The Design of Modern Steel Bridges - TEDI

58 The Design of Modern Steel Bridges
all bridges on public roads are designed for the same loading, whereas in the
other countries, the design loading depends on the type of route. Most codes
prescribe the simultaneous action of a single concentrated or truck loading, and
a uniformly distributed load. Some codes consider the effect of a heavier axle
or wheel load. The impact effect is already included in the design loads in
some codes, but in the other codes the design loading has to be increased by
a factor which generally decreases with the length of the member. The
intensity of loading decreases with the increase in the loaded length in most of
the codes. Most codes also allow a reduction when several traffic lanes have to
be loaded.
This study also included a valuable numerical exercise of calculating the total
bending moments caused by the live loads of the various codes on a simply
supported bridge. Separate calculations were made for the bridge carrying two,
three and four traffic lanes and spanning 10–100 m. The total load on the whole
bridge was considered for this comparison, as if the bridge was supported by
one single beam. The impact factor and the reduction due to multiple land
loading were taken into account; any difference between the various codes on
the allowable stress levels was also allowed for by multiplying the bending
moment by a ratio:
yield stress of steel specified in the national material specification
allowable stress of steel in the bridge design code
The bending moment M thus obtained was converted into an equivalent
uniformly distributed load qeq in kN/m, given by 8M/L 2 . These qeq values
indicate the structural strength of bridges built according to the loading specifications
of the various countries. Figures 3.4(a) and (b) show these qeq values
for bridges with two and four lanes in the carriageway, respectively. Very wide
differences between different countries are evident, the AASHTO loading
being by far the lightest for spans over 25 m.
3.4 Recent developments in bridge loading
In recent years it has been found in several countries that the standard loading
does not satisfactorily reflect the effect of a long queue of vehicles in a traffic
jam situation, particularly for long loaded lengths of, say, over 40 m. In the
USA, proposals[8] were made for a new loading standard which will consist of
a uniformly distributed load U and a concentrated load P for each lane, both
of which depend on the length of the bridge to be loaded for the worst effect.
U depends also on the percentage of heavy goods vehicles in the traffic.
Table 3.6 gives typical values. No allowance for impact needs to be added, as
the loading represents a static jam situation. In multiple lanes, a second
lane shall have 70% of the basic lane load and all other lanes shall each have
40%.