COST 334

The vertical dynamic loads are determined by the vertical vehicle motions and itssuspension system. Horizontal dynamic loads are determined by the forces needed for e.g.acceleration, braking, steering or ascent of the vehicle.Due to the definition of the load scale, at equal loads there is no difference betweendifferent tyres at this scale (although different tyres may cause different dynamic loads atthe same level of static load). Therefore it falls mainly outside of the scope of COST 334,as stated in section 4.2.At a distance from the load, the main influencing factor for the stresses and strains in thepavement is the magnitude of the wheel load, independently from the way it is applied(this follows from the application of St. Venant’s principle, see 4.3.5.1). This isparticularly the case for the lower pavement layers and for the subgrade of the pavement.(Huhtala et al 1989, Halliday et al 1997, among others). So, at this distance, the stressesexhibit no noticeable difference between the tyre types (at equal wheel load) and, thus,will not be further discussed within this chapter. However, this distance (where only theload magnitude counts, and not its distribution) may well be larger than the pavementthickness, especially for thin pavements (Huhtala et al. 2000a). In those cases, the stressesand strains which are relevant for asphalt fatigue or rutting in granular layers (see 4.3.5)are influenced by the tyre-pavement contact area and the stress distribution thereupon,which are discussed in the following sections.4.3.4.3 The tyre scaleAt this scale, distinction is made between the number of areas (tyre footprints) over whicha load is distributed. This distinguishes between one contiguous area for wide base andordinary single tyres, and two areas, separated by some distance, for dual tyre assemblies.The size of these contact areas, or the stress distribution within these areas is notconsidered at this scale.The significance of this scale lies in its influence on the stresses and strains in thepavement. The distance between the tyres of a dual assembly (and the absence of suchdistance for a wide base single) widens the area over which the load is distributed,reducing stresses and strains at many points in the pavement structure.24version 29 November 2001