Longitudinal loading on Ballasted railroad bridges 1

[SDBRG]

I am a frustrated highway bridge engineer trying to figuure out how to distribute the longitudinal loading from a E80 cooper load from the track through the ballast onto the bridge deck. According to the AREMA manual the load is distributed with respect to the relitive stiffness of the members, which is great but how is it distributed thourgh the ballast to the members? It seems to me the load would reduce as some of it would be absorbed by the ballast. By the way the bridge is 3-span with the members simply supported. Any help on this would be appreciated.

 

[Qshake]

I am sorry not to be of much help, but it does seem to me that whether the ballast reduces the load isn't the main question to the code writers. Rather it was how much should the longitudinal load be reduced? Since the answer probably isn't known quite exactly an arbitrary percentage is used. For example in highway bridges we use 5%. Where did that come from and is it really 5% or is it more or less doesn't seem to be real important as accounting for some load in that direction.

 

[SDBRG]

Qshake, in the AREMA manual there is no referance as to how much the longitudinal force can be reduced due to the ballast, simply that the force shall be distributed to the various members of the supporting structure. But in order to do this the force needs to be transmitted from the rail to the ties into the ballast and finally onto the structure itself. The formula for calculating the force at the railing is specific an takes into accout the frictional value between the rail and the train.

In your opinion what would be a reasonable vaue to reduce the force? Since the ties are acting aganinst the passive resistance of the ballast and that transmission to the structure is probably limited by a frictional value of the ballast against the concrete would a frictional value of sliding for ballast against concrete make sense?

Any help would be appreciated

 

[cage]

By "relative stiffness of the members" I think that they mean "relative stiffness of the substructures. i.e a tall slender pier would provide less resistance to longitudinal force than a short stubby pier.