such that, if well designed, they are likely to outlast the roadway and trusses, consequently the latter can easily be replaced without other support than that afforded by the cables themselves.

For spans approaching the present limits of lengths, the necessary practice is for such efficient construction, capacity, and maintenance that there is little prospect of any need for replacement of the superstructures; and, should it become necessary, there would probably be little choice of methods. Any scheme that would be possible, however, should be entitled to favorable consideration.

Erection of Concrete-Girder Bridges

Concrete-girder bridges are usually concreted in situ by ordinary methods and with standard equipment involving regular operations that do not present any special economic features. There may, however, be a decided economic consideration in the question of using pre-cast members for such structures. The large capacity of derrick cars and of other equipment that is available makes it possible in some cases to cast long girders and heavy floor slabs in multiple, in the Contractor's yard, remote from the bridge site and under conditions more favorable to good work and economical operation than those existing at the latter, and afterwards to transport the well-seasoned units to position and set them in place ready for service. This method is especially applicable to railway structures.

The economic considerations are likely to involve not only freight cost but also quality of structure and rapidity of operations as affecting the interruption of traffic.

Concrete Arch Spans

Arch spans must be concreted in situ on forms rigidly supported against settlement and provided with devices for striking the center so as to swing the arch free of its construction support when the concrete has sufficiently hardened. For short spans, low heights, and where obstructions underneath are permissible, the forms can usually be most economically supported on ordinary falsework-bents.

Where the height is great, or where passage must be maintained underneath for stream flow, navigation, or traffic, it is often necessary to carry these forms on arch-center trusses. For short spans these may be of either timber or steel; but for medium and long spans they are almost always constructed of riveted steelwork, usually arranged in sets braced together and often moved transversely from side to side of a wide bridge and longitudinally from span to span as the work progresses, thus making one set suffice for many places.