« AnteriorContinuar »
five feet were added to the clear elevation proposed in the plans. This requirement, together with an increase of five feet in the breadth, added 8 per cent. to the original estimates. Other changes in the designs, which swelled the actual cost to nearly $15,000,000, were the adoption of solid masonry for the approaches, instead of light iron trusses, and the sinking of caissons for the towers, instead of erecting them on a foundation of piles. There were many hindrances and delays in the work of construction. In addition to the enormous and to a considerable extent unexpected technical difficulties, differences frequently occurred between the two municipalities, and occasionally appropriations were not granted in time, so that the work had to be suspended for long periods. The piers were built up by the aid of caissons of unprecedented size. The dimensions of the towers at the base are 140 by 59 feet. The New York tower was founded on the bed-rock, 78; feet below the surface of the water. The Brooklyn tower was built up from the clay, 444 feet below the surface. The lowering of the Brooklyn caisson began in May, 1870, and was completed in March, 1871. The New York caisson was towed into position in October, 1871, and sunk to the rock by the May following. The erection of the enormous towers was a work of time. The Brooklyn tower was finished in May, 1875, and the New York tower in July, 1876. The towers are each pierced by two archways 31% feet wide at the height of 118 feet above high-water mark. Through these openings passes the floor of the bridge. Above the arches, which are 1203 feet high, the partitions reunite, and the towers rise 80 feet higher, to support the saddles which sustain the cables. The total height of the towers above the surface of the water is 276 feet. The height of the bridge-floor over high-water mark is 118 feet at the towers, and 135 feet in the center of the span. The four cables are 16 inches in diameter, and contain about 5,000 single wires each. The wire is of +-inch size; 278 single wires were grouped into a rope and 19 ropes bunched to form a cable. The wires were carried forward and back from anchorage to anchorage over the towers. The sun, expanding the more exposed wires, and the wind, rendered the nice work of forming the ropes with mathematical exactness exceedingly difficult. The work of stringing the wires began in June, 1877, and was completed in October, 1878. On one occasion a bundle of wires broke away from the anchorage and shot across the tower, falling into the river. The iron saddles on which the cables rest are made movable, to permit of expansion and compression on a saddle-plate of iron firmly imbedded and anchored in the towers. The saddles are 13 feet long, 4 broad, and 44 thick. They glide through minute distances in response to strains and changes of temperature, upon 40 iron rollers.
The anchorages are 930 feet from the towers on each side; they are solid masses of masonry, each 132 by 119 feet at base and top, 89 feet high, and weighing 60,000 tons. During the construction of the bridge 20 fatal and many disabling accidents occurred. The compressed air of the caissons caused over 100 cases of caisson-disease. The victim of the first accident was Engineer Roebling, who died from lockjaw resulting from a crushed foot, received when laying the foundations of one of the ..o. July 22, 1869. His son, Washington A. Roebling, took charge of the work, but in 1871 he was prostrated with a peculiar form of caisson-disease which destroyed the nerves of motion, the result of a fire in the Brooklyn caisson in 1870. This fire necessitated the flooding of the caisson, and delayed the work two months. After his accident Mr. Roebling never was capable of active work. His intellectual faculties, however, were unimpaired, and he was able to make the plans and calculations, and to superintend the construction, through the mediation of his wife. It was not until 1876 that he was sufficiently restored to be removed, after which he remained in view of the bridge, directing the work, though for a long time after that he was still incapable of locomotion. The total length of the bridge and approaches is 5,989 feet. Of this the middle span takes up 1,595 feet, the distance between the towers and the anchorages on each side 930 feet, and the approaches 1,5624 feet on the New York and 972+ feet on the Brooklyn side. The length of the suspended structure, from anchorage to anchorage, is 3,454 feet. Its total weight is 6,470 tons. The maximum load which it is designed to bear is 1,740 tons. The ultimate resistance is calculated at 49,200 tons. (For other measurements see “Annual Cyclopaedia” for 1882.) The bridge is divided into five avenues. The central one, 15+ feet in width, is the path for foot-passengers. The two outer ones, 19 feet wide, are for vehicles. The others are laid with the rails for the cars, which are drawn by an endless chain. They are attached and detached by means of a “grip ’’ arrangement. Niagara Cantilever Bridge.—A double-track railroad-bridge over Niagara river, about 300 feet above the railroad suspension-bridge, completed in November for the New York Central and Michigan Central Railroads, is constructed on the new cantilever principle, which is that of a balanced beam. In the perfect cantilever, represented by the bridge now building over the Forth, in Scotland, of which a description is given below, the diagonally-braced frame of the cantilever is exactly poised on the upright iron columns in the center. In the Niagara bridge, designed by C. C. Schneider and Edmund Hayes, the abutting banks are made use of to attach the shore ends to a mass of masonry which counterpoises the extra weight of the river arms, and stays and anchors the
entire structure, and the two arms of the cantilever are different in length and in details of construction. The cantilever type of highlevel bridges is a development of the use of cast-steel, which combines with rigidity a tensile elasticity that enables it to resist lateral strains to a certain degree. Like the suspension-bridge, the cantilever span can be carried over places where, as in the Niagara chasm, it is impossible to erect temporary supports. The two gigantic steel towers which bear up the eantilevers of the Niagara bridge are 1325 feet high, and rest on stone piers 39 feet high. They are composed of four columns of plates and angles riveted together, braced with horizontal struts and ties. They converge upward with a batter of 1 in 24 in the direction of the length of the bridge and 1 in 8 at right angles to the middle line of the bridge. The cantilevers are each 395 feet in length. A space of 120 feet between the river ends of the cantilevers is spanned by a girder resting on the extremities of the arms. The total length of the bridge is 910 feet between the centers of the anchorage-piers. The clear span between the towers is 470 feet. The height of the bridge is 239 feet from the surface of the river to the rail. The cantilevers are composed of two trusses, 28 feet apart, having a depth of 56 feet at the towers, 26 feet at the extremities of the river arms, and 21 feet at the shore ends. The materials used in the bridge are steel and wrought-iron, the former for the towers and the lower chords, center posts, and all the pins, and the latter for all the tension members. The steel pins connecting the members fit into the bored holes with the utmost accuracy. The lower chords and center posts are latticed channel-plates. The upper chords are heavy eye-bars. A compression member is packed between the chords of the shore arms. The shore ends of the beams are anchored to masonry abutments by short links, which serve also as expansion-joints. Joints are provided also at the connection of the intermediate span with the river ends, to allow for contraction and expansion due to changes of temperature. The floor-beams are wroughtiron plates and angles, 4 feet deep, riveted between the vertical posts. On these rest four lines of stringers, consisting of plate-girders 2} feet deep. The width of the floor is 32 feet, a plank walk and iron railing at the side of the tracks being supported by the white-oak ties, one half of which project beyond the tracks for the purpose. Each column of the towers stands on a limestone pier, 12 feet square at the top and battering 1 in 24. The piers are connected by walls 34 feet wide at top. The courses of the piers are 2 feet deep. The foundations are a solidified mass of bowlders, béton, and cement, 20 by 45 feet and 8 feet deep under each pair of piers. The anchoragepiers are 11 by 374 feet under the coping, and consist of blocks of masonry, each measuring 460 cubic yards and weighing 1,000 tons, raised
upon 12 iron plate-girders 24 feet deep and 36 feet long, resting in turn on 18 15-inch I-beams through which the anchorage-rods pass in such a way that the pressure is distributed evenly over the entire mass of masonry. The maximum uplifting force of the cantilevers is 678,000 pounds, or only about one third of the weight of the piers. After the towers were built, the shore arms were constructed by the aid of temporary structures, in the usual way. After they were completed and attached to the anchorages, the river arms were built out over the river, one panel at a time, by means of huge traveling steam-derricks. When each panel was constructed and braced, the traveler was moved forward and the next panel erected. The intermediate 120-foot span was specially designed with bottom compression members so that it also could be built out from the end of each arm by the aid of temporary stays, which were removed when the two halves of the girder were fitted together in the middle. The bridge is designed to bear a running load of a ton per lineal foot, that being one fifth of the calculated ultimate resistance, and for a wind pressure of thirty pounds per square foot on twice the exposed face of the truss, floor, and train. Forth Railway-Bridge.—The completion of the Niagara cantilever bridge lends interest to a description of the one over the river Forth at Queens Ferry, in Scotland, which was begun in 1883. The engineers of Great Britain, to whom the development of the cantilever principle is due, have never taken kindly to the suspension principle, just as they are in general skeptical of the stability of the lighter structures which American engineers design for equal stresses. Yet, after condemning the principle for a whole generation, while the Niagara suspension-bridge stood as a practical demonstration of its soundness, at last Sir Thomas Bouch adopted the American idea in his design for the projected bridge over the Forth. The river was to be bridged by two suspension spans, with towers nearly 600 feet high, one on each bank and two on the island of Inchgarvie in the middle of the estuary. The foundations were already dug, when the Tay bridge disaster first brought to the knowledge of engineers facts relating to the intensity of wind-strains which meteorologists had already published to the world. The designs for the suspension-bridge, whose author was the designer also of the collapsed Tay structure, were discarded. Fowler and Baker, the new engineers, drew plans for a cantilever truss double-span bridge, all of steel, which will be the most stupendous structure of its kind. The material is tested for an ultimate resistance of thirty tons per square inch in tension, and thirty-four tons in compression, and the structure is planned to sustain four times the combined strain of a wind pressure of fifty-six pounds to the square foot and a maximum running load of two tons to the foot, or 3,400 tons on a span. The breadth to be spanned in the Forth is not much more than half that of the Tay at Dundee, where the new bridge, which, will cost about £750,000, is making rapid progress and is expected to be finished in 1885. But the channels on the two sides of the island of Inchgarvie are about 200 feet deep, and must therefore be crossed by spanning the entire breadth of some 1,600 feet on each side. Three balanced cantilevers are sufficient to accomplish this. Two of them rest on piers erected at the edge of the channel on each side of the river, and one on the island. The cantilever has the shape of an elongated diamond. There are four masonry piers to support the four gigantic legs on which are poised the balancing arms, which extend 675 feet on each side of the base. The uprights converge upward, being 120 feet apart at the base, 33 feet at the top, where the middle
oint of the girder rests on the ends of the
egs. The middle cantilever is longer than the other two, its base being 270 feet long, while theirs are 155 feet. The four legs are steel tubes, 12 inches in diameter and 320 feet long. The height of the bridge above the piers is 330 feet. While a lattice-girder forms the upper side of the cantilever, the under side of the enormous truss is a hollow curve, approaching in form a quadrant of a circle, drawn from the base of the legs, or struts, to the ends of the cantilever. The ends of the beams do not touch each other within 350 feet. The intermediate space is bridged by lattice-girders resting on the ends of the arms. On the shore sides of the outside cantilevers the weight of these girders is counterpoised by an equal weight of metal. The bridge will present the appearance of two distended arches and a half-arch at each end. The shore sections will
consist of girders resting on stone piers. There will be eight piers within high-water mark and two on land, on the south side, and six on the north side, all on land. Garabit Wiaduct.—A bridge, begun in 1881 and to be finished in 1884, which is intended to carry a railroad over a river at Garabit,
France, is even loftier than the lately completed Kinzua bridge in Pennsylvania. The French viaduct has a great arch in the center. The height from the bed of the river to the rail is 413 feet, while in the Kinzua valley viaduct the level of the rail is 301 feet above the stream-bed. In length the French structure is 1,880 feet, or 171 feet less than the other. American Transcontinental Railroads.-The completion of the Northern Pacific railroad in October gives the United States three or properly four great transcontinental lines, while two more are far advanced in construction. The Atchison, Topeka, and Santa Fé, the last link of which was finished two months before the Northern Pacific, is the fourth to reach completion. The Northern Pacific has its eastern terminus at Duluth, where it connects with land and water routes to the seaboard. The road has recently been extended eastward to Superior City, with the intention of ultimately crossing the Sault Ste. Marie and finding an outlet on the seaboard by one of the new trunk lines. The western terminus and the difficult section across the Cascade range were changed from the original plan. owing to the combination under the Villard management with the Oregon company. Instead of terminating at Puget Sound, the Pacific section follows Columbia river down to Portland. The first of the transcontinental lines that was built was the Union Pacific from Omaha to Ogden, continued by the Central Pacific, to San Francisco. This road was chartered in 1863 and completed in 1869. The Southern Pacific, in connection with the Texas and Pacific, forms a third transcontinental route. The eastern terminus of the Southern Pacific railroad is at Galveston, and that of the Texas and Pacific at New Orleans. The Southern Pacific railroad has recently supplied the link which gives it a northern terminus at Vicksburg in the direction of its natural ocean outlet at Savannah. It approaches the Pacific ocean near San Diego, which is its natural terminus, but it is now carried up through California to San Francisco. The Atlantic and Pacific, called sometimes the thirty-fifth parallel road (as the last mentioned is called the thirty-second parallel route), terminates through its continuation, the St.Louis and San Francisco road, at St. Louis. It emerges on the Pacific coast at the same place as the Southern Pacific, and uses its prolongation up the coast to meet the ocean commerce at San Francisco. The Atchison, Topeka, and Santa Fé, which line was completed in 1883, forms with the Sonora railroad, lately acquired by purchase, running southward through Mexico to Guaymas, on the Gulf of California, a fifth transcontinental line, connecting with the Eastern railroads at Kansas City. The Canadian Pacific is rapidly approaching completion. South of the United States there are, besides the Panama railroad, three Mexican interoceanic lines chartered and partly constructed. The most northerly crosses Tampico to San Blas and is called the Mexican Central. The Mexican National railroad crosses from Vera Cruz, by way of the city of Mexico, to Manzanillo. The third is the line across the Isthmus of Tehuantepec, which was begun with the aid of subsidies by an American company, but became forfeit by lapse of the term stipulated for completion, and was confiscated and carried on by the Mexican Government. The southern route for a Pacific railway now followed by the Southern Pacific, the Atchison, Topeka, and Santa Fé, and the new Atlantic and Pacific lines, was proposed when the project of a transcontinental railway was first under discussion, but was rejected by Congress. It has the advantage of avoiding the elevations north of Colorado and in the Nevada plateau. The Northern Pacific, where it crosses the Rocky mountains, is a remarkable example of railroad engineering. There is a gradual ascent on the western side through a magnificent forest-region to Clark's Fork. At Missoula, in this valley, it assumes the character of a mountain railway, which is preserved up to the point where it emerges in the valley of the Yellowstone. It crosses the summit range, the Cascades, at Mullen Pass, through a tunnel nearly 4,000 feet in length, at an elevation of 6,560 feet. The descent on the opposite slope is by moderate gradients through the valleys of the head-waters of the Missouri, the Jefferson, the Madison, the East and West Gallatin, and finally the Yellowstone, which it leaves at Glendive. The construction of a transcontinental railroad across South America is an engineering §. the conditions of which are entirely ifferent from those of the North American routes. Henry Meiggs, when building in Peru his first Andes railroad, till then the most magnificent mountain railway in the world, which ascends to altitudes as great as Mont Blano, in which the barometric pressure is only 400 millimetres, and fire will scarcely burn, intended it as a link in a railroad across the continent. This work of genius was thrown into the shade by his Lima-Oroya railroad, which ascends on each side through 44 tunnels and over dizzy viaducts, to the summit-level in the CimaJalexa tunnel, 1,860 metres long and 4,769 metres above the level of the sea. The project of a transcontinental railway was not only prenature, but the location of the route by Meiggs was through countries which, though possessinz unlimited natural resources, were socially backward. An interoceanic road between the more progressive states of Chili and the Ar
gentine Republic is much more feasible as a work of engineering, and commercially more promising. The project was under consideration at the time when Meiggs carried out his stupendous works. A convention between the two governments has been in existence many years, but they do not seem to desire such close commercial communication. The distance between the two capitals in a direct line is only 375 miles. The dangers from water and an exuberent vegetation, which are found in the Brazilian route, are here absent. There was no known practicable pass in the Andes, but no technical examination of the mountains had yet been made. Lately a gap in the chain has been discovered farther south, in Northern Patagonia, which would afford an easy passage.
EPIDEMIC DISEASES IN 1883. With the exception of cholera and yellow fever, there has been no wide-spread epidemic of disease during the year; but these have been manifested with their usual virulence and activity. Eu
rope and the United States escaped an epidemic,
but Egypt suffered almost as much from the devastations of cholera as from the effects of her civil war, and certain towns in Mexico were almost depopulated by yellow fever and the resulting panic. Concerning the cholera in Egypt, it is officially stated that the deaths from the disease were in excess of 48,000, and probably reached 50,000. This epidemic first appeared at Damietta, on the 24th of June, 1883. The city itself had a population of about 32,000. On the 27th of June cholera was reported from Port Said, and on the 30th of June at Samanoud, but it was not until the 15th of July that the disease reached Cairo. A great panic prevailed throughout Egypt, and on the earliest report of the existence of cholera at Damietta, the people fled in great numbers; a majority of them departing for Turkey. The Porte required all refugees from Egypt to undergo quarantine, either at Beirut or Bourla at the entrance of the Bay of Smyrna. Consul-General Heap reported to the State Department from Constantinople, on the 21st of July, that “the limited accommodations at both places were enlarged by the erection of temporary wooden barracks and tents, but the panic-stricken refugees from Alexandria came in such numbers that these structures soon became insufficient to accommodate such crowds, and as each day brought fresh arrivals, the sufferings of these people from exposure to the burning sun and the chilling night dews became very great, and threatened to create the very evil it was the intention to guard against. Those arriving at Beirut were the greatest sufferers, and the Turkish authorities were finally compelled to telegraph to Alexandria, to give warning that no more refugees could be received, or would be allowed to land.” Sanitary cordons were established in a somewhat desultory manner by the local authorities of each place, from time to time, and such as