twenty (20) to sixty (60) feet, of plate girders; spans from sixty (60) to ninety (90) feet, of open-webbed, riveted girders of single cancellation, or pin-connected "A" trusses; and spans exceeding ninety (90) feet, of pin-connected trusses.

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.

FORMS OF TRUSSES.

The forms of trusses to be used are as follows:

For pin-connected spans up to ninety (90) feet, the "A" truss.

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

For deck-spans carrying joists on the top chords, the Warren or Triangular girder with verticals dividing the panels of the top chords.

For spans between ninety (90) 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 width of bridge and the live load to be carried.

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 joists or stringers.

Steel stringers are, preferably, to be riveted to the webs of the cross-girders, but wooden joists are generally to rest on top of the latter.

In general, all trusses shall have main end posts inclined.