Distribution modes and parameters
This article describes the available distribution modes and the parameters used to define custom distributions in Archie-M. For background see "Available width, effective width, and transverse distribution".
Document history
- 2025-02-10: First complete version, including details of forthcoming modes in Archie-M 2.7
Standard distribution modes
CS454
Implements the distribution model set out in CS454 (and previously BD21).
- Transverse: axlewidth + 1500 + depth
- Longitudinal: 300 + depth
- Sine conversion factor: 1.681
Note that for road loads, the wheel contact patch width is within the 1500mm distribution with zero cover, so the axle width in the load files should be centre to centre.
Railway
Renamed "Legacy Railway" from Archie-M version 2.7.
- Transverse: axlewidth + 2 * depth for depths up to 300mm, axlewidth + 300 + depth for depths > 300mm.
- Longitudinal: 250mm + depth
- Sine conversion factor: 1.681
Note that railway load files should include transverse distribution in sleeper (0.8m) as part of axle width, so wheel loads have "axle" width 0.8m, axle loads have width 2.235m 1.435 between centres plus 0.8m distribution in the sleeper).
Note that Archie-M does not move half of the load onto neighbouring sleepers.
NR/GN/CIV/025
New in Archie-M 2.7, this follows '025 to the letter. The old Railway distribution was more conservative at fill depths greater than 300mm.
- Transverse: axlewidth + 2 * depth
- Longitudinal: 250mm + depth
- Sine conversion factor: 1.681
Note that railway load files should include transverse distribution in sleeper (0.8m) as part of axle width, so wheel loads have "axle" width 0.8m, axle loads have width 2.235m 1.435 between centres plus 0.8m distribution in the sleeper).
Note that Archie-M does not move half of the load onto neighbouring sleepers.
EN1991-2:2003
New in Archie-M 2.7, this implements the distribution set out in EN1991-2:2003. This distribution model is intended for use with fill over slabs, where it might be assumed that the assessment model will account for further distribution in the slab itself. It is significantly more onerous than the CS454 distribution at all depths, and than the Railway distribution at high depths.
- Transverse: axlewidth + 0 + 0.5 * depth
- Longitudinal: 250 + 0.5 * depth
- Sine conversion factor: 1.681
Note use of a 250mm constant for longitudinal distribution. This value is not defined in EN1991-2:2003. Use of a different constant is possible with the custom distribution mode (see below).
Note that Archie-M does not move half of the load onto neighbouring sleepers.
Custom
New in Archie-M 2.7, bridge files containing custom distribution parameters will not be loadable in Archie-M 2.6 or earlier.
Custom distribution allows the parameters to the distribution to be set by the user. For each of transverse and longitudinal distribution, the user may set a constant and a multiplier on the depth of fill. The distributions are then:
- Transverse: <axle length from load file> + <transverse constant> + <transverse multiplier> * <depth>
- Longitudinal: <longitudinal constant> + <longitudinal multiplier> * <depth>
Longitudinal distribution is converted to a sine curve using the standard Archie-M parameter.
The custom distribution parameters default to those for CS454 distribution.
Archie/Multi - deprecated
The Archie/Multi distribution mode is included only to allow old assessments to be replicated. It will be removed in a future release as it is now 25 years since Archie-M was released, and the distribution can be replicated using the (new in Archie-M 2.7) Custom distribution model.
Prior to version 2.6, the Archie/Multi distribution mode had remained the default, and is still set in many bridge files. From version 2.6, Archie-M will warn and suggest changing to CS454 distribution when loading a bridge file using Archie/Multi distribution.
Note that Archie/Multi distribution uses a non-standard parameter to transform the uniform distribution to a sine wave cycle.
Up to Archie-M 2.6 there was the possibility of setting custom parameters for the Archie/Multi distribution. These parameters were very rarely used, and were not saved in the bridge file. They have been removed from Archie-M 2.7 in favour of a separate custom distribution mode. If you need to adapt the old distribution parameters to the new, use (where A, B, C are the old parameters):
- Transverse
- Constant: A * 1500
- Multiplier: B
- Longitudinal
- Constant: 300 + (surfacing + overlay) * 1.5
- Multiplier: 0.5
Note that you will need to update the the longitudinal constant whenever the surfacing or overlay depths are changed.
Conversion to sine curve (longitudinal only)
As described in the standards, the longitudinally distributed load from an axle is a UDL with hard edges. This makes integration onto the back of the arch needlessly difficult and causes unrealistic kinks in the line of thrust. Archie-M converts the distributed length into a single cycle sine wave. This tapers out to zero and is easy to integrate, and is no less realistic than the UDL.
The wavelength of the sine curve is found by multplying the distributed length from the respective standard by the factor given in the sections above. In all cases except the legacy Archie/Multi mode, this factor is 1.681. That particular factor has two properties that ensure the influence of the change is limited:
- It keeps 90% of the area under the sine curve within the longitudinal distribution from the basic rule.
- It keeps the centroid of the half curve in the same place.
As drawn in '025 (reproduced below) the longitudinal distribution is projected down to contact with the extrados at both sides. Doing this using a UDL would shift the load to the downhill side. Archie-M uses the depth directly below the axle to find the distributed length, and applies the distributed load centred on the axle.
Overlay as railway track
The Archie-M option "Use overlay as railway track" causes the distribution to start at the bottom of the overlay rather than the top, and allows overlay to be used to represent load of track and any ballast above bottom of sleeper level.
Distributed dead load is favourable unless strength is critical. If strength is not critical, it is safe to ignore the weight of track, but legitimate to include it. Be aware that to do so, you will need to account for the effective strip, which may vary in width as loads are moved over the bridge.