In this post we’ll discuss about the traffic loads on road bridges according to Eurocodes.

The first step when evaluating the loads is the division of the carriageway into notional lanes. The width of the carriageway (w) has to be measured between the inner limits of vehicle restraint systems. Should not be included in the width:

– the distance between fixed vehicle restraint systems;

– kerbs of a central reservation;

– widths of the vehicle restraint systems.

The width, taken as per above, should then be divided into notional lanes. The width of the notional lanes and their number is defined accordingly to the Code instructions.

The number of notional lanes depends on the width:

– w < 5.4 m => **n1 = notional lane**;

– 5.4 m ≤ w < 6 m => **n1 = 2 notional lanes**;

– w ≥ 6 m => **n1 = Int (w/3) notional lanes**.

Accordingly, their width is:

– w < 5.4 m => **3 m**;

– 5.4 m ≤ w < 6 m => **w/2**;

– w ≥ 6 m => **3 m**.

For what regards the width of the remaining area:

– w < 5.4 m => **w – 3 m**;

– 5.4 m ≤ w < 6 m => **0**;

– w ≥ 6 m => **w – 3 x n1**.

For example, for a carriageway width equal to 11 m:

– n1 = Int (11/3) = 3;

– wl = 3 m;

– remaining area = 11 – 3 x 3 = 2m.

Hence, a carriageway of 11 m is composed by 3 notional lanes having a width of 3 m and the remaining area is 2 m.

If the width is variable, the number of notional lanes has to be variable as well, hence:

– 1 notional lane where w < 5.4 m;

– 2 notional lanes where 5.4 m ≤ w < 9 m;

– 3 notional lanes where 9 m ≤ w < 12 m;

– etc.

Where the carriageway on a bridge deck is physically divided into two parts separated by a central reservation, then:

– each part should be separately divided in notional lanes if this system is permanent;

– the whole carriageway, central part included, should be divided in notional lanes if this system is temporary.

Where the carriageway consists of two separate parts on two independent decks, each part is considered as carriageway and separate numbering is used for the design of each deck.

For the numbering, the lane giving the most unfavorable effects is numbered lane 1, followed by lane 2, etc. As traffic loads are variable actions, they are placed in such a way that the most adverse effects are obtained. That means that they are placed longitudinally and transversely only in regions where the influence line of the relevant effect (internal moment, shear, support reaction, deformation, etc.) is either positive or negative. Their position is not necessarily numbered in sequence (1-2-3) but may be different (1-3-2).

Where the carriageway consists of two separate parts on the same deck, only one numbering should be used for the whole carriageway. This means that if the carriageway is divided into two separate parts, there is only one Lane Number 1, which can be alternatively on the two parts.

As said, where the carriageway consists of two separate parts on two independent decks, each part should be considered as a carriageway. Separate numbering should then be used for the design of each deck. If the two decks are supported by the same piers and/or abutments, there should be one numbering for the two parts together for the design of the piers and/or the abutments.

There are different load models to be applied, as discussed in a future post on The Structural Engineer’s Corner.

For any issues or questions, you can contact the author at:

*Eng. Onorio Francesco Salvatore*

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