Railway bridges are not very common in the IStructE exam, perhaps because there are so many standard designs available. Structures carrying light railway fall into a grey area, and could be designed as highway bridges with lower span to depth ratios.
This cheat sheet sets out some common types of railway bridge that are suitable for different span ranges, along with some key considerations when laying them out. Depths are given directly instead of span to depth ratios, but you will notice that the ratios are much lower than for highway bridges.
When designing to the Eurocodes and CIV/020, railway bridges are often limited by deflection/stiffness requirements rather than strength at the ultimate limit state. CIV/020 defines a railway live load deflection limit of span/600, which is very onerous. Many other complicated deflection/rotation/acceleration requirements exist, but this one is suitable for hand calculation and could potentially be used to size a structure. It is worth noting in your answer that the design may be limited by deflection even if no calculations are carried out.
The following sources of information were used to produce the cheat sheet:
- SCI P318: Design Guide for Steel Railway Bridges
- Network Rail standard designs
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