Railway Loads

Document history

• 2023-04-17 Added discussion of RA1 load model, alternative load models, distribution, and centrifugal loading
• Initial, very short, version only noting that railway loads are wheel loads and should be used with a reduced bridge width.

RA1 Loading and Route Availability

Assessment of under-line railway bridges is set out in NR/GN/CIV/025. The general approach is to establish a route availability (RA) number for the structure. The way the system is presented is convoluted.

The main load model used is called "RA1 loading". Route availability numbers run from RA0 to RA10 and beyond. If a bridge has a capacity of 11 British Standard Units of RA1 loading, it gets a route availability number RA1. These two uses of "RA1" are quite separate.

The RA1 load models

The RA1 load models are presented in NR/GN/CIV/025. For ease of discussion, the figure is reproduced below. It should go without saying that this should not be relied on: refer to the latest version of the standard.

Note that what is shown is not RA1 loading - it is 20 british standard units of RA1 loading. Which corresponds with a route availability of RA10.

Two variant models are provided, the standard and the "short lengths" version. The short lengths model is most often applicable in masonry bridge assessment, but we consider the standard version first.

025 presents this load model without any background. It should raise the immediate question "what does this represent?". It certainly doesn't look like any common real load. That may be important, because details of loading geometry matter in masonry bridge assessment.

The answer is beautifully illustrated by Alan Hayward, founder of Cass Hayward Consulting Engineers, in an article for the Great Central Railway journal.

So the standard load is a double headed pair of 0-8-0 steam locos with a freight train. This load model first appeared in BS153:1922. The configuration it represents is not a common sight now!

The odd spacings are imperial: 1.524m and 1.829m are 5ft and 6ft respectively. 2.743m is 9ft, 3.962m is 13ft.

025 issue 3 states that, for masonry bridges, "The assessment loading should be a single axle or group of axles based on the Type RA1 load train defined in Section 4, or representing specific trains as indicated in the Assessment Remit."

"Group of axles" is not clearly defined. It could reasonably be interpreted to mean any number of adjacent axles, or it might refer to the obvious groups of 4. These "long" loads have not been distributed with Archie-M (true up to and including version 2.5.1). The short load is expected to be at least as onerous for many masonry bridges.

The "short loads" model

The RA1 load model includes a "short loads" variant with 2 no. 25 tonne axles at 1.829m (6ft) spacing. This is a good representation of a standard bogie on a 100 tonne freight wagon at capacity. It is likely to be more onerous for many masonry bridges than the full load model and as such it should always be checked.

Other load models

025 issue 3 states that, "The assessment loading should be a single axle or group of axles based on the Type RA1 load train defined in Section 4, or representing specific trains as indicated in the Assessment Remit."

The main "specific train" load that would be of concern is that of heavy freight in bogie wagons. (Note that the pattern of loading produced by modern freight generates adverse behaviour in viaducts that is not represented in current assessment models.)

For many years, Archie-M has included a version of the "D4" load model (the axle spacings are very slightly different from the those in the current UIC standard, which are round numbers in the metric system). 025 does not explicitly allow for assessment to the UIC D4 load model, but it is a good representation of modern heavy freight, and we would recommend that it always be checked and reported where freight traffic is expected.

The critical load for masonry bridges except those few with very long spans will be either one bogie, or a pair of bogies across a coupling. Subject to interpretation of 025 wording re assessing for "a single axle or group of axles", consistency requires consideration of not only the single bogie and the pair, but also the 1 and 3 axle cases. It is quite possible in that 4 axles might be less onerous than 3 on certain spans. The standard Archie-M D4 load family offers all 4 cases with impact variants.

Longitudinal distribution

025 allows longitudinal distribution:

1. through the rails to the sleepers,
2. through the sleepers, and
3. through ballast and fill.

Archie-M implements items 2 and 3 (use "Railway" distribution model). Item 1 would have to be implemented in the load files, and those we provide do not do so.

We would argue that no coherent interpretation of item 1 is possible. Loads move; sleepers do not. Axle spacings do not match sleeper spacings. So for any given load position, some axles may be positioned over a sleeper as the diagram in 025 suggests, but others will be at some distance between. As the load moves, the relationship between axles and sleepers will vary.

Given this lack of coherence, and the considerable load file complexity created by this additional distribution as described, standard Archie-M load files do not implement it.

Transverse distribution (dispersal)

Archie-M calculates the effective width for a given load position; it is not necessary to calculate an effective width, only to provide the maximum available width over which loads can distribute. Because of the need to handle centrifugal effects (see below), the railway load files are for wheel loads, and the "bridge" width should be the width available to carry one wheel.

Centrifugal effects

025 requires that centrifugal effects be taken into account. These occur where track over a bridge is on a plan curve. In the 2D model of masonry bridge assessment, the transverse load is neglected, but the effect also transfers some load from the wheel on the inside of the curve to that on the outside at each axle.

To represent this in a fundamentally 2D analysis based on the effective strip model, 025 requires use of a wheel load and a correspondingly reduced width. Archie-M standard railway loads are therefore wheel loads.

Archie-M calculates the effective strip based on the distribution model chosen; the input bridge width is the maximum width over which the load can be allowed to distribute. For assessment using wheel loads, the available "bridge width" runs from the middle of the four foot to the first limit encountered outside the track. That limit may be the bridge elevation, a construction joint, or the mid-point to near rail of an adjacent track. Longitudinal cracks may require a further reduction in available width.

Archie-M does not calculate the additional load from centrifugal effects. Use the load factors to augment the static loads.

Where centrifugal effects are relevant, the bridge width used should normally be for a wheel on the outside of the curve. It may be that a very limited available width on the inside of the curve would result in a more onerous result.

025 notes that the vertical effect of centrifugal force may be reduced by track cant. It is surprising that no guidance is provided on the calculations involved.

Where cant is present, it will result in unequal load on rails in the opposite sense at low speeds; this is not mentioned.

Archie-M railway loads

The Archie-M 2.5.1 load collection includes a set of railway loads. These are all wheel loads. They include:

• The RA1 short load, without impact and with impact on each axle.
• A "D4" load, with variants from 1 to 4 axles, each without impact and with impact on each axle.

All of the loads are for 25 tonne axles (12.5 tonne wheel loads), and are named "RA10" to make this clear. Assessment for other route availability numbers can be achieved by factoring down or up.

The normal bridge width for use with railway wheel loads is 1.7m. More can be provided if it is available outside the track, but the bridge width used must not extend past the middle of the four foot.