Innovative Bridge Concept
3
The stresses, deformations and associated bridge deck accelerations induced in
a bridge are increased and decreased under the effects of moving traffic. Quasistatic analysis can be usually carried out with classified vertical loads representing
normal rail traffic. Resulting static load effects are then multiplied by the dynamic
factor Φ (j) taking account of the dynamic magnification of stresses and vibration
effects in the structure, but unable to predict resonance effects from high speed trains.
Generally the dynamic factor Φ 1 is taken as either Φ 2 , according to the quality of
track maintenance and determinant length L Φ allowing these factors to be used for
other structural members with different support conditions.
1.1.2 Dynamic Behaviour Under Traffic Loadings
This simplified procedure cannot be adopted at all times for higher speed, as the
ratio of maximum stress due to a train crossing at greater speed to the maximum
stress due to the same train journey very slowly is usually a function of several
parameters. The susceptibility of appearance of resonance in a determined bridge
deck depends thus not only on the value of the train loads, but also on their spatial
distribution, the train speed, the rigidity of the convoy, the irregularities of the rail
and the natural frequencies of the deck itself. Therefore, complex modelling not only
of the bridge, but also of the train groups is used in order to properly analyse their
dynamic interaction. It is first important to make sure that the wheel and rail contact
is still maintained despite the oscillations of the structure and the train dynamic
trajectory As a result, the arising vertical acceleration onto axles and the track twist
due to the girder torsional movements have to be restricted. Secondly, it is also
necessary to check that the girders dynamic oscillations do not cause a reduction
in track stability or loss of track geometry. Safety and operational comfort aspects
impose more strict limitations on the values of the acceleration. To prevent any
discomfort when a train is crossing a bridge, passengers should not be subject to
excessive levels of vertical acceleration. These accelerations are generated, on the
one hand, by bridge oscillations and, on the other, by the damping from vehicle body
suspensions. Depending on the level of comfort required, the vertical accelerations
on vehicle bodies are limited to a value between 0.1, 0.13 or 0.2 g.
1.1.3 Serviceability Requirements
Safety and comfort are two major requirements to determine also the deformability
limits of rail bridges. Therefore, new design bridges should not be made more flexible
than existing ones. To check the permissible vertical deflection for speeds less than
200 km/h to minimise track maintenance and to avoid dynamic studies, α = 1.0
shall be adopted. Even if α = 1.21 or 1.1 was taken into consideration for ULS.
This simplified rule for speeds less than 80 km/h, permissible values for deflections
calculated under LM 71, multiplied by dynamic factor Φ, should not exceed L/600.
3
The stresses, deformations and associated bridge deck accelerations induced in
a bridge are increased and decreased under the effects of moving traffic. Quasistatic analysis can be usually carried out with classified vertical loads representing
normal rail traffic. Resulting static load effects are then multiplied by the dynamic
factor Φ (j) taking account of the dynamic magnification of stresses and vibration
effects in the structure, but unable to predict resonance effects from high speed trains.
Generally the dynamic factor Φ 1 is taken as either Φ 2 , according to the quality of
track maintenance and determinant length L Φ allowing these factors to be used for
other structural members with different support conditions.
1.1.2 Dynamic Behaviour Under Traffic Loadings
This simplified procedure cannot be adopted at all times for higher speed, as the
ratio of maximum stress due to a train crossing at greater speed to the maximum
stress due to the same train journey very slowly is usually a function of several
parameters. The susceptibility of appearance of resonance in a determined bridge
deck depends thus not only on the value of the train loads, but also on their spatial
distribution, the train speed, the rigidity of the convoy, the irregularities of the rail
and the natural frequencies of the deck itself. Therefore, complex modelling not only
of the bridge, but also of the train groups is used in order to properly analyse their
dynamic interaction. It is first important to make sure that the wheel and rail contact
is still maintained despite the oscillations of the structure and the train dynamic
trajectory As a result, the arising vertical acceleration onto axles and the track twist
due to the girder torsional movements have to be restricted. Secondly, it is also
necessary to check that the girders dynamic oscillations do not cause a reduction
in track stability or loss of track geometry. Safety and operational comfort aspects
impose more strict limitations on the values of the acceleration. To prevent any
discomfort when a train is crossing a bridge, passengers should not be subject to
excessive levels of vertical acceleration. These accelerations are generated, on the
one hand, by bridge oscillations and, on the other, by the damping from vehicle body
suspensions. Depending on the level of comfort required, the vertical accelerations
on vehicle bodies are limited to a value between 0.1, 0.13 or 0.2 g.
1.1.3 Serviceability Requirements
Safety and comfort are two major requirements to determine also the deformability
limits of rail bridges. Therefore, new design bridges should not be made more flexible
than existing ones. To check the permissible vertical deflection for speeds less than
200 km/h to minimise track maintenance and to avoid dynamic studies, α = 1.0
shall be adopted. Even if α = 1.21 or 1.1 was taken into consideration for ULS.
This simplified rule for speeds less than 80 km/h, permissible values for deflections
calculated under LM 71, multiplied by dynamic factor Φ, should not exceed L/600.