Design considerations for steel railroad bridges differ somewhat from those for highway bridges. Railroad bridges have a higher live-to-dead load ratio because the mass of the railroad loading is generally large, relative to that of the bridge. In case of accidents, rail traffic cannot steer away from damaged bridge components, but highway traffic can frequently be moved to other lanes while repairs are made. Rail traffic cannot be readily detoured;
it is impossible on some rail lines and very disruptive and expensive on others. Thus, railroad bridge design should consider the ease of bridge repairs. Unit trains, a consist made up of cars of the same kind and weight, can create a high number of similar loadings in a component with the passage of one train. Thus, the fatigue life of design details (Art. 11.38) is especially important under these conditions.
Open Deck Bridges
In railroad bridges of open deck design where the track is supported on a pair of stringers, the stringers should be spaced not less the 6.5 ft apart. The nominal bridge tie length is 10.0 ft. Where multiple stringers are used, they should be spaced to uniformly support the track load and provide stability.
Stringer and Floorbeam End Connections
Stringer and floorbeam end connections should be designed to provide for flexure in the outstanding leg of the connection angles. Connection angles should be not less than 1â„2 in thick and the outstanding leg should be 4 in or greater in width. For stringers, in open and ballast deck construction, the gage distance, in, from the back of the connection angle to the first line of fasteners, over the top one-third of the depth of the stringer, should be not less than squer(Lt /8) where L is the length of the stringer span, in, and t is the angle thickness, in.
Deflections
Simple span deflection should be computed for the live load plus impact that produces the maximum bending moment at midspan. The maximum deflection should not exceed 1â„640 of the span length, center-to-center of supports. The gross moment of inertia may be used for prismatic flexural members.
Safety
Safety devices required by the owner and by regulations must be provided for in the earliest stage of design. Safety devices may include such items as walkways, hand railings, vandal fences, ladders, grab-irons, bridge end-posts, clearance signs, refuge booths, stanchions, and fall protection fittings. A bridge located within 300 ft of a switch generally requires a walkway.
Skewed Bridges
Many railroads restrict the bridge skew angle. Generally, all bridge ends must be designed to provide structural support, at right angle to the centerline of track, for the end ties. This requires the bridge backwall to be designed at the same time as the spans.
Clearances
Appropriate clearances must be provided for in the design of all structures. Through-girder and through-truss bridges should provide a minimum of 9.0 ft horizontal side clearance, measured from the centerline of track. A minimum vertical clearance of 23.0 ft above the plane of the top of the high rail should be provided in through-truss bridges. The designer
should verify clearance requirements with the owner.
Bridge Bearings
Masonry plates should have a minimum of 6 in of clearance from the free edge of concrete or masonry supports. Improved specifications for railroad bridge bearings are being developed to better utilize the available materials. Refer to Chapter 19 of the AREMA Manual for current requirements.
