(BS EN 1991-2:2003 + UK NA.)

A double-axle load (called the Tandem System) is applied in each traffic lane in conjunction with a uniformly distributed load (called the UDL System).

The UK use a 300kN axle load with a uniformly distributed load of 5.5kN/m^{2}. If there is more than one lane of traffic then the axle load is reduced in adjacent lanes (200kN in lane 2, 100kN in lane 3 and 0kN in other lanes).

This loading covers most of the effects of the traffic of lorries and cars.

A single-axle load is applied anywhere on the carriageway.

The UK use a 400kN axle load which includes a factor to allow for dynamic amplification effects. When the action is applied within 6m of an expansion joint then an additional dynamic amplification factor is applied. This load model is more predominant on short span members up to about 7m; and includes members such as deck slabs spanning between main beams. Effects under one 200kN wheel load should also be considered.

If the structure is to be designed for abnormal loads then vehicles from Load Model 3 will need to be considered.

The UK National Annex describes two groups of vehicles, SV and SOV vehicles.

- SV model vehicles (SV80, SV100 and SV196) are in accordance with the Special Types General Order (STGO) Regulations

Highways England's Document BD100/16 (http://www.standardsforhighways.co.uk) recommend that the levels of SV loading are as follows:

Motorways & Trunk Roads = SV80, SV100, SV196

Principal Roads = SV80, SV100

Other Public Roads = SV80

These represent vehicles with nominal axle weights not exceeding 16.5 tonnes.

SV80 has a maximum gross weight of 80 tonnes with a maximum basic axle load of 12.5 tonnes.

SV100 has a maximum gross weight of 100 tonnes with a maximum basic axle load of 16.5 tonnes.

SV196 has a maximum gross weight of 196 tonnes with a maximum basic axle load of 16.5 tonnes.

- SOV model vehicles (SOV250, SOV350, SOV450 and SOV600) in accordance with the Special Order (SO) Regulations.

Vehicle

Max. total weight of trailers

Trailer Bogie - 1

Trailer Bogie - 2

SOV-250

250 tonnes

6 axles x 225kN @ 1.5m

5 axles x 225kN @ 1.5m

SOV-350

350 tonnes

8 axles x 225kN @ 1.5m

8 axles x 225kN @ 1.5m

SOV-450

450 tonnes

10 axles x 225kN @ 1.5m

10 axles x 225kN @ 1.5m

SOV-600

600 tonnes

14 axles x 225kN @ 1.5m

13 axles x 225kN @ 1.5m

Vehicle SOV-250 (250 tonnes)

Trailer Bogie 1

6 axles x 225kN @ 1.5m

Trailer Bogie 2

5 axles x 225kN @ 1.5m

Vehicle SOV-350 (350 tonnes)

Trailer Bogie 1

8 axles x 225kN @ 1.5m

Trailer Bogie 2

8 axles x 225kN @ 1.5m

Vehicle SOV-450 (450 tonnes)

Trailer Bogie 1

10 axles x 225kN @ 1.5m

Trailer Bogie 2

10 axles x 225kN @ 1.5m

Vehicle SOV-600 (600 tonnes)

Trailer Bogie 1

14 axles x 225kN @ 1.5m

Trailer Bogie 2

13 axles x 225kN @ 1.5m

Each axle of the SV and SOV vehicles has to be multiplied by a Dynamic Amplification Factor (DAF) which varies from 1.2 to 1.07 for axles loads from 100kN to 225kN respectively.

Only one SV or SOV vehicle is applied to the structure. Load Model 1 is applied in combination with the SV or SOV vehicle loading. The "frequent" value of LM1 is used and positioned in adjacent lanes and within 5m of the front and rear axles of the SV or SOV vehicle.

A uniformly distributed load of 5kN/m^{2} used to represent crowd loading and may be applied to both road bridges and footway/cycleway bridges. Unless specified otherwise the ULD load may be reduced for footway/cycleway bridges with loaded lengths greater than 10m. The UK NA also applies this reduction to crowd loading on road bridges with loaded lengths greater than 30m.

Load Models 1 to 4 may be combined to form 'Groups' of traffic loads. The Groups are referenced gr1a, gr1b, gr2, gr3, gr4, gr5 and gr6 and the load models used in each group are listed in Table N.A.3 of the UK NA (this overwrites Eurocode EN1991-2 Table 4.4a).

Load Model 1 is combined with footway loading. The footway loading is reduced to 3kN/m^{2} (0.6 x 5kN/m^{2}).

The diagram above illustrates Group gr1a for a single span two lane carriageway with two footways. The Tandem Systems can be positioned anywhere along the length of the traffic lane so as to produce the worst load effect. The position shown above will produce the worst effect for the mid-span bending moment. If the worst shear in the deck is required then the Tandem Systems will need to be positioned adjacent to the support.

When analysing for global effects the tandem systems are positioned in the centre of the notional lanes (0.5m from each edge of a 3.0m lane).

When analysing for local effects then the two tandem systems are positioned so that the minimum distance between them is not less than 0.5m [*see Fig. 4.2b) and clause 4.3.2(5) of EN 1991-2:2003*].

This consists of the 400kN axle shown in Load Model 2 and is not combined with any other load model.

The 'Frequent' value of Load Model 1 is combined with Braking and Acceleration Forces (Clause 4.4.1) and Centrifugal (Clause 4.4.2) and Transverse Forces (Clause NA.2.20). The Frequent value of Load Model 1 is obtained by multiplying axle loads and UDL by 0.75 (Ψ_{1} = 0.75 from Table NA.A2.1 in NA to BS EN 1990:2002+A1:2005).

The diagram above illustrates Group gr2 for a single 10m span deck with a two lane carriageway. Although the axle loads and UDL values have been shown as reduced by Ψ_{1} it is usually more convenient to apply the 0.75 factor to the load effects rather than the loads. The longitudinal force can be reversed; similarly for the transverse force.

From Clause 4.4.1 the longitudinal force = 0.6 x 1 x 2 x 300 + 0.1 x 1.0 x 9.0 x 3.0 x 10.0 = 387kN.

*Note*: UK NA to BS EN 1991-2:2003 Table NA.1 contains a note which says α_{q1} = 1.0.

From Clause NA.2.20 the transverse braking force = 50% of the longitudinal braking force = 194kN.

The deck shown above is straight, therefore there will be no centrifugal force.

Group gr2 will generally be required for design of the bearings and substructure, and will not usually have a significant effect on the design of the deck.

This consists of Load Model 4 and is applied to the footways only; it is not combined with any other load model. The UDL may be applied to one or both of the footways so as to achieve the worst load effect.

This consists of Load Model 4 and is applied to the footways, carriageways and central reserve; it is not combined with any other load model.

The 'Frequent' value of Load Model 1 (LM1) is combined with Load Model 3 (LM3). The Frequent value of LM1 is obtained by multiplying axle loads and UDL by 0.75 (Ψ_{1} = 0.75 from Table NA.A2.1 in NA to BS EN 1990:2002+A1:2005). Tandem Systems (TS1 and TS2 shown below) can be interchanged if a worse load effect is achieved. Loading from LM1 is omitted from the Lane, or Lanes, occupied by LM3 for a distance within 5 metres of the front and rear axles.

The diagram above illustrates Group gr5 for a single span two lane carriageway with two footways. The SV80 vehicle has been shown to represent the Special Vehicle for LM3 for the purpose of this example. Although the axle loads and UDL values for LM1 have been shown as reduced by Ψ_{1} it is usually more convenient to apply the 0.75 factor to the load effects rather than the loads.

Load Model 3 (LM3) is combined with Braking and Acceleration Forces (Clause NA.2.18.1) and Centrifugal (Clause NA.2.18.2) and Transverse Forces (Clause NA.2.20).

The diagram above illustrates Group gr6 for a single span two lane carriageway with two footways.

Vehicle SV80 has been shown to represent the Special Vehicle for LM3 for the purpose of this example.

From Clause NA.2.18.1 the braking force for each axle = Q_{lk,S} Hence total force = 6 x 0.5 x 130 = 390kN.

From Clause NA.2.18.1 the acceleration force = 10% x 6 x 130 = 78kN < 390kN Hence longitudinal force = 390kN.

From Clause NA.2.20 the transverse braking force = 50% of the longitudinal braking force = 0.5 x 390 = 195kN.

The deck shown above is straight, therefore there will be no centrifugal force.

Group gr5 will generally be required for design of the bearings and substructure, and will not usually have a significant effect on the design of the deck.

Contact David Childs