The Structural Engineer's Corner

Eng. Onorio Francesco Salvatore

Traffic loads on road bridges: a practical example

Written By: Lexatus - Nov• 12•13

Traffic loads 15 - Onorio

After the recent posts on the traffic loads as defined by EN 1991-2, let’s make an example. We have a bridge covering a span of 50.0 m with a static scheme of simple abutment. The roadway is 7.50 m wide flanked on each side by walkways having a width of 2.0 m. The walkways are separated from the central road by safety barriers.

The first step is to define the width w of the roadway and the number of notional lanes, in accordance with the EN 1991-2 “Actions on structures. Part 2: general actions – Traffic loads on bridges“.

The walkways are accessible only to pedestrians, hence the width w is delimited by the net distance between the aforeside guardrails.

Being the roadway 7.50 m, hence w > 6.0 m, the number of conventional lanes is:

Traffic loads 12 - Onorio

The width of the residual area is:

Traffic loads 13 - Onorio

The load models are:

– LM1 => applicable (for global and local verifications);

– LM2 => applicable (for local verifications);

– LM3 => not applicable (it concerns the transit of special vehicles over the bridge and must be considered only when expressly required);

– LM4 => applicable (pedestrian traffic).

On each lane must be applied a pair of tandem axles. We have:

– Lane 1: Qk = 300 kN ; qk = 9.0 kN/m²;

– Lane 2: Qk = 200 kN ; qk = 2.5 kN/m²;

– Residual area: Qk = 0 kN ; qk = 2.5 kN/m².

The location of the notional lanes is not linked with their numbering, so that number and location of the notional lanes are selected each time in order to maximize the considered effect.

Should be noted that the Eurocode 1 allows to have:

Traffic loads 14 - Onorio

Hence, a value multiplied by 0.8 for the first conventional lane and 1.0 for the others. When the coefficients are taken equal to 1, they correspond to a traffic for which a heavy industrial international traffic is expected, representing a large part of the total traffic of heavy vehicles. For more common traffic compositions (highway or motorways), a moderate reduction of α factors applied to TS and UDL on Lane 1 may be applied (10 to 20%). The unity value is also used for bridges without road signs restricting vehicle weights.

The rules to follow are:

– in each notional lane only one tandem system should be considered, located in the most severe position;

– the tandem system travels in the direction of the longitudinal axis of the bridge, centrally along the axis of the notional lane;

– when present, the tandem system should be considered in full, i.e. with all its wheels;

– the uniformly distributed loads apply, longitudinally and transversally, only on the unfavorable part of the surface of influence;

– the two load systems can insist on the same area, so they are promiscuous;

– the impact factor is included in the load values;

– when relevant, and only for local verifications, the transverse distance between adjacent tandem system should be reduced up to a minimum of 40 cm.

For what regards the pedestrian traffic load, the regulation prescribes a nominal of 5.00 kN/m², but recommends 2.5 kN/m² as combination value.

As said, LM1 and LM4 loads must be distributed in the least favourable way (both transversally and longitudinally) for a determined effect.

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

Eng. Onorio Francesco Salvatore

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