The Design of Modern Steel Bridges - TEDI

Table 3.6 Typical values of U and P for various bridge lengths
Loads on Bridges 61
Loaded length (m) P (kN) U (kN/m) for percentage of HGV in traffic
7.5% 30% 100%
15.25 0 38 38 38
122 320 10.4 13.9 17.1
488 534 7.1 10.8 12.3
1950 747 5.8 9.9 10.5
1977, goods moved by road, measured by tonne–km, virtually doubled.
Statistics of vehicle population by gross weight indicate that the number of
vehicles with gross weight greater than 28 tonnes increased from an insignificant
number in 1962 to 90 000 in 1977. Vehicle population with gross weight
less than 11 tonnes fell substantially in that period, whereas those between 11
and 28 tonnes also increased, but the rate of increase of over 28 tonnes
completely outstripped that of the others.
This is the context in which a thorough review of design for live loading of
bridges was undertaken in early 1980s. These reviews took into account the
growth of traffic and change in traffic mix predicted for the 1990s, and produced
new proposals for loading for the entire range of loaded lengths. For the
shorter spans, the review was primarily on a deterministic basis, although an
element of probability for illegal overloading and lateral bunching was taken
into account. The extreme loading obtained from this part of the exercise was
considered to be just possible in a rare event, i.e. to be used as design load in
the ultimate limit state without multiplying by any further partial safety factor.
The results were thus divided by 1.5 to get nominal loading. For the longer
spans, a fully statistical basis was used to derive a characteristic loading, i.e.
95% probability of not being exceeded in 120 years. The nominal loading for
design, i.e. a 120 year return period load, was taken as the characteristic loading
divided by 1.2. A level 3 overall safety analysis was then performed to obtain
an appropriate partial safety factor for loading consistent with the assurance
that a structure will not have more than 1 in a million chance of collapse in
service in its lifetime. The partial safety factor on the nominal load was found
to be approximately 1.5, thus confirming the value already in BS 5400. On top
of the nominal design loading derived in this fashion, a 10% allowance for
future contingencies was provided throughout the whole span range. On a
multi-lane bridge deck the loading over the third lane onwards was increased
from 33% to 60% of the above design load in the first two lanes.
In 1986 the European Commission decided to adopt a single regulation for
maximum vehicle weights and dimensions throughout all the countries belonging
to the European Economic Community. For the UK this meant an increase
in the maximum axle weight from 10.5 to 11.5 tonnes, and in the maximum