Engineering and Construction

Many existing small-span railway bridges weather cast or wrought iron girders to which rails are fixed directly without use of ballast or half-through girders with trough floors.As these bridges have reached the end of their lives they have been replaced in steelwork in modern form with ballasted track. The legacy of the  original decks with their very shallow construction depth has influenced modern underline bridge practice in replacements and new bridges.The majority of railway underline bridges therefore tend to be of half-through type. The direct fastening of track without ballast gives the cheapest possible form of railway bridge and is appro- priate for rapid transit or tramway bridges where speeds are not high.

Fig. 4.11 Railway bridges. (a) Half-through plate girders; (b) half-through box girders; (c) composite deck type (spans > 30m)

Girder depths are generally greater than for highway bridges due to the heavier loading and because limits for deformation are necessary. A span-to-girder depth ratio of 12 to 15 is typical. For short spans half-through plate girders are used with composite steel cross girders forming rigid U-frames and supporting a concrete floor with ballasted single or double track.Through or half-through construction is appro- priate for medium spans exceeding 50m using trusses. For spans up to 39m, the railway authorities use a standard box girder design with a steel ribbed floor of minimal depth which is achieved by spanning between the inner webs of trapezoidal box girders proportioned so as to fit closely within the station platform space. The type has advantages in using components entirely of steel,which are bolted together at site and commissioned during temporary possession of existing tracks.Where suf- ficient depth is available then deck construction is preferable and more economic, with either twin plate girders or a box girder beneath each rail track.

Simply-supported spans are widely used for railway bridges because:

(a) individual spans can be erected or replaced quickly during temporary track  possession,
(b) uplift is more likely to occur if spans are unequal under heavy railway loading,
(c) fatigue is potentially less critical. Fatigue still tends to govern the design of steel elements having spans less than about 24m. Thus cross girders, railbearers and other short-span members need to use lower working stresses.