designed with special reference to extreme rigidity in all parts.

For spans exceeding one hundred and seventy-five (175) feet, pin-connected spans.

The use of pony-truss bridges of any kind is prohibited, excepting only half-through, plate-girder spans, in which the top flanges are held rigidly in place by brackets riveted to cross-girders that are spaced generally not to exceed fifteen (15) feet apart.

In general, double-track bridges shall have only two trusses, in order to avoid spreading the tracks.

FORMS OF TRUSSES.

The forms of trusses to be used are as follows:

For pin-connected spans up to one hundred and twenty-five (125) feet, the "A" truss.

For open-webbed, riveted girders, the Warren or triangular girder with verticals dividing the panels of the top chords; also the Pratt truss.

For deck-spans having top chords supporting wooden ties, the Warren or triangular girder with verticals dividing the panels of the top chords.

For spans between one hundred and twenty-five (125) feet, and about two hundred and fifty (250) feet, Pratt trusses with top chords either straight or polygonal.

For spans exceeding two hundred and fifty (250) feet, Petit trusses.

It is understood that these limiting lengths are not fixed absolutely, as the best limits will vary somewhat with the number of tracks and weight of trains.

MAIN MEMBERS OF TRUSS-BRIDGES.

All spans of every kind shall have end floor-beams, riveted rigidly to the trusses or girders, for supporting the stringers. Stringers are to be riveted to the webs of the cross-girders. In general, all trusses shall have main end posts inclined.  All  trusses  shall  be  so  designed  as  to  admit  of  accurate