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The specifications given in the preceding chapter for fixed spans apply also to draw-spans, except where otherwise stated in the following pages.
GENERAL DESCRIPTION.
MATERIALS.
The specifications previously given apply also to drawspans, except that cast iron may be used for the centre castings on top of pivot-piers, for anchor-pieces in the masonry, for shafting-boxes, and for rail-chairs, and some other castings of minor importance. The use of high steel for drawbridges will not be permitted.
STYLES OF BRIDGES FOR VARIOUS SPAN LENGTHS.
For spans up to one hundred and sixty (160) feet in length, plate-girder spans should be used. These may be made to act as continuous girders over the pivot-pier, or may have pin-connections over the drum, so that when the live-load is applied they will act as two separate spans. The latter style is generally preferable, because there is no tendency for the far end of the span to rise when the live load is being brought on.
For spans between one hundred and sixty (160) feet and two hundred and seventy-five (275) feet, pin-connected Pratt trusses with parallel top chords and stiff diagonals in panels where there is reversion of stress, or riveted trusses of single cancellation, are to be used.
For spans between two hundred and seventy-five (275) feet
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