"There are no ambiguities in the calculations in reference to the distribution of the load; and the distance required from base of rail to masonry is generally less than that required for a rim-bearing bridge with proper distribution of the load over the drum. Any irregular settlement of the masonry does not materially affect its operation.

"On the other hand, the rim-bearing bridge requires a circular girder or drum of difficult and expensive construction, a ring of accurately-turned rollers, and circular tracks that necessitate great care in their construction and delicate adjustment in their erection, in order to make the bridge operate satisfactorily. Repairs are troublesome and expensive; and any irregular settlement of the masonry will throw the whole turning apparatus out of order."

As an illustration of diametrically opposite opinion, the following quotation from a printed statement by the late C. H. Cartlidge, formerly Bridge Engineer of the Chicago, Burlington, and Quincy Railway, is directly to the point. "The writer's experience with center-bearing draw-spans has been such as to prejudice him against them for spans of any magnitude. It seems difficult at any reasonable cost to proportion the pivot-bearing so that it will not wear; and any wear on a pivot-bearing is expensive to repair. On one draw the wearing away of the bronze bearing in the pivot allowed the upper and lower castings to rub, making the turning of the draw a very noisy operation, while the few wheels provided to steady the span during turning were overworked and cut the circular track badly."

There is a combination of the rim-bearing and the center-bearing swings advocated by some engineers; but the author, on general principles, objects to hybrid designs, and especially in this case where there must exist a great uncertainty concerning the distribution of load between rim and pivot.

Bob-Tailed Swing versus Ordinary Swing

While there is apparently a saving in first cost by cutting down the length of one arm of a swing-span so as to convert it into a "bob-tailed" structure, that saving is generally absorbed by the adoption of more power and heavier machinery (with which to operate against unbalanced wind loads), heavy counterweights, and the special metal needed to support the said counterweights.

There are other questions of economics in swing spans, such as plate-girder versus truss-span structures, arid continuous versus non-continuous trusses over pivot piers; but in view of the fact that the author is opposed on principle to building any more swing spans, it were useless to carry further this economic dissertation, especially as the subject of swing bridges is treated quite thoroughly in Chapter XXIX of "Bridge Engineering."