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Steel Designers' Manual - 6th Edition (2003)
If so then bearing stiffeners should be provided and designed in accordance with
clause 4.5.2 of the Code.
17.4.8 Gantry girders
Plate girders used to support cranes should be designed in accordance with clauses
4.4 and 4.11 of the Code.
17.5 Initial choice of cross-section for plate girders used in bridges
17.5.1 Choice of span
Initial choice of cross-section for plate girders used in bridges 483
Plate girders are frequently employed to support railway and highway loadings on
account of their economic advantages and ease of fabrication, and are considered
suitable for spans in the region of 25–100 m. Plate girders are considered when rolled
sections are not big enough to carry the loads over the chosen span.
Spans are usually fixed by site restrictions and clearances. If there is freedom for
the designer, simply-supported spans within the range of 25–45 m will be found to
be appropriate; the optimum for continuous spans is about 45 m, as 27 m long girders
can be spliced with pier girders 18 m long. However, plate girders can be employed
for spans of up to about 100 m in continuous construction.
17.5.2 Span-to-depth ratios
For supporting highways a composite concrete decking, having a concrete thickness
in the region of 250 mm, is commonly employed. A span-to-depth ratio of 20 for
simply-supported spans serves as an initial choice for such girders; where continuous
spans are employed the depth of the girder can be reduced at least by one-third
compared with a simply-supported span.
Through girders of constant depth are rarely employed in continuous construction.
For simply supported plate girders designed for highway loading, a span-todepth
ratio of 20 can serve as an initial choice; for through girders supporting railway
loading larger depths are required, and a span-to-depth ratio of 15 is more appropriate.
Variable-depth plate girders are more appropriate where spans in excess of
30 m are required.
For continuous girders in composite construction, a span-to-depth ratio of 25
at the pier and 40 at mid-span is suitable. For highway bridges provided with
an orthotropic deck, the corresponding values are increased to 30 and 60
respectively.