G B

Figure 6.7: Transverse distribution of actions by the sleepers and ballast, track without cant

(3) On bridges having ballasted track with cant the actions may be distributed transversely as shown in Figure 6.8. Load distribution under the rails may be

Figure 6.8: Transverse distribution of actions by the sleepers and ballast, track with cant

6.3.5.4 Equivalent vertical loading for earthworks

(1) In the absence of more accurate calculations, the equivalent vertical loading for earthworks under the track may be taken as uniformly distributed over a width of 3,00 m at a level 0,70 m below the running surface of the track.

(2) No dynamic effect needs to be applied to the above uniformly distributed load.

6.3.6 Footpath loading

6.3.6.1 Non-public footpaths

(1) Non-public footpaths are used only by authorised persons.

(2)P Pedestrian and cycle loads shall be represented by an uniformly distributed load with a characteristic value q^ - [5] kN/m2. This load should be applied over the length and width of the footpath which gives the most adverse effect.

6.3.6.2 Public footpaths

(1) The loading on public footpaths shall be in accordance with section 5.

6.4 Dynamic effects

6.4.1 Introduction

(1) The static stresses and deformations induced in a bridge are increased and decreased under the effects of traffic by the following:

- the rapid rate of loading due to the speed of traffic crossing the structure and the effects of inertia of the structure which are not taken into account in static calculations,

- variations in wheel loads resulting from track or wheel irregularities,

- the passage of successive loadings with approximately uniform spacing which can excite the structure and, in certain circumstances, create resonance (where the frequency of excitation matches the natural frequency of the structure, there is a possibility that the vibrations caused by successive axles running onto the structure will be excessive).

(2)P For structural analysis (stresses, deflections, etc) these effects shall be taken into account.

6.4.2 Factors influencing dynamic behaviour

(1) The following are the principal factors which influence dynamic behaviour:

- the natural frequency of the structure,

- the spacing of axles,

- the traffic speed across the bridge,

- damping of the structure

- regularly spaced supports of the deck slab and construction (cross girders, sleepers, ...)

- vertical imperfections of the track

These factors are taken into account as described in 6.4.3 and 6.4.4 below.

6.4.3 Dynamic factor $ {fa, <J>3) 6.4.3.1 Field of application

(1)P The dynamic factor takes account of the dynamic magnification of stresses and vibration effect in the structure but do not take account of resonance effects and excessive vibrations of the deck.

The dynamic factor applies only for speeds V * [220] km/h and where the natural frequency of the structure is within the limits shown in Figure 6.9.

The upper limit of n0 is given by n0 =94,76 x L*0'748

The lower limit of n0 is given by for 4m £ L £ 20m no =23,58 x L-°'592 for 20m < L £ I 00m

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