complete at one end of the bridge and as nearly as possible at the required level, the rear end is counterweighted, and (especially if there is only a single span) the forward end is extended by a temporary pilot truss, when such extension is necessary. Each span is pushed forward longitudinally on rollers until the forward end or extension takes roller bearing on the next pier, so as to support it during the farther advance until the span comes to position and is lowered to bearing. If several spans are erected in the same way, they are temporarily connected to form a continuous-girder structure during protrusion.

Where a number of spans of substantially uniform and moderate lengths are constructed in one bridge, they have, under certain circumstances, been erected on a platform suspended from a temporary, overhead, movable span that travels from pier to pier as the work progresses.

Where the ground under the bridge is accessible, unobstructed, and comparatively level, and when materials can be delivered there, it may be possible, as has occurred in some cases, to distribute the steel in advance at low level, erect the spans there complete in the proper alignment, and raise them to required position and elevation as the piers are built up.

Considerable use has been made of the floating method whereby the spans, having been erected in the usual manner on shore or on falsework built in comparatively shallow and sheltered waters, have been transferred to the decks of scows, towed to position between the piers, and aligned with their seats on the latter, usually at low-level elevation, and lowered to place by the use of water ballast or tidal fluctuations or both.

In very difficult and unusual conditions, as in some mountain railroads, temporary suspension spans have been built, the permanent spans erected on them, and the suspension spans removed.

The above methods and their combinations, variations, and modifications have all been successfully used on different occasions; and together they cover the principal features of ordinary erection for medium, long, and short spans. Any one of them under special conditions may become the economic method for erection in that case; but where it is practicable, the method of erection on ordinary falsework with a derrick car or traveler is likely to be most desirable and economic.

Long Spans

Up to about 700 feet in length and 150 feet in clear height, spans have been erected on framed-timber falsework, which, of course, for such extreme dimensions becomes very costly, but permits more rapid and satisfactory assembling of the span than do the other methods, thus making presumption of economic desirability for this method when it can be used.

The very fact that conditions require spans of more than 400 feet is quite likely to indicate deep water, bad bottom, swift current, great height, or wide, unobstructed openings between the piers that will make falsework impracticable.