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664. Abutments. The forms and dimensions of the abutments will depend upon the manner in which they may be connected with the chains. When the locality will admit of the chains being anchored without deflecting them vertically, the abutments may be formed of any heavy mass of rough masonry, which, from its weight, and the manner in which it is imbedded, have sufficient strength to resist the tension in the direction of the chain. If it is found necessary to deflect the chains vertically to secure a good anchoring point, it will also generally be necessary to build a mass of masonry of an arched form at the point where the deflection takes place, which, to present sufficient strength to resist the pressure caused by the resultant of the tension on the two branches of the chain, should be made of heavy blocks of cut stone well bonded. If the abutments are not too far from the foundations of the piers, it will be well to connect the two, in order to give additional resistance to the anchoring points.

665. Main Chains, etc. The suspending curves, or arches, may be made of chains formed of flat or round iron, or may consist of wire cables constructed in the usual manner.

The main chains of the earlier suspension bridges were formed of long links of round iron made in the usual way; but, independently of the greater expense of these chains, they were found to be liable to defects of welding, and the links, when long, were apt to become misshapen under a great strain, and required to be stayed to preserve their form. Chains formed of long links of flat bars, usually connected by shorter ones, as coupling links, have on these accounts superseded those of the ordinary oval-shaped links.

The breadth of the chains has generally been made uniform, but in some bridges erected in England by Mr. Dredge, the chains are made to increase uniformly in breadth, by increasing the number of bars in a link, from the centre to the points of suspension. In addition to this change in the form of the main chains, Mr. Dredge places the suspending chains in a vertical plane parallel to the axis of the bridge, but obliquely to the horizon, inclining each way from the points of suspension towards the centre of the curve. This system has never come into general use. At the present day nearly all cables. of suspension bridges are made of wire.

Some of the links of the main chains should be arranged with adjusting screws, or with keys, to bring the chains to the proper degree of curvature when set up.

The chains may either be attached to, or pass over a movable cast-iron saddle, seated on rollers on the top of the piers,

so that it will allow of sufficient horizontal displacement to permit the chains to accommodate themselves to the effects of a movable load on the roadway. The same ends may be attained by attaching the chains to a pendulum bar suspended from the top of the pier.

The chains are firmly connected with the abutments, by being attached to anchoring masses of cast iron, arranged in a suitable manner to receive and secure the ends of the chains, which are carefully imbedded in the masonry of the abutments. These points, when under ground, should be so placed that they can be visited and examined from time to time.

666. Suspending-Chains. The suspending-rods, or chains, should be attached to such points of the main chains and the roadway-bearers, as to distribute the load uniformly over the main chains, and to prevent their being broken or twisted off by the oscillations of the bridge from winds, or movable loads. They should be connected by suitably-arranged articulations, with a saddle piece bearing upon the back of the main chain, and at bottom with the stirrup that embraces the roadway-bearers.

The suspending-chains are usually hung vertically. In some recent bridges they have been inclined inward to give more stiffness to the system.

667. Roadway. Transversal roadway-bearers are attached to the suspending-chains, upon which a flooring of timber is laid for the roadway. The roadway-bearers, in some instances, have been made of wrought iron, but timber is now generally preferred for these pieces. Diagonal ties of wrought iron are placed horizontally between the roadwaybearers to brace the frame-work.

The parapet may be formed in the usual style either of wrought iron, or of timber, or of a combination of cast iron and timber. Timber alone, or in combination with cast iron, is now preferred for the parapets; as observation has shown that the stiffness given to the roadway by a strongly-trussed timber parapet limits the amplitude of the undulations caused by violent winds, and secures the structure from danger.

In some of the more recent suspension bridges, a trussed frame, similar to the parapet, has been continued below the level of the roadway, for the purpose of giving greater security to the structure against the action of high winds.

When the roadway is above the chains, any requisite number of single chains may be placed for its support. Frames formed of vertical beams of timber, or of columns of cast

iron united by diagonal braces, rest upon the chains, and support the roadway-bearers placed either transversely or longitudinally.

668. Vibrations. The undulatory or vibratory motions of suspension bridges, caused by the action of high winds, or movable loads, should be reduced to the smallest practicable amount, by a suitable arrangement of bracing for the roadway-timbers and parapet, and by chain-stays attached to the roadway and to the basements of the piers, or to fixed points on the banks whenever they can be obtained.

Calculation and experience show that the vibrations caused by a movable load decrease in amplitude as the span increases, and, for the same span, as the versed sine decreases. The heavier the roadway, also, all other things being the same, the smaller will be the amplitude of the vibrations caused by a movable load, and the less will be their effect in changing the form of the bridge.

The vibrations caused by a movable load seldom affect the bridge in a hurtful degree, owing to the elasticity of the system, unless they recur periodically, as in the passage of a body of soldiers with a cadenced march. Serious accidents have been occasioned in this way; also by the passage of cattle, and by the sudden rush of a crowd from one side of the bridge to the other. Injuries of this character can only be guarded against by a proper system of police regulations.

Chain-stays may either be attached to some point of the roadway and to fixed points beneath it, or else they may be in the form of a reversed curve below the roadway. The former is the more efficacious, but it causes the bridge to bend in a disagreeable manner at the point where the stay is attached, when the action of a movable load causes the main chains to rise. The more oblique the stays, the longer, more expensive, and less effective they become. Stays in the form of a reversed curve preserve better the shape of the roadway under the action of a movable load, but they are less effective in preventing vibrations than the simple stay. Neither of these methods is very serviceable, except in narrow spans. In wide spans, variations of temperature cause considerable changes in the length of the stays, which makes them act unequally upon the roadway; this is particularly the case with the reversed curve. Both kinds should be arranged with adjusting screws, to accommodate their length to the more extreme variations of temperature.

Engineers at present generally agree that the most efficacious means of limiting the amplitude, and the consequent

injurious effects of undulations, consists in a strong combination of the roadway-timbers and flooring, stiffened by a trussed parapet of timber above the roadway, and in some cases in extending the framework of the parapet below it. These combinations present, in appearance and reality, two or more open-built beams, as circumstances may demand, placed parallel to each other, and strongly connected and braced by the framework of the roadway, which are supported at intermediate points by the suspending rods or chains. The method of placing the roadway-framing at the central line of the open-built beams, presents the advantage of introducing vertical diagonal braces, or ties, between the beams beneath the roadway-frame. The main objections to these combinations is the increased tension thrown upon the chains from the greater weight of the framework. This increase of tension, however, provided it be kept within proper limits, so far from being injurious, adds to the stability and security of the bridge, both from the effects of undulations and of vibrations from shocks.

As a farther security to the stability of the structure, the framework of the roadway should be firmly attached at the two extremities to the basements of the piers.

669. Preservative Means. To preserve the chains from oxidation on the surface, and from rain or dews which may lodge in the articulations, they should receive several coats of minium, or of some other preparation impervious to water, and this should be renewed from time to time, and the forms of all the parts should be the most suitable to allow the free escape of moisture.

Wires for cables can be preserved from oxidation, until they are made into ropes, by keeping them immersed in some alkaline solution. Before making them into ropes, they should be dipped several times in boiling linseed oil, prepared by previously boiling it with a small portion of litharge and fampblack. The cables should receive a thick coating of the same preparation before they are put up, and finally be painted with white-lead paint, both as a preservative means, and to show any incipient oxidation, as the rust will be detected by its discoloring the paint.

670. Proofs of Suspension Bridges. From the many grave accidents, accompanied by serious loss of life, which have taken place in suspension bridges, it is highly desirable that some trial-proof should be made before opening such bridges to the public, and that, moreover, strict police regulations should be adopted and enforced, with respect to them,

to guard against the recurrence of such disasters as have several times taken place in England, from the assemblage of a crowd upon the bridge. In France, and on the continent generally, where one of the important duties of the public police is to watch over the safety of life, under such circumstances, regulations of this character are rigidly enforced. The trial-proof enacted in France for suspension bridges, before they are thrown open for travel, is about 40 lbs. to each superficial foot of roadway in addition to the permanent weight of the bridge. This proof is at first reduced to onehalf, in order not to injure the masonry of the points of support during the green condition of the mortar. It is made by distributing over the road surface any convenient weighty material, as bricks, pigs of iron, bags of earth, etc. Besides this after-trial, each element of the main chains should be subjected to a special proof to prevent the introduction of unsound parts into the system. This precaution will not be necessary for the wire of a cable, as the process of drawing alone is a good test. Some of the coils tested will be a guarantee for the whole.

From experiments made at Geneva, by Colonel Dufour, one of the earliest and most successful constructors of suspension bridges on the Continent, it appears that wrought bar iron can sustain, without danger of rupture, a shock arising from a weight of 44 lbs., raised to a height of 3.28 feet on each, .0015dths of an inch of cross-section, when the bar is strained by a weight equal to one-third of its breaking weight; and he concludes that no apprehension need be entertained of injury to a bridge from shocks caused by the ordinary transit upon it, which has been subjected to the usual trial of a dead weight; and that the safety, in this respect, is the greater as the bridge is longer, since the elasticity of the system is the best preservative from accidents due to such causes. Mr. Whoeler, an engineer in Germany, concluded, after a long series of carefully conducted experiments, that good wrought iron would sustain any number of continuous shocks, provided that it was in no case strained more than 10,000 pounds per square inch of section.

671. Durability. Time is the true test of the durability of the structures under consideration. So far as experience goes there seems to be no reason to assign less durability to suspension than to cast-iron or even stone bridges, if their repairs and the proper means of preserving them from decay are attended to. Doubts have been expressed as to the durability of wire cables, but these seem to have been set at rest

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