The Structural Engineer's Corner

Eng. Onorio Francesco Salvatore

Traffic loads on road bridges: the Load Model 1 (LM1) in Eurocodes

Written By: Lexatus - Nov• 10•13

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As discussed in the previous post, the Load Model 1 should be used for both global and local verifications. It is intended to cover flowing, congested or traffic jam situations with a high percentage of heavy lorries. In general, when used with the basic values, it covers the effects of a special vehicle of 600 kN.

The Load Model 1 consists of two partial systems:

– Double axle concentrated loads, called Tandem System (TS), with weight αQi x Qk per axle;

– Uniformly distributed loads (UDL), with weight αqi x qik.

Should be noted that:

– No more than one Tandem System should be taken into account per notional lane;

– One complete Tandem System should be taken into account, meaning that all four wheels should be loaded, even if some of them produce a favorable effect when they load the influence line in the opposite direction;

– Each Tandem System travels centrally along the notional lane;

– The axle load is equally divided into the two wheels, each one being loaded by 0.5 x αQi x Qik;

– Amplifications due to dynamic effects are included in the characteristic values of Qik and qik, and thus no further magnifications are required;

– The contact surface of each wheel is taken as square with a surface of 0.4 x 0.4 m².

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The loads are the following:

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The adjustment factors represent the return period for traffic on the main roads in Europe. An adjustment factor aq equal to 1 is equivalent to a return period of 1000 years for the characteristic values and corresponds to a heavy industrial international traffic (fatigue excluded). The above equates to a probability of exceedance in 50 years equal to 5%. For lighter traffic conditions, such as case of highways and motorways, reduced adjustment factors may be found in the National Annexes.

For frequent values, the return period on European roads is 1 week.

For local verifications, for example, for the design of the slab, the wheel loads may be considered as uniformly distributed taking into account the contact area of the wheel and the dispersal of the load through the pavement and the concrete slab.

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

Eng. Onorio Francesco Salvatore

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