Part 34

A rival scheme for carrying a ship canal across the isthmus that divides the Atlantic and Pacific Oceans is that known as the Nicaragua Canal, as the proposed route is to cross Lake Nicaragua, an extensive sheet of water situated some 400 or 500 miles north-west of the Panama Canal. The lake is 110 miles long and 45 miles broad, and is on its western side separated from the Pacific by a strip of land only 12 miles wide, having at one point an elevation not exceeding 154 feet, which is probably the lowest on the isthmus. The lake drains into the Caribbean Sea on the east, by the San Juan river, a fine wide stream, 120 miles in length, which is navigable for river boats from the Caribbean Sea up to the lake, except near its upper part, where some rapids at certain times prevent the passage of the boats. This canal project first took definite form in 1850, when a survey was made and routes reported on. The scheme attracted some attention in the United States, and in 1872, and again in 1885, further surveys and estimates were made at the instance of the States Government. The earlier schemes provided for the rise and fall between sea and lake-–108 feet, a considerable number of locks—eleven on each side, making the total length from sea to sea 181 miles. The report of the latter advocated the canalization of the San Juan by a very bold measure, namely, the construction of an immense dam, by which the waters were to be retained in the valley for many miles at the level of the lake. A company was formed to promote the project, and again in 1890 there were more surveys and estimates made. This company actually expended a considerable sum of money in attempting to improve the harbour at Greytown, which would have formed the eastern terminus, but had become silted up. But it was found afterwards that it would be better to recommend the formation of an artificial harbour at another point, by constructing two long piers running out into the sea, although this change would involve the abandonment of a few hundred yards of canal already excavated by the company near Greytown. The company has also laid down about 12 miles of railway along the proposed route, with wooden and iron sheds as workshops, offices, etc., and, moreover, had dredges and other appliances at work. At this stage it was proposed that the United States Government should guarantee the bonds of the Nicaragua Canal Company to the extent of more than twenty million pounds sterling. By an Act of Congress passed in March, 1895, a commission of engineers was appointed for the purpose of ascertaining the feasibility, permanence, and cost of construction and completion of the Nicaragua Canal by the route contemplated. The report of this commission is an elaborate and exhaustive review of the whole scheme based upon a personal examination of the route, and on the plans, surveys, and estimates made for the company, whose records, however, are stated in the report to be deficient in the supply of many important data. The Canal Company’s project provided for the improvement of Greytown harbour, as already stated, and from that place the canal was to proceed westward at the sea-level to the range of high ground on the eastern side of the isthmus, which elevation was to be ascended by three locks of unusual depth, and a deep cut more than 3 miles in length, through rock to a maximum depth of 324 feet. After passing this enormous cut, the route provides for a series of deep basins, in which the water is confined by numerous dams or embankments, the canal excavations being confined to short sections through higher ground separating these basins. The total length of these embankments will be about 6 miles, and their heights will vary from a few feet to more than seventy. About 31 miles from Greytown the canal reaches the San Juan river, which, however, by means of an enormous dam across the valley at a place called Ochoa, 69 miles below the point at which it receives the waters of Lake Nicaragua, is there practically converted into an arm of the lake. This dam, which would raise the water of the river 60 feet above its present level, and would, of course, flood the valley back to the lake, is the most notable feature of the project. Its maximum height would be about 105 feet, and the weirs on its crest, to discharge the surplus water, would require a total length of nearly a quarter of a mile. Twenty-three smaller embankments would also be needed for retaining the waters; the river would have to be deepened in the upper part, and a channel dredged out in the soft mud of the lake for 14 miles beyond the river. The big Ochoa dam is said to have no precedent in engineering construction, on account of its great height and the enormous volume of the waters it is intended to retain. No doubt its construction and safe maintenance are within the range of engineering skill, when a thoroughly exhaustive survey of the site has been made, and the necessary funds are forthcoming. From the western shore of the lake its level would also be extended by another great dam crossing the valleys of the Tola and the Rio Grande, with a length of 2,000 feet and a height of 90 feet. The canal would then be carried to the sea-level by a series of locks. The length of the canal from sea to sea would be 170 miles, but of this only 40 miles of channel would require to be excavated. The total cost of the work, as estimated by the Nicaragua Canal Company, would be about fifteen million pounds sterling, but the State Commission of Engineers thinks about double that amount would be a safer calculation, and taking into account the imperfection of the data, even this might be exceeded in certain contingencies. The Government of the United States has been urged to expend a few thousand pounds on another engineering commission, to make complete surveys, and consider all the practical problems involved, including the final selection of a route.

IRON BRIDGES.

The credit of having invented the arch is almost universally assigned to the ancient Romans, though the period of its introduction and the date of its first application to bridge building are unknown. That some centuries before the Christian era, the timber bridges of Rome had not been superseded by those of more permanent construction is implied in the legend of the defence of the gate by Horatius Cocles—a tale which has stirred the heart of many a schoolboy, and is known to everybody by Macaulay’s spirited verses, in which

“Still is the story told, How well Horatius kept the bridge, In the brave days of old.”

Some of the arched bridges built by the Romans remain in use to this day to attest the skill of their architects. The Ponte Molo at Rome, for example, was erected 100 B.C.; and at various places in Italy and Spain many of the ancient arches still exist, as at Narni, where an arch of 150 ft. span yet remains entire. Until the close of the last century the stone or brick arch was the only mode of constructing substantial and permanent bridges. And in the present century many fine bridges have been built with stone arches. The London and Waterloo Bridges across the Thames are well-known instances, each having several arches of wide span, attaining in the respective cases 152 ft. and 120 ft. The widest arch in England, and one probably unsurpassed anywhere in its magnificent stride of 200 ft., is the bridge across the Dee at Chester, built by Harrisson in 1820. At the end of last century _cast_ iron began to be used for the construction of bridges, a notable example being the bridge over the Wear at Sunderland, of which the span is 240 ft. But with the subsequent introduction of _wrought_ iron into bridge building a new era commenced, and some of the great results obtained by the use of this material will be described in the present article. In order that the reader may understand how the properties of wrought iron have been taken advantage of in the construction of bridges, a few words of explanation will be necessary regarding the strains to which the materials of such structures are exposed.

Such strains may be first mentioned as act most directly on the materials of any structure or machine, and these are two in number, namely, extension and compression. When a rope is used to suspend a weight, the force exerted by the latter tends to stretch the rope, and if the weight be made sufficiently great, the rope will break by being pulled asunder. The weight which just suffices to do this is the measure of the _tenacity_ of the rope. Again, when a brick supports a weight laid upon it, the force tends to compress the parts of the brick or to push them closer together, and if the force were great enough, the brick would yield to it by being crushed. Now, a brick offers so great a resistance to a crushing pressure, that a single ordinary red brick may be capable of supporting a weight of 18 tons, or 40,320 lbs.—that is, about 1,000 lbs. on each square inch of its surface. Thus the bricks at the base of a tall factory chimney are in no danger of being crushed by the superincumbent weight, although that is often very great. The _tenacity_ of the brick, however, presents the greatest possible contrast to its strength in resisting pressure, for it would give way to a pull of only a few pounds. Cast iron resembles a brick to a certain extent in opposing great resistance to being crushed compared to that which it offers to being pulled asunder, while wrought iron far excels the cast metal in tenacity, but is inferior to it in resistance to compression.

The following table expresses the forces in tons which must be applied for each square inch in the section of the metals, in order that they may be torn apart or crushed:

┌───────────────────────┬──────────────────────┬──────────────────────┐ │ │ Tenacity per square │Crushing pressure per │ │ │ inch, in tons. │square inch, in tons. │ ├───────────────────────┼──────────────────────┼──────────────────────┤ │Cast iron │ 8 │ 50 │ │Wrought iron │ 30 │ 17 │ │Iron wire │ 40 │ ... │ └───────────────────────┴──────────────────────┴──────────────────────┘

Besides the direct strains which tend to simply elongate or compress the materials of a structure or of a machine, there are modes of applying forces which give rise to transverse strains, tending to twist or wrench the pieces or to bend them, or rupture them by causing one part of a solid to slide away from the rest. Strains of this kind no doubt come into play in certain subordinate parts of bridges of any kind; but if we divide bridges according to the nature of the strains to which the essential parts of the structure are subject, we may place in a class where the materials are exposed to crushing forces only, all bridges formed with stone and brick arches; and in a second class, where the material is subjected to extension only, we can range all suspension bridges; while the third class is made up of bridges in which the material has to resist both compression and extension. This last includes all the various forms of girder bridges, whether trussed, lattice, or tubular. The only remark that need be here made on arched bridges is, that when cast iron was applied to the construction of bridges, the chief strength of the material lying in its resistance to pressure, the principle of construction adopted was mainly the same as that which governs the formation of the arch; but as cast iron has also some tenacity, this permitted certain modifications in the adjustment of the equilibrium, which are quite out of the question in structures of brick and stone.

The general principle of the construction of girder bridges is easily explained by considering a simple case, which is almost within everybody’s experience. Let us suppose we have a plank supported as in Fig. 139. The plank will by its own weight sink down in the centre, becoming curved in the manner shown; or if the curvature be not sufficiently obvious, it may always be increased by placing weights on the centre, as at _g_. If the length of the plank had been accurately measured when it was extended flat upon the ground, it would have been found that the upper or concave surface, _a b_, had become shorter, and the lower or convex surface, _c d_, longer when the plank is supported only at the ends—a result sufficiently obvious from the figure it assumes. It is plain, then, that the parts of the wood near the upper surface are squeezed together, while near the lower surface the wood is stretched out. Thus, the portions in the vicinity of the upper and lower surfaces are in opposite conditions of strain; for in the one the tenacity of the material comes into play, and in the other its power of resisting compression. There is an intermediate layer of wood, however, which, being neither extended or compressed, receives no strain. The position of this is indicated by the line _e f_, called the _neutral line_. If the plank, instead of being laid flat, is put upon its edge, as in Fig. 140, the deflection caused by its weight will hardly be perceptible, and it will in this position support a weight which in its former one would have broken it down. There is in this case a neutral line, _e f_, as before; but as the part which is most compressed or extended is now situated at a greater distance from the neutral line, the resistance of the material acts, as it were, at a greater leverage. Again the portions near the neutral line are under no strain; they do not, therefore, add to the strength, although they increase the weight to be supported, and they may, for that reason, be removed with advantage, leaving only sufficient wood to connect the upper and lower portions rigidly together. The form of cast iron beams, Fig. 141, which were used for many purposes, depends upon these principles. The sectional area of the lower flange, which is subjected to tension, is six times that of the upper one, which has to resist compression, because the strength of cast iron to resist pressure is about six times greater than its power of resisting a pull. If the upper flange were made thicker, the girder would be weaker, because the increased weight would simply add to the tension of the lower one, where, therefore, the girder would be more ready to give way than before. If we suppose the vertical web divided into separate vertical portions, and disposed as at Fig. 142, the strength of the girder, and the principle on which that strength depends, will be in no way changed, and we at once obtain the box girder, which on a large scale, and arranged so that the roadway passes through it, forms the tubular bridge. It is only necessary that the upper part should have strength enough to resist the compressing force, and the lower the extending force, to which the girder may be subject; and wrought iron, properly arranged, is found to have the requisite strength in both ways, without undue weight. The various forms of trussed girders, the trellis and the lattice girders, now so much used for railway bridges, all depend upon the same general principles, as does also the Warren girder, in which the iron bars are joined so as to form a series of triangles, as in Fig. 143.

Girders have been made of wrought iron up to 500 ft. in length, but the cost of such very long girders is so great, that for spans of this width other modes of construction are usually adopted.

_GIRDER BRIDGES._

The Britannia Bridge, which carries the Chester and Holyhead Railway across the Menai Straits, is perhaps the most celebrated example of an iron bridge on the girder principle. It was designed by Stephenson, but the late Sir W. Fairbairn contributed largely by his knowledge of iron to the success of the undertaking, if he did not, in fact, propose the actual form of the tubes. Stephenson fixed upon a site about a mile south of Telford’s great suspension bridge, because there occurred at this point a rock in the centre of the stream, well adapted for the foundation of a tower. This rock, which rises 10 ft. above the low-water level, is covered at high water to about the same depth. On this is built the central tower of the bridge, 460 ft. from the shore on either side, where rises another tower, and at a distance from each of these of 230 ft. is a continuous embankment of stone, 176 ft. long. The towers and abutments are built with slightly sloping sides, the base of the central or Britannia tower being 62 ft. by 52 ft., the width at the level where the tubes pass through it, a height of 102 ft., being reduced by the tapering form to 55 ft. The total height of the central tower is 230 ft. from its rock foundation. The parapet walls of the abutments are terminated with pedestals, the summits of which are decorated by huge lions, looking landwards. As each line of rails has a separate tube, there are four tubes 460 ft. long for the central spans, and four 230 ft. long for the shorter spans at each end of the bridge. Each line of rails, in fact, traverses a continuous tube 1,513 ft. in length, supported at intervals by the towers and abutments. The four longer tubes were built up on the shore, and were floated on pontoons to their positions between the towers, and raised to the required elevation by powerful hydraulic machinery. The external height of each tube at the central tower is 30 ft., but the bottom line forms a parabolic curve, and the other extremities of the tubes are reduced to a height of 22¾ ft. The width outside is 14 ft. 8 in. Fig. 144 shows the construction of the tube, and it will be observed that the top and bottom are cellular, each of the top cells, or tubes, being 1 ft. 9 in. wide, and each of the bottom ones 2 ft. 4 in. The vertical framing of the tube consists essentially of bars of ⟙-iron, which are bent at the top and bottom, and run along the top and bottom cells for about 2 ft. The covering of the tubes is formed of plates of wrought iron, rivetted to ⟙- and ∟-shaped ribs. The thickness of the plates is varied in different parts from ½ in. to ¾ in. The plates vary also in their length and width in the different parts of the tubes, some being 6 ft. by 1¾ ft., and others 12 ft. by 2 ft. 4 in. The joints are not made by overlapping the plates, but are all what are termed _butt_ joints, that is, the plates meet edge to edge, and along the juncture a bar of ⟙-iron is rivetted on each side, thus: ╬. The cells are also formed of iron plates, bolted together by ∟-shaped iron bars at the angles. The rails rest on longitudinal timber sleepers, which are well secured by angle-iron to the ⟙-ribs of the framing forming the lower cells. More than two millions of rivets were used in the work, and all the holes for them, of which there are seven millions, were punched by special machinery. The rivets being inserted while red hot, and hammered up, the contraction which took place as they cooled drew all the plates and ribs very firmly together. In the construction of the tubes no less than 83 miles of angle-iron were employed, and the number of separate bars and plates is said to be about 186,000. The expansion and contraction which take place in all materials by change of temperature had also to be provided for in the mode of supporting the tubes themselves. This was accomplished by causing the tubes, where they pass through the towers, to rest upon a series of rollers, 6 in. in diameter, and these were arranged in sets of twenty-two, one set being required for each side of each tube, so that in all thirty-two sets were needed. There are other ingenious arrangements for the same purpose at the ends of the tubes resting on the abutments, which are supported on balls of gun-metal, 6 in. in diameter, so that they may be free to move in any manner which the contractions and expansions of the huge tubes may require. Each of the tubes, from end to end of the bridge, contains 5,250 tons of iron. The mode in which these ponderous masses were raised into their elevated position is described in the article on “Hydraulic Power,” as it furnishes a very striking illustration of the utility and convenience of that contrivance. The foundation-stone of the central tower was laid in May, 1846, and the bridge was opened in October, 1850. The tubes have some very curious acoustic properties: for example, the sound of a pistol-shot is repeated about half a dozen times by the echoes, and the tubular cells, which extend from one end of the bridge to the other, were used by the workmen engaged in the erection as speaking-tubes. It is said that a conversation may thus be carried on with a person at the other end of the bridge, a distance of a quarter of a mile. The rigidity of the great tubes is truly wonderful. A very heavy train, or the strongest gale, produces deflections in the centre, vertical and horizontal respectively, of less than one inch. But when ten or a dozen men are placed so that they can press against the sides of the tube, they are able, by timing their efforts so as to agree with the period of oscillation proper to the tube, to cause it to swing through a distance of 1¼ in.—an illustration of facts of great importance in mechanics, showing that even the most strongly built iron structure has its own proper period of oscillation as much as the most slender stretched wire, and that comparatively small impulses can, by being isochronous with the period of oscillation, accumulate, as it were, and produce powerful effects. Bridges are often tried by causing soldiers to march over them, and such regulated movements form the severest test of the freedom of the structures from dangerous oscillation. The main tubes of the Britannia Bridge make sixty-seven vibrations per minute. The expansion and contraction occurring each day show a range of from ½ in. to 3 in. The total cost of the structure was £601,865.

A stupendous tubular bridge has also been built over the St. Lawrence at Montreal, and the special difficulties which attended its construction render it perhaps unsurpassed as a specimen of engineering skill. The magnitude of the undertaking may be judged of from the following dimensions: Total length of the Victoria Bridge, Montreal, 9,144 ft., or 1¾ miles; length of tubes, 6,592 ft., or 1¼ miles: weight of iron in the tubes, 9,044 tons; area of the surface of the ironwork, 32 acres; number of piers, 24, with 25 spans between the piers, each from 242 ft. to 247 ft. wide.