EFFECTIVE LENGTHS AND DEPTHS.

See Specifications for Railroad Structures.

STYLES OF BRIDGES FOR VARIOUS SPAN LENGTHS.

For spans up to one hundred and forty (140) feet in length, plate-girder spans are to be used. These plate-girder spans 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 former style is generally preferable as a matter of economy in time of operation, there being no important reason for raising the ends to any great extent, as there is in the case of railroad draw-spans.

For spans between one hundred and forty (140) and two hundred and twenty-five (225) feet, pin-connected Pratt trusses with parallel chords are to be used.

For spans between two hundred and twenty-five (225) feet and three hundred (300) feet, pin-connected Pratt trusses with broken top chords are to be employed.

For spans of over three hundred (300) 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 width of bridge and the live load to be carried.

The proper truss depths for all cases cannot well be specified, as they will depend upon various considerations, such as appearance, economy, width of structure, etc.

In all cases the top chords are to be of rigid members, and inclined posts are to be used at ends and over drum, as specified for railroad draw-spans.

MAIN MEMBERS OF TRUSS DRAW-SPANS.

See Specifications for Highway Bridges.

LOADS.

See Specifications for Railroad Draw-Spans.