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A four-axle vehicle runs over a still standing track, i.e. all sleepers belongs to an Euler coordinate system of type e_fix. Also the masses which builds up the bridge model belongs to coordinate system e_fix. The rails are modeled with the beam_3-element.
In the beginning of the simulation there are fluctuations in the vertical track forces. The reason for these fluctuations is that no initial values have been calculated. When the simulation starts all degrees of freedom are in their zero position. The wheels are just touching the rails, but no vertical track forces are produced. When the simulation starts the vehicle falls down, until the vertical track forces corresponds to the weight of the vehicle. After approx 0.1[s] the vertical track forces are stable.
At low speed the vertical track forces are fairly constant. The dynamics of the vertical track forces for this case is low. The vertical wheel/rail forces are illustrated in the four vertical white arrows.
When a wheelset is over the bridge it can be seen that it deflects downwards.
Animation of the low speed case:
At higher speed there are more dynamics in the vertical track forces.
Before the first wheelset enters the bridge, it can be seen that the vertical track forces not are stable. The reason for the varying track forces is that the vertical track stiffness is not constant. The track model consists of discrete sleepers, with a sleeper distance of 0.65[m]. The track is vertically stiffer when a wheelset is above a sleeper, compared to when the wheelset is in between two sleepers.
When the first wheelset enters the bridge it can be seen that the vertical wheel forces reduces,
and the wheelset falls downwards.
Just before the pillar, when the vertical stiffness of the bridge increases,
a peak in the vertical forces occurs, and the wheelset moves upwards.
When passing the pillar the vertical forces reduces and the wheelset falls down again.
A second peak in the vertical forces occurs when the wheelsets leaves the bridge, and the wheelset is forced to rise up to the same vertical position as before the bridge.