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Steel Designers' Manual - 6th Edition (2003)
126 Bridges
tural behaviour and the absolute necessity to include the effects of the erection procedure
in the design process.
This chapter on bridges gives emphasis to initial design, an important stage of the
process, because the basic decisions as to member proportions, spacings and splice
positions vitally affect economy of the structure. It is essential that the detailed
analysis is based upon optimized sizes which are as accurate as possible. If this is
not achieved then the detailed design will be inefficient because time consuming
repetitive work will have been expended, adversely affecting the economy of the
design and the construction costs. Guidance is given on the initial design of highway
bridges using composite deck construction, which is a significant proportion of the
number of steel bridges built, although since 1990 a significant number of railway
bridges have been constructed for new-build schemes such as the Channel Tunnel
Rail Link and in replacement of life expired structures.
Steel is particularly suitable for the strengthening and repair of existing bridges
arising from increases in highway traffic loadings and the incidence of accidental
impact damage from road vehicles. Steel is suitable for such work using welded or
bolted strengthening.
4.2 Selection of span
The majority of bridges fall within the category of short span because for many
crossings of rivers, railways or secondary highways a single span of less than 30 m is
sufficient. For multiple-span viaducts a decision on span length must be made, which
depends on factors shown in Table 4.4.
For long viaducts it is necessary to carry out comparative estimates for different
spans to determine the optimum choice, as shown in Fig. 4.1.
Table 4.4 Factors which decide choice of span for viaducts
Factor Reasons
Location of obstacles Pier positions are often dictated by rivers, railway tracks and
buried services
Construction depth Span length may be limited by the maximum available
construction depth
Relative superstructure and Poor ground conditions require expensive foundations;
substructure costs economy favours longer spans
Feasibility of constructing (a) Tidal or fast-flowing rivers may preclude intermediate piers
intermediate piers in river (b) For navigable waterways, accidental ship impact may
crossings preclude mid-river piers
Height of deck above ground Where the height exceeds about 15 m, costs of piers are
significant, encouraging longer spans
Loading Heavier loadings such as railways encourage
shorter spans