Highway Loads Tab

The Highway Loads tab allows you to include road traffic actions in retaining wall design, either as a surcharge acting through the retained soil, or as direct bridge abutment actions applied to the wall. This tab supports both British Standard and Eurocode based approaches.

It is assumed you are familiar with the intent and application of the relevant highway loading documents and how these actions are applied to retaining walls.

British Standard Highway Loadings (BD 30/87 and BD 37/01)

Highway loadings can be added to the design of the retaining wall. MasterSeries currently accepts loads and load combinations in accordance with the BD 30/87 and BD 37/01 publications.

Load factors

You can edit the loading factors used for each design load case and load type. The default values are pre set using Table 1 from the relevant document, and can be amended where required to match your design assumptions.

Bridge abutment loads

You can add bridge abutment actions directly to the wall as individual loads. For each load, enter:

Load magnitude
Direction (horizontal or vertical)
Position relative to the top of the wall, in both horizontal and vertical directions
Load type

Up to 10 loads can be defined.

Eurocode Highway Loadings (to EN 1991 2)

Highway actions can also be applied using Eurocode traffic loading principles, with load cases and combinations based on several documents and the Bridge Design & Assessment website:-

BS EN 1991-2 + UK NA, eg, page 14 table NA3 showing groups of loads gr1 to gr6.
PD 6694, eg, tables B1 to B6.
Website http://bridgedesign.org.uk , eg, Tutorials > Abutment Design Example to EC + UK NA and http://bridgedesign.org.uk/tutorial/eu-abutment-example.php

The tab provides seven loading cases, each with three combinations, and the loading case titles are displayed for clarity.

Stability combinations

The stability check combinations for cases 1 to 7 follow the published EC abutment example approach. These are shown within the tab as the case set.

Stability Check load combinations cases 1 – 7 taken from http://bridgedesign.org.uk/tutorial/eu-abutment-example.php

Bridge abutment loading inputs

For Eurocode bridge abutment loading, you enter:

Load magnitude
Direction (horizontal or vertical)
Horizontal and vertical position relative to the top of the wall
Load type selected from the list provided

Additional options are available using the tick boxes provided on the tab.

MasterKey Retaining Wall Road Traffic Loading

MasterKey Retaining wall design includes for the option to design a retaining wall as a highway loaded structure. This can be either;

1. Highway retaining wall (road parallel to the wall)

A retaining wall with a road running parallel on the retained soil side, which will be referred to as a ‘Highway retaining wall’.

In this case, the highway actions are usually entered as surcharges and line loads on the retained soil surface using the Loads tab. The highway loading will transfer through the soil creating additional lateral earth pressure on the soil retained side of the wall.

2. Bridge abutment retaining wall (deck supported at wall top)

Retaining wall supporting a bridge deck on the top of the wall and highway loading on surface of the retained soil from the road leading up to the bridge will be referred to as a ‘Bridge Abutment Retaining Wall’.

A bridge abutment wall may carry:

Bridge deck reactions at the top of the wall
Highway loading on the retained soil surface from the approach road

For the purposes of retaining wall design the vehicle loading is split up into two main categories.

British Standard (BD30/87 BD37/01)

Vehicle actions are generally represented as HA and HB loading.

Eurocode EN 1991 2

Vehicle actions are generally represented using load models (LM):

LM1, normal traffic loading (lane UDL and associated actions)
LM3, special vehicles (SV) and special order vehicles (SOV), represented by axle load sets

The UK National Annex provides different axle patterns and loads for special vehicle types (for example SV80, SV100, SV196, where the number refers to gross vehicle weight in tonnes).

MasterKey Retaining Wall does not offer the ability to select different types of SV or SOV vehicles, but rather you are required to calculate the load and input them under the LM3 load category.

💡

A key point to note is that the design is for a unit length of retaining wall, therefore all point loads from LM3 SV/SOV axles and UDL from LM1 lanes will need to be converted to equivalent load per 1m unit length of retaining wall.

Bridge Deck Loading

For ‘Bridge Abutment Retaining Wall’ bridge deck loading applied to the top of the wall, you should calculate the total vertical and horizontal reactions of your SV or SOV loading and input these as a ‘LM3 Deck Loading’ (vertical) and ‘LM3 Braking & Acceleration’ (horizontal) load type. You should ensure the correct direction is selected of the chosen load types. Similarly for LM1 loading.

Retained soil highway loading (approach road and highway retaining walls)

Highway loading on the retained soil surface is applied using the Loads tab, where you can enter:

Vertical line loads (parallel to the wall)
Vertical area surcharges (full or partial), using start and end distances X1 and X2

These loads are transferred through the soil and automatically converted into additional lateral earth pressure on the wall.

Typical modelling guidance

Highway retaining wall (road parallel to wall)

LM1 is commonly represented using a partial surcharge area load over the lane width, with X1 and X2 defining the lane position relative to the wall
LM3 may be represented using one or more line loads to approximate wheel lines, or an equivalent partial surcharge where appropriate

Bridge abutment retaining wall (road approaching the abutment)

LM1 is commonly represented using a full surcharge on the retained surface where appropriate
LM3 is often most critical when the vehicle is close to the wall stem, so the chosen conversion to per metre run should reflect the proximity and the loaded length contributing to the wall response

For ‘Bridge Abutment Retaining Wall’ and ‘Highway retaining wall’ the highway loading on the surface of the retained soil can be applied in the ‘Loads’ tab. This offers both vertical line loads (parallel to wall) and vertical full/partial area loading (with partial having a Start X1 and end X2 distances specified) on the surface of the retained soil. These loads are transferred through the soil as part of the soil pressure analysis to produce additional lateral earth pressures. The loading from LM1 or LM3 should be converted to kN/m of line loads and kN/m2 for area loads. In both cases the horizontal distances of the load from the top of the wall stem can be specified.

For the ‘Highway retaining wall’ this would refer to the distance of the parallel road from the top of the wall stem. For LM1 loading a partial surcharge area load would be appropriate with X1 and X2 distances specified. For LM3 a set of line loads could be used to represent loading from a line of wheels on one side of an SV/SOV axle, however an equivalent partial surcharge area loading may yield similar accuracy depending on the proximity of the road to the wall. The example below shows the case of one 3m wide lane positioned 2m from the wall stem, with SV80 LM3 loading applied through two line loads and an LM1 UDL over the lane width.

For the ‘Bridge Abutment Retaining Wall’ SV LM3 this would refer to the distance of the vertical load from the wall stem, while for LM1 loading this would likely be a full surcharge (without distances X1 & X2) specified. It is likely for the LM3 SV loading the most critical condition will be when the vehicle is at a short distance from the wall stem. In this case the LM3 SV loading and LM1 lane loading both run perpendicular to the wall, and hence is more discrete in relation the overall length of the wall. You may have a single SV load over a 3m wide length, while the wall is 10m long. How you convert these discrete loads into per m run of wall loads requires engineering judgement?

PD 6694 simplified surcharge method (optional approach)

For this purpose, PD6694-1:2011 Road Traffic Surcharge Loading has proposed a simplified alternative for dealing with bridge vehicle lane loading on the surface of the soil retained side. The values of the load are fixed for Normal Vehicle loading (LM1) and SV and SOV (LM3) types. It applies the loading to the back of the wall through an equivalent top edge lateral line load combined with a full lateral UDL pressure per vehicle lane. The SV loading in this method covers SV100 and SV196, as the soil pressure produced by both vehicle is similar due the axle loads being equal (yes 196 is heavier but it is also longer and loads further from the wall on the soil retained side will have no impact on the wall). The PD comments that SV80 is less critical than normal LM1 loading (as noted in the UK NA) and is therefore ignored. If this method is used the equivalent type LM1 and LM3 line and surcharge UDL in the 'Loads' tab need not be input and should be removed.