Medium spans up to 300 or 400 feet long, and including some short spans, when for any reason it is impracticable to handle the trusses as complete units, are generally, in the case of new work, best erected on ordinary framed wooden falsework by means of one or two derrick cars.

When there are pile foundations which can be placed in advance, the derrick car can rapidly put in the framed-falsework bents; after which two derrick cars, one at each end of the span, if two are available, can place the floor system, assemble the lower chords, erect the web members on them, and finish the erection by placing the top chords, top laterals, and sway bracing as the cars retreat from the center to the ends of the span.

For heavy structures where the top chords involve too great a load for a pair of derrick cars, or where locomotive cranes are used instead, they can be supplemented advantageously by a simple gantry traveler to handle the heaviest members.

It is entirely practicable to execute the erection wholly with the gantry traveler; but that traveler is costly to construct, difficult to transport from job to job, and not as rapid or economical as derrick cars when the latter are available.

Alternative Methods

If the elevation of the span is exceedingly high above the water; if there is great danger from ice or floods; if the bottom is very treacherous or difficult; if the current is too fierce; or if the space underneath the span must not be obstructed by falsework (as when it is required to be left open for navigation or for heavy city or railroad traffic beneath), falsework becomes too dangerous or expensive or is wholly inadmissible, and some other system of erection must be devised.

The most common method is by cantilever erection from each end of the span, the truss members being made heavier or temporarily reinforced until the center-panel connections are made and the structure is transformed into a simple, self-supporting span. This method involves either the provision of special anchorages and counterweights or the erection of alternate spans in advance so that they may serve as anchorages. Cantilever erection is always objectionable when it can be avoided, because it is much slower and more costly than falsework erection, and as there is greater danger of injury to the uncompleted structure by sudden storms or from various accidents than there is when falsework is used.

Sometimes falsework of various types can be provided eccentric from the alignment of the bridge, and the permanent span may be erected on it by the ordinary method-then, when complete, moved transversely to the required position and permanently seated on the substructure.

The method of protrusion is occasionally employed abroad and has infrequently been adopted in America.  When it is used,  the span is erected