The Design of Modern Steel Bridges - TEDI

62 The Design of Modern Steel Bridges
vehicle weight from 38 to 40 tonnes. The increase in the maximum axle weight
required a slight increase in the previously derived design HA loading for the
short spans. The increase in the maximum vehicle weight was, however, offset
by an increase in the minimum axle spacing, thus requiring no further increase
in the design loading.
In the new HA loading[9], the uniformly distributed load was increased to
W ¼ 336L 0:67
for L450
¼ 36L 0:1
for 50 < L41600
where L is the loaded length in metres and W is the load per metre in kN. The
new HA loading is shown in Fig. 3.5 on which are also shown the HA loading
in BS 153[4] and BS 5400[2]. The increase for all loaded lengths is obvious,
but in the middle range of 25–60 m it is not very substantial. For the shorter
loaded lengths, the apparently substantial increase is not in fact critical, since
30 units of HB loading produces worse loading effects, even after allowing for
the appropriate partial factors. For loaded lengths above, say 60 m, dead load
starts to become more dominant than live load, and hence the total increase in
loading will not be as much as indicated in this graph.
Figure 3.5 British bridge loading.