the girders together, one at each end, provided the towers do not need removal; but in the latter case, particularly if the viaduct is very high and the connecting spans are long, the problem becomes one of great difficulty and expense.

This replacement can be, and has been, accomplished by supporting the structure on falsework; by suspending it from overhead spans long enough to reach from tower to tower, and carrying successively both old and new structures; and by providing new towers, or portions of new towers, before the old towers are wholly or partly removed.

All of these methods are likely to be slow, hazardous, and expensive; and they must be very carefully planned in detail for each structure considered.

When the old and the new structures can successively support each other during the construction, or when it is possible to by-pass the traffic, or to transpose old and new structures transversely in sections, or when it is possible to build new towers intermediate between the old ones or adjacent to them, the difficulties are likely to be considerably diminished, but such favorable conditions do not frequently prevail.

As the economics of design necessitate a rapid increase of span-lengths and weight with increasing height of track, the units handled in reconstructing a high viaduct become very large, and the derrick-car method and the translation of the structure as a whole are likely to be impracticable.

Replacing Short Spans on Old Substructure

Several methods of replacement of short and medium-length spans have been devised and repeatedly executed until they are to a certain degree standardized; and under ordinary conditions they may frequently be selected by inspection and modified, if necessary, to suit the special conditions and requirements of the case. Under such circumstances the method that provides the most simple and rapid operations with the least temporary construction is likely to be the most economic.

Where the spans are over water that is not too swift nor too much obstructed, and where navigation permits, it is frequently possible to place both the old and the new span on barges or their equivalents, and, at the given time, disconnect the old span from its original support and carry it immediately transversely out of position, and simultaneously or successively move the new span into the former position of the old one, seating it on its permanent foundations, the old span being removed to any suitable location. This system is frequently used for drawbridges; and under favorable circumstances the entire operation can be concluded with only a very short interruption to traffic on the structure. The spans are usually raised and lowered by regulating the amount of water ballast in the barges.

Replacing by transverse displacement has been successfully accomplished for  spans  up  to  200  feet  or  more  in  length.  By this method the