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applied to some bridges in Europe , where the floor, in consequence of a too lightconstruction, too easily yields to the weight of a transitory load, and where thelength of suspenders has been accurately determined for a catenary or parabola.The great assistance rendered by stays to cables, will cause the latter to sink atthe centre and rise at the ends. The catenary curve of the cables will thereforebe changed, the length of suspenders will not agree with our calculations, andwe must resort to a practical mode of adjusting them. But these little difficul-ties are easily overcome, and the trouble will be amply rewarded by the greaterperfection of the work. The few stays applied on the Monongahela, are fulfillingtheir object and add much to the stiffness and solidity of that structure. Thesystem of stays proposed for the Cincinnati bridge, will not only increase itsstrength and stiffness, but also add much to its appearance.
The floor will be constructed of well-seasoned white pine timber and plank,and will be divided into a roadway of 20 feet 6 inches wide, and two side-walksof 6 feet wide each. The distance across the floor, between the railings, will be34 feet. The floor itself will be composed of two courses of plank, the lowercourse running in the direction of the bridge, the second course laid across.The first course on the roadway will be two and one-half inch white pine, thesecond course two and one-half inch white oak. The two courses on the side-,walks will be two inches each, and of white pine. The bearers upon whichthe floor rests will be arranged in pairs of four feet apart from centre to centre.The roadway will be divided into two tracks, but so that the division will formno impediment to driving from one to the other. If, for instance, a driver onhorseback, or a light carriage, wishes to be ahead of a heavy slow-drivingteam, they can turn out upon the second track and then return upon the first.The side-walks will be separated from the roadway by the cables, suspenders,stays, and fender-rails.
The plan to be adopted for the construction of the floor will, in its mainfeatures, be the same as was applied on the Monongahela bridge, but moreperfect in all its parts, and, in consequence, more expensive. The rigid econ-omy which had to be observed on the Monongahela, did not admit of someimportant improvements, which I contemplate to introduce on the Cincinnati bridge, and which are the result of long study and experience. The floor ofthe Monongahela bridge, when heavy teams are passing over it, is about as stiffand stable as a common wooden arch bridge of the same spans. The vibrationscaused by more light and rapidly moving loads are greater, but not so great asto be very perceptible. A higher degree of stiffness will be required for theCincinnati bridge. It does, however, not follow that the flexibility of the floorof a suspension bridge increases with the length of a span. There are means atthe disposal of the engineer, which will enable him to impart to a large spana greater stiffness and more stability than is at all requisite for a smaller one. Atransitory load bears a smaller proportion to the weight of a large span than toa small span. By disposing of this larger quantity of material in such a mannerthat every portion will add to the stiffness of the structure, it will become appa-rent why large spans can be made as stiff or stiffer than smaller ones.