Impact Factor | Dynamic Allowance in Design | Causes | Characteristics | Bridge Loads

Impact Factor

Vehicular Live loads are multiplied with an Impact Factor to accommodate this dynamic effect in the design of bridges. The dynamic loads for moving vehicles are considered Impact in bridge engineering because of the relatively short duration.

The magnitude of the impact factor depends on the bridge span, stiffness and surface roughness, and vehicle dynamic characteristics such as moving speed and isolation system. Impact factor is often referred as Dynamic Allowance, IM, Impact Load in different design codes.

Impact Factor is considered for Vibration induced by Truck on Bridge
Impact Factor is considered for Vibration induced by Truck on Bridge

Causes of Impact

The wearing surface of bridges are not usually perfectly smooth, thus the moving vehicle/train suspension must accommodate this roughness by compression and decompression of the suspension system or tires. Vehicles such as trucks and trains passing bridges at certain speeds will induce two major dynamic effects

  1. Global vibration and
  2. Local hammer effects

This oscillation creates additional axle forces that exceed the static weight of the vehicle when acceleration is upward. This is less than the static weight when the acceleration is downward. Although called impact, this phenomenon is referred to as dynamic loading.

Moving vehicles mainly causes vertical vibration of the bridge, Unlike earthquake loads which can cause vibration in bridge longitudinal, transverse, and vertical directions. Impact effect has influence primarily on the superstructure and some of substructure members above the ground because the energy will be dissipated effectively in members underground by the bearing soils.

The interaction between moving vehicles and bridges is a complex phenomenon. The dynamic effects of moving vehicles on bridges are accounted for by a dynamic load allowance or impact factor, in addition to static live load in most of the bridge design specifications.

Impact factor or Dynamic Allowance for various Limit states as per AASHTO LRFD

Limit StateImpact/ Dynamic Allowance
Deck joint75%
Fatigue and fracture limit state15%
All other limit states33%

Characteristics of Impact Factor

  • In most of the cases, Impact factor increases as vehicle speed increases.
  • Impact decreases as bridge span increases.
  • It increases remarkably with increasing surface roughness from “good” to “poor”.
  • Impact decreases as vehicles travel in more than one lane.
  • The probability of maximum dynamic response occurring at the same time for all vehicles is small.
  • Impact factor for exterior girders is much larger than for interior girders this is because the excited torsion mode shapes contribute to the dynamic response of exterior girders.

Impact factor in Different Design Codes

  • In British Standards, Impact factor is already considered in the vehicle definition. For railway bridges loading like RL loading, code provides formulas to calculate the dynamics factors.
  • Impact percentage are specified in Indian standards for Class AA and 70 R loadings to accommodate dynamic effects of Live loads.