and three hundred and fifty (350) feet, pin-connected Pratt trusses with broken top chords are to be employed.

For spans of over three hundred and fifty (350) feet, pin-connected trusses with subdivided panels are to be adopted.

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

The height of towers should generally be between one sixth (1/6) and one seventh (1/7) of the total length of span, measuring from centre to centre of end-pins; although in certain cases it may, for the sake of appearance, be made a little greater. The truss depth at the inner hips should be from one ninth (1/9) to one tenth (1/10) of the total length of span. The truss depth at outer hips for spans up to four hundred (400) feet will generally be determined by the clearance required. For longer spans it should be between one fourteenth (1/14) and one fifteenth (1/15) of the total span length.

The length of the centre panel will, in most cases, be made equal to the perpendicular distance between central planes of trusses.

In spans having horizontal top chords all panels of the latter must be made of stiff members, excepting only the centre panel over the pivot-pier; and the diagonals next to the middle panel are to be tension-members.

Broken top chords must be made of stiff members from ends to inner hips, but the portion between the inner hips is to be made of eye-bars. Inclined posts extending from inner hips to drum are to be used in all cases where top chords are broken.

LOADS.

The loads to be considered in designing draw-spans are the following:

A. Live Load.

B. Impact Allowance Load.

C. Dead Load.

D. Uplift at Ends.

E. Direct Wind Load.

F. Indirect Wind Load or Transferred Load.