cautions the danger can be pretty nearly eliminated. About as long a plate-girder as has ever been shipped in one piece was one of one hundred and thirty-two (132) feet. It required four flat-cars to transport it. Longer plate-girder spans than this have been built, notably tubular bridges and swing spans, but they were shipped in parts and assembled at site. This expedient for simple spans is really permissible only in case of bridges to be sent to foreign countries, and it is to be avoided if possible even then, because it is sometimes difficult to obtain a satisfactory job of field-riveting when making the splices, although the use of pneumatic riveters tends to reduce materially the force of this objection.

As far as economics is concerned, it may be stated that, if deck plate- girders are feasible for any opening, they are more economical than truss spans up to a length that is prohibitory for shipment. As the depth of a very long plate-girder is generally from one-tenth (1/10) to one-twelfth (1/12) of the span, the requirements of underneath clearance often bar out deck plate-girders and necessitate either half-through plate-girders or through trusses.

Again, the great depth required for very long plate-girder spans often sets the limit for span-length because of shipping requirements. Some railroads have tunnels and overhead crossings which are lower than custom is now requiring; and very deep girders loaded on flat-cars might not be able to pass—nor could such girders be placed flat, because then the horizontal clearance would be encroached upon.

Half-Through Plate Girder-Spans

The economic limit of length for this type of structure is materially less than that of the type just treated, because of the necessity for using a steel floor. On this account it has not the advantage over the through-truss bridge which the deck-plate girder structure possesses. For a length of one hundred (100) feet the weight of metal in the latter type exceeds that in the former, by from five (5) to fifteen (15) per cent, the smaller figure being for the lightest live-loads and the larger for the heaviest. Of course, the cheaper metal of the plate-girder type would tend to offset its greater weight, but, in order to make the costs of the two 100-ft. steam-railway-bridge spans the same, the ratio of pound prices for metal erected in the girders and trusses themselves would have to be from 1.1 to 1.3—a condition of market that is unusual. But as, for various good reasons, it hardly seems advisable to build through, steam-railway spans shorter than one hundred (100) feet, it is well to adopt this length as the superior limit for half-through plate-girders and deck plate-girders in standard railway bridges. For electric-railway bridges and highway bridges, this limit might advantageously be reduced to about seventy-five (75) feet.