Impact Factor 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%

Impact Factor 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-

Types of Road Bridges as Per Different Criteria

IRC 5-2015 [PDF download]

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

IRC 112-2011 [PDF Download]

External Resources-

Indian Road Congress

Indian Institute Of Bridge Engineers

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