The Concept of Moving Loads Based on Eurocode ②

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1. What is moving load?

2. Various types of loads to be aware of for moving load design

3. What is EUROCODE?

Eurocodes are the European Standards for the design of structures. They consist of a series of 10 European Standards, EN 1990 – EN 1999, divided into 58 parts and encompassing over 3,000 pages. They are the European reference design codes, providing European common structural design rules for everyday use. That includes the design of whole structures and construction products.

The EN Eurocodes are the result of a long procedure of bringing together and harmonising the different design traditions in the Member States. Before Eurocodes, EU Member States all had different design methodologies. These differences have been taken into account in Eurocodes: A number of clauses called NPD (Nationally Determined Parameters) in the individual Eurocodes still allow nationally established values to be chosen. These values are listed in the National Annexes.

1) Structure of the EUROCODE

Eurocode is composed as follows:

• EN 1990 Eurocode : Basis of structural design
• EN 1991 Eurocode 1: Actions on structures
  • EN 1991-1: Actions on structures
  • EN 1991-2: Traffic loads on bridges

• EN 1992 Eurocode 2: Design of concrete structures
• EN 1993 Eurocode 3: Design of steel structures
• EN 1994 Eurocode 4: Design of composite structures
• EN 1995 Eurocode 5: Design of timber structures
• EN 1996 Eurocode 6: Design of masonry structures
• EN 1997 Eurocode 7: Geotechnical design
• EN 1998 Eurocode 8: Design for earthquake resistance
• EN 1999 Eurocode 9: Design of aluminium structures

The first page of EN 1991-2 (2003)

EN 1991-2 defines imposed loads (models and representative values) associated with road traffic, pedestrian actions and rail traffic which include, when relevant, dynamic effects and centrifugal, braking and acceleration actions and actions for accidental design situations. For the design of buried structures, retaining walls and tunnels, provisions other than those in EN 1990 to EN 1999 may be necessary.

2) Classification of actions

Let's start by touching the loads we will mainly deal with. This part is defined in Section 1 General of EN 1991-2.

(Section 1 gives definitions and symbols.) First, let's talk about the general loads.

Permanent actions(G) : self-weight of structures, fixed equipment and road surfacing, and indirect actions caused by shrinkage and uneven settlements, Prestressing action (dead load)

Variable actions(Q) : traffic loads, wind loads, snow loads, etc. (live load)

Accidental actions(A) : explosions, collision, etc

Seismic actions(A_Ek) : earthquake action

For normal conditions of use, the traffic and pedestrian loads should be considered as variable actions. The following are various representative values of variable actions:

Characteristic value, Q_k, which are either statistical, i.e. corresponding to a limited probability of being exceeded on a bridge during its design working life, or nominal, see EN 1990, 4.1.2(7) ;

Combination value, Ψ₀Q_k is intended to take account of the reduced probability of the simultaneous occurence of two or more variable actions.

Frequent value, Ψ₁Q_k is such that it should be exceeded for only a short period of time.

Quasi-permanent value, Ψ₂Q_k may be exceeded for a considerable period of time.

These representative values of variable actions are used in the Ultimate Limit State (ULS) and Serviceability Limit State (SLS) load combinations.

These representative values of variable actions are used in the Ultimate Limit State (ULS) and Serviceability Limit State (SLS) load combinations.

Representative values of variable actions

4. Calibration of the main Load Models (LM)

The first thing we have to do is Calibration of the main Load Models. In Table 2.1, some information is given on the bases for the calibration of the main Load Models (fatigue excluded) for road bridges and footbridges.

Load Models 1 and 2 are deemed to represent the most severe traffic met or expected in practice, other than that of special vehicles requiring permits to travel, on the main routes of European countries.

1) Divisions of the carriageway into notional lanes

The location and numbering of the lanes should be determined in accordance with the following rules:

EN 1991-2, Table 4.1 - Number and width of notional lanes

Explaining with a diagram based on the Eurocode standards, it would look like the following.

Example of Division of the carriageway

• W = 11m

• Width of a notional lane : 3 m

• Number of notional lanes : 11m/3=3.67, Int(3.67) = 3

• Remaining area: 11m – 9m = 2m

2) Location and numbering of the lanes for design

For each individual verification (e.g. for a verification of the ultimate limit state of resistance of a cross-section to bending), the number of lanes to be taken into account as loaded, their location on the carriageway and their numbering should be so chosen that the effects from the load models are the most adverse.

The lane giving the most unfavourable effect is numbered Lane Number 1, the lane giving the second most unfavourable effect is numbered Lane Number 2, etc. (see Figure 4.1).

EN 1991-2, Figure 4.1 Example of the Lane Numbering in the most general case

• W Carriageway width

• W1 Notional lane width

• ① Notional Lane Nr. 1

• ② Notional Lane Nr. 2

• ③ Notional Lane Nr. 3

• ④ Remaining area

3) Application of the load models on the individual lanes

(1) For each individual verification, the load models, on each notional lane, should be applied on such a length and so longitudinally located that the most adverse effect is obtained, as far as this is compatible with the conditions of application defined below for each particular model.

(2) On the remaining area, the associated load model should be applied on such lengths and widths in order to obtain the most adverse effect.

4) Vertical loads - Characteristic values

The load models for vertical loads represent the following traffic effects:

• Load Model 1 (LM1) : Concentrated and uniformly distributed loads, which cover most of the effects of the traffic of lorries and cars. This model should be used for general and local verifications.

• Load Model 2 (LM2) : A single axle load applied on specific tyre contact areas which covers the dynamic effects of the normal traffic on short structuralt members.

• Load Model 3 (LM3) : A set of assemblies of axle loads representing special vehicles (e.g. for industrial transport) which can travel on routes permitted for abnormal loads. It is intended for general and local verifications.

• Load Model 4 (LM4) : A crowd loading, intended only for general verifications.

• Load Models 1,2 and 3, where relevant, should be taken into account for any type of design situation (e.g. for transient situations during repair works)

Now let’s see each case by model.

5-1) Load Model 1 (EN1991-2, 4.3.2 Load Model 1)

Load Model 1 consists of two partial systems:

Double-axle concentrated loads (tandem system TS), each axle having the following weight: α_QQ_k

Uniformly distributed loads (UDL system), having the following weight per square metre of notional lane: α_qq_k

In LM1 (Load Model 1), the following considerations are taken into account:

• Dynamic amplification included in the characteristic values of Q_ik and q_ik.

• The values of α_Qi , α_qi and factors are given in the National Annex.

• LM1 is intended to cover flowing, congested or traffic jam situations with a high percentage of heavy lorries.

• No more than one tandem system should be taken into account per notional lane.

• Only complete tandem systems should be taken into account.

• For the assessment of general effects, each tandem system should be assumed to travel centrally along the axes of notional lanes.

• The uniformly distributed loads should be applied only in the unfavourable parts of the influence surface, longitudinally and transversally.

The characteristic values of Q_ik and q_ik, dynamic amplification included, should be taken from this Table 4.2.

The details of Load Model 1 are illustrated in Figure 4.2a.

EN 1991-2, Figure 4.2a - Application of Load Model 1

Let's consider the following example. 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/m². 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).

Example of Load Model 1

5-2) Load Model 2 (EN1991-2, 4.3.3 Load Model 2)

LM2 : Load Model 2 consists of a single axle load.

• Single axle load: β_QQ_ak

• Q_ak= 400kN (dynamic amplification included)

• The National Annex may give the value of β_Q

Load Model 2 (LM2) represents standard vehicles that induce maximum dynamic effects on short spans (3-7m). The contact surface of each wheel should be taken into account as a rectangle of sides 0,35 m and 0,60 m (see Figure 4.3).

EN 1991-2, Figure 4.3 - Load Model 2

5-3) Load Model 3 (EN1991-2, 4.3.4 Load Model 3)

Where relevant, models of special vehicles should be defined and taken into account.

• The National Annex may define Load Model 3 and its conditions of use. Annex A gives guidance on standard models and their conditions of application.

• Depending on the models under consideration, these models may be assumed to move at low speed (not more than 5 km/h) or at normal speed (70 km/h).

• Where the models are assumed to move at low speed, only vertical loads without dynamic amplification should be taken into account.

• At normal speed, a dynamic amplification should be taken into account. The following formula may be used:

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.

Highways England's Document CD 350 recommend that the levels of SV loading:

i.Motorways & Trunk Roads = SV80, SV100, SV196

ii.Principal Roads = SV80, SV100

iii.Other Public Roads = SV80

*Technical approval authority (TAA)

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.

SOV Vehicles

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.

5-4) Load Model 4 (EN1991-2,4.3.5 Load Model 4)

Crowd loading, if relevant, should be represented by a Load Model consisting of a uniformly distributed load (which includes dynamic amplification) equal to 5 kN/m².

Example of crowd loading on a bridge deck

A uniformly distributed load of 5kN/m² 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.

Topics

Moving Load

Eurocode

Dynamic Force

Vehicle Load

Load Model

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