new span is completely erected as close alongside the old span as possible, and the floor is placed on it ready for traffic. Upper and lower sets of track-rails, transverse to the bridge axis and extending completely under and beyond both the old and the new span, are placed beneath the ends of both spans, and are separated by live rollers. Both the old and the new span are seated on the upper track-rails and are moved, usually simultaneously, transversely until the old span is carried clear of the alignment, the new span following it up until it occupies the required permanent position, when it is lowered to its masonry seat and the old span is dismembered and removed. The operations are accomplished with hydraulic jacks and power tackles; and the work can sometimes be done so rapidly as to involve only a few minutes' interruption to traffic. The rapidity of the work is often an important economic consideration that may outweigh considerable extra expense in the way of labor, material, and equipment.

For spans of more than 100 feet, it is often desirable to support the old span on falsework that is designed also to carry the new span during erection, and, after the removal of the old trusses, to erect the new span on the falsework, frequently while the traffic is carried on the old floor that is still supported by the said falsework.

If the old span is strong enough to sustain the new span during construction, plus the weight of the minimum traffic necessary, the new span may be so designed as to permit its construction by this method; and, after completion, it may, if necessary, be moved a short distance either horizontally or vertically, in order to bring it into the exact alignment after the old structure has been removed.

In other cases the new span may be designed with transverse dimensions, greater than those of the old structure, that enable it to be built outside of the latter and practically independent of it, so that the new construction may be substantially completed while traffic is maintained on the old structure; after which it will receive traffic and will support it and the weight of the old structure whilst the latter is removed piece-meal.

Replacing Long Spans

Comparatively few very-long spans have been replaced by new ones in the same alignment. The principles governing the operation are substantially those developed for moderate-length spans, with the exception, perhaps, of the application of the cantilever method, by which the trusses of new long spans have been built out as cantilever arms which clear the old structure and are self-supporting during erection.

If traffic can be diverted from the old structure during its replacement, the old trusses will be likely to be found strong enough to support the new span during erection; and, if there are more than two lines of trusses, the old ones may be successively replaced by the new ones.

In the case of a suspension bridge, the factor of safety of the cables is