The 'impact factor' for bridge girder design for short spans is applied because:
Choose the correct answer
Bridges vibrate more than roads
Dynamic amplification of wheel loads due to shock, vibration, road surface unevenness — dynamic load = static LL × (1+I)
Concrete is stronger than steel
To account for wind load only
Correct Answer
B. Dynamic amplification of wheel loads due to shock, vibration, road surface unevenness — dynamic load = static LL × (1+I)
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Impact factor (I): additional dynamic load due to vehicle shock, vibrations, and pavement unevenness. I = 4.5/(6+L) for spans up to 3 m (I approaches 1 for very short spans); I = 9/(13.5+L) for spans 3–45 m. Total dynamic live load = static LL × (1+I). Longer spans have less impact (mass dampens shock). Pedestrian bridges: no impact.
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