Cooper E90 surcharge on bridge approach fill

[VoyageofDiscovery]

Can someone please explain to me how to interpret Ch.8 Part 5 Section 5.3.1. b. of AREMA which states:

"In calculating the surcharge due to track loading on an abutment and on wingwalls that are in line with the abutment backwalls, the entire load shall be taken as distributed uniformly on the surface of the ballast immediately below the tie, over a width equal to the length of the tie. With increased depth, the width for distribution can be increased on slopes of 1 horizontal to 2 vertical, with surcharge loads from the adjacent tracks not being permitted to overlap."

I see an ambiguity or omission with respect to individual axle loads at their respective spacings as it does not mention slopes and overlap of these pressures longitudinally along the track.

Am I interpreting this wrong or is there an omission?

Thanks

VoyageofDiscovery

 

[purdue86]

The key phrase to understanding this is "the entire load shall be taken as distributed uniformly on the surface". The method generally used is to take the maximum axle load and distribute it in the longitudinal direction by the axle spacing. For a standard E-80 design that would be 80k / 5 ft = 16k/ft. Assume an 8' long tie to make it easy and you end up with a 2 ksf surcharge at the bottom of the tie. The surcharge load will be 8' wide and infinitely long. I realize that you could decrease the surcharge load for the lighter axles that are further away from the heavy axles but you are looking at a lot of work for very little benefit.

If you have 2' of ballast under the tie, the width would increase 1' horizontal on each side of the tie. Therefore your width would increase to 10' and a surcharge of 1.6 KSF.

If you have two tracks at 12' centers (unlikely but I need a demonstration), and 4' of ballast under the tie (10' tie). The width of distribution would be 10' + 4' x (1' horz. / 2' vertical) * 2 sides = 14'. But this won't work because you will have 2' of surcharge overlapping between the tracks. I would distribute the load over 26'. Width outside of the centerline = 7', width between centerlines = 12' and width outside centerline = 7' = 26'.

 

[VoyageofDiscovery]

Hi purdue86,

Thanks for the response, yes I too have made these calculations, but I still see a lack of explanation or omission wrt longitudinal distribution in the AREMA clause.

What you say about "distributed uniformly on the surface" and neglecting lateral effects of overlapping soil pressures is a bit of a leap in my view without the AREMA Code specifying it.

Respectfully

VOD