Question

I'm analysing a bridge with a near-surface slab that could affect the load spread from a vehicle - can LimitState:RING consider this?

Answer

Yes, for medium span bridges, we've found the following procedure to provide acceptable results:

Longitudinal Direction

Spread the load from one axle over three axles, spaced longitudinally at a distance of span/6 and with a ratio of loading of 25% - 50% - 25%. If the axle in question is subject to dynamic effects, then the ‘dynamic factor’ field should be set to ‘true’ for all three axles. If the bridge is relatively long spanning then you may wish to review the axle spacing such that it spreads the load to a more realistic extent.

Transverse Direction

In the transverse direction, you can alter the effective bridge width and / or angle of load spread through the fill accordingly. To take account of the increased load from the slab, adding the weight to the top fill layer is likely the best approach to take, as it is then treated as dead loading.