Management Strategies for Special Permit Vehicles for Bridge Loading

An examination of weigh-in-motion (WIM) data collected recently at sites in five European
countries has shown that vehicles with weights well in excess of the normal legal limits are
found on a daily basis. These vehicles would be expected to have special permits issued by the
responsible authorities. It can be seen from the WIM measurements that most of them are
travelling at normal highway speeds (around 80 km/h). Photographic evidence indicates that,
while many are accompanied by an escort vehicle in front and/or behind, normal traffic is
flowing alongside in other lanes of the highway. As European freight volume grows, the
frequency of these special vehicles can be expected to increase. Hence, the probability of them
meeting a heavy truck travelling in the opposite direction on a bridge also increases.
Gross vehicle weights in excess of 100 t have been observed at all sites, and are a daily
occurrence in the Netherlands. Most of these extremely heavy vehicles are either mobile cranes
or low loaders carrying construction equipment. Both types have multiple axles at very close
spacing, and the gross weight and axle layout have implications for bridge loading.
This paper presents findings based on a simulation model which incorporates the load effects for
all observed truck types on short to medium span bridges. It is evident that special vehicles
govern the lifetime maximum bridge loading, and the occurrence of extremely heavy trucks is
sufficiently frequent that meeting events can be expected during the design lives of the bridges.
The effects of different management strategies for special permit vehicles are modelled and the
results are presented.