Excerpt from General Specifications for Iron and Steel Railroad Bridges and Viaducts
1. All parts of the structures shall be of wrought-iron or steel, except tics and guard rails. Cast-iron or steel may be used in the machinery of movable bridges and in special cases for bed-plates.
Generally "double tracks through" bridges will have but two trusses, to avoid spreading the tracks at bridges.
In calculating strains the length of span shall be understood to be the distance between centres of end pins lor trusses, and between centres of bearing plates for all beams and girders.
3. The girders shall be spaced, with reference to the axis of the bridge, as required by local circumstances, and directed by the Engineer of the Railroad Company. (§ 5.) Longitudinal floor girders shall in 110 case be less than three feet and three inches from centre line of tracks. (§ 6.)
4. For all through bridges and overhead structures there shall be a clear head-room of 20 feet above the base of the rails.
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