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GENERAL DESCRIPTION.
MATERIALS.
All parts of the structure, except ties, foot-planks, and guard-timbers, shall, for all spans of ordinary lengths, be of medium steel, excepting only that rivets and bolts are to be of soft steel, and adjustable members of either soft steel or wrought iron. For very long spans high steel may be used for top chords, inclined end posts, pins, eye-bars in bottom chords, and those in main diagonals of panels where there is no reversion of stress when impact is included. It may be used also for the web-members of cantilever and anchor arms in cantilever bridges where the variation of stress is comparatively small and where the impact cannot be great. Excepting for purely ornamental work, cast iron will not be allowed to be used in the superstructure of any bridge, trestle, or elevated railroad, cast steel being employed wherever important castings are necessary.
CROSS-TIES, FOOT-PLANKS, AND GUARD-TIMBERS.
Cross-ties, foot-planks, and guard timbers shall be of long-leaf, Southern yellow pine or other timber which, in the opinion of the Engineer, is equally good and serviceable. The wooden floor shall be so designed as to ensure safety from passing trains for the railroad employees. The spaces between ties shall, in general, not be less than five (5) inches nor more than six (6) inches wide. The sizes of ties shall be such as to give the requisite resistance to bending, under the assump-
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