Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/348

Rh 304 BRIDGES [SUSPENSION BRIDGES. can be bought in large quantities of a quality which does not break with less than a stress of from 55 to GO tons per square inch of section ; charcoal iron wire of the sizes used will bear 40 tons per square inch ; common sizes of wire for the purpose are from O lG to 14 inches, or say, No. 9 or 10 Birmingham wire gauge. Three or four thousand wires are not unfrequently used in one cable, and it is very essential that each wire shall take an equal part of the whole stress. It used to be thought necessary to ensure this by straining each wire separately either over the actual piers, or piers similarly placed, and binding them together when hanging, strained by their own weight with the dip proposed for the bridge. It was also thought essential that each rope should bs an aggregate of parallel wires, not spun as in a hempen rope. Experiment has shown, however, that wire ropes spun with machines which do not put a twist into each wire, but lay it helically and untwisted, and with no straight central wire, are as strong as wire ropes of equal weight made with straight wires. They are, however, much more easily made. A number of ropes of this kind may, therefore, with more convenience and economy be bound together into one cable in the manner previously practised for single wires. Care should be taken to fill every interstice of the ropes with a bitumin ous compound. When the chains are made of links of iron their ultimate strength cannot be taken as more than 30 tons per square inch, even if the very best material is secured. It is doubtful if this ultimate strength can at present be surpassed by steel links, for although many steel links of greater strength could certainly be obtained, occasionally a comparatively weak link will be produced even by the i/3 o best manufacturers. In designing the links care must be taken to provide sufficient cross section at the eye, AB + CD, fig. 36, as well as at EF. The diameter of the pin BC must be such as will allow it to resist the shearing stress on it, and the surface of the pin and eye from B to C must be sufficient to bear the crushing stress, Otherwise, although the pin may not be shorn it may be squeezed flat, and the head of the link may bulge out and be much distorted under the stress. To obtain the necessary surface, without unduly increasing the diameter of the pin, the link may be rolled with a head Fig. 37. broader than the body of the link. The section at GH and IJ must also be sufficient to resist shearing. When two or more parallel chains are used, care must be taken that the rods suspending the platform bear equally on the several chains. Fig. 37 shows a plan of securing this. Chains of unequal dip should not be used to support one platform, for the strain cannot be equally divided between them, inasmuch as they must deflect unequally with any passing load, or with any increase of temperature. 33. Merits and Defects of Suspension Bridges. The great merit of a suspension bridge is its cheapness, arising from the comparatively small quantity of material required to carry a given passing load across a given span. This merit may be easily seen by considering an elementary example. A man might cross a chasm of 100 feet hanging to a steel wire - 21 inches in diameter, dipping 10 feet; the weight of the wire would be 12 75 Ib. A wrought iron beam of rectangular section, three times as deep as it is broad, would have to be about 27 inches deep and 9 inches broad to carry him and its own iveight. It would weigh 87,500 Ib. An iron I beam of the best construction, 10 feet deep, would weigh about 120 Ib, without allowing anything for the stiffening of the centre web which would in practice be required. In each case four feet in length have been allowed for bearings at the ends of the span. The enormous difference would not exist if the beam and wire had only to carry the man, although even then there would be a great difference in favour of the wire ; the main difference arises from the fact that the bridge has to carry its ou-n weight. The chief merit of the suspension bridge does not, therefore, come into play until the weight of the rope or beam is considerable when compared with the platform and rolling load ; for although the chain will for any given load be lighter than a beam, the saving in this respect will for small spans be more than compensated by the expense of the anchorages. In large spans tho advantage of the suspension bridge is so great that we find bridges on this principle of 800 or 900 feet span con structed at much less cost per foot run than girder bridges of half the span. The disadvantages of the suspension bridge are, however, very great. A change in the distribution of the load causes a very sensible defor mation of the structure ; for the chain of the suspension bridge must adapt its form to the new position of the load, whereas in the beam the deformation is hardly sensible, equilibrium being attained by a new distribution of the stresses through the material. This flexibility of the sus pension bridge renders it unsuitable for the passage of a railway train at any considerable speed. The platform rises up as a wave in front of any rapidly advancing load, and the masses in motion produce stresses much greater than those which could result from the same weights when at rest ; moreover, the kinetic effect of the- oscillations produced by bodies of men marching, or even by impulses due to wind, may give rise to strains which cannot be foreseen, and which have actually caused the failure of some suspension bridges. On the 16th of April 1850 a suspension bridge at Angers gave way when 487 soldiers were passing, and of these 226 were killed by the acci dent. Another danger peculiar to suspension bridges is that the platform may be lifted by the wind, when its oscillation will produce most dangerous strains. This accident may be prevented by tying the platform down to the piers or abutments. Lateral oscillation produced by the wind is also dangerous, and even gathered ice and snow may be a serious increment to the load on these bridges, forming a much more considerable frac tion of the whole weight than where the supporting structure is itself massive. Suspension bridges must be well cross-braced to resist the action of the wind. They can be much stiffened laterally by placing the chains in inclined planes, converging downwards to the platform. 3i. Modifications of the Simple Siispension Bridge. Many efforts have been made to design a bridge which