The design of steel railway bridges should allow for a change in length due to temperature change of 1 in per 100 ft of span. Also, provision should be made for change in length from live load. In truss spans more than 300 ft long, allowance should be made for expansion of the floor system. The use of high adhesion locomotives may justify a more vigorous review of floor system expansion in even shorter spans.
In accord with current practice, spans more than 70 ft long should have hinged bearings at both ends and rollers or rockers at the expansion end. Spans 70 ft or less should be designed to slide on self-lubricating bronze plates at least 1â„2 in thick. All end bearings should be secure against lateral and vertical movement and, when founded on masonry, should be raised above the seat on metal bolsters or pedestals. New provisions are being developed by a committee of AREMA established to formulate specific and detailed recommendations for the construction of bearings for non-movable railway bridges.
Bridge Deck Drainage
On ballast deck construction, the deck drainage is part of the steel design. In general, deck drainage is gathered in scuppers or drops and carried in closed conduits to drops at the piers or abutments. Deck drains are frequently specified to be ductile iron pipe, supported in brackets to secure it against movement from vibration. Drainage systems require a grade of at least 1%.
Water accumulation must be kept away from expansion ends on ballast deck bridges where the bridge movement is accommodated by steel plates sliding under the ballast. Stainless steel plate and fasteners should be used in these expansion joints. At abutments, where bridges are built in a track grade, the water coming to the bridge should be intercepted in designed drains in the embankment behind the back wall.
Steel bridges should be protected with a quality paint system, metallic coating, or other protective system approved by the owner. A full system shop coating may be appropriate for some structures. Many bridge owners have developed their own coating specifications.
Design details should enhance the service life of the coating by providing for drainage and avoiding accumulation of dirt and debris.
Railway bridges may require provisions for walkways, handrails, access ladders, lights, signals, and signs; supports for conduits, communication lines, and fiber optic ducts; as well as track equipment, and other ancillary devices. The bridge designer should make provision for such items as directed by the owner. Bridge plates showing ownership and date of erection may also be required.