Puslapio vaizdai
PDF
„ePub“
[merged small][merged small][graphic][merged small]

heart becomes excited, and many persons become affected by what is known as the "caisson disease," which is accompanied by extreme pain and in many cases results in more or less complete paralysis. The careful observations of eminent physicians who have given this disease special attention have resulted in the formulation of rules which have reduced the danger to a minimum.

The execution of work within a deep pneumatic caisson is worth a moment's consideration. Just above the surface of the water is a busy force engaged in laying the solid blocks of masonry which are to support the structure. Great derricks lift the stones and lay them in their proper position. Powerful pumps are forcing air, regularly and at uniform pressure, through tubes to the chamber

more than a few hours. The water from without is only kept from entering by the steady action of the pumps far above and beyond their control. An irregular settlement might overturn the structure. Should the descent of the caisson be arrested by any solid under its edge, immediate and judicious action must be taken. If the obstruction be a log, it must be cut off outside the edge and pulled into the chamber. Boulders must be undermined and often must be broken up by blasting. The excavation must be systematic and regular. A constant danger menaces the lives of these workers, and the wonderful success with which they have accomplished what they have undertaken is entitled to notice and admiration.

Another process, which has succeed

ed in carrying a foundation to greater shall descend evenly and always maindepths than is possible with compressed tain its upright position. The dredge air, is by building a crib or caisson, is handled and operated entirely from with chambers entirely open at the top, the surface. The very idea is startling, but having the alternate ones closed of managing an excavation more than at the bottom and furnished with cut- a hundred feet below the operator, en

[graphic][subsumed][merged small]

ting edges. These closed chambers are weighted with stone or gravel until the structure rests upon the bottom of the river; the material is then excavated from the bottom through the open chambers, by means of dredges, thus permitting the structure to sink by its weight to the desired depth. When that depth is reached, the chambers which have been used for dredging are filled with concrete, and the masonry is constructed upon the top of this structure. The use of this system has enabled the engineer to place foundations deeper than has been accomplished by any other device, one recently built in Australia being 175 feet below the surface of the water. Illustrations on page 15 show this method of construction.

Even more remarkable than the pneumatic caisson is this method of sinking these great foundations. The removal of material must be made with such systematic regularity that the structure

tirely by means of the ropes which connect with the dredge, and doing it with such delicacy that the movement of an enormous structure, weighing many tons, is absolutely controlled. This is one of the latest and most interesting advances of engineering skill.

While it is true that the avoidance of large expenditure, when possible, is a mark of the best engineering, yet great structures often become absolutely necessary in the development of railway communication. Wide rivers must be crossed, deep valleys must be spanned, and much study has been given to the best methods of accomplishing these results. In the early history of railways in Europe substantial viaducts of brick and stone masonry were generally built; and in this country there are notable instances of such constructions. proach to the depot of the Pennsylvania Railroad, in the city of Philadelphia,

The ap

Truss Bridge of the Northern Pacific Railway over the Missouri River at Bismarck, Dakota.-Testing the central span.

is an excellent example. Each street crossed by the viaduct is spanned by a bold arch of brick. Upon a number of our railways there are heavy masonry arches and culverts, and at some places these are of a very interesting character. The arches in the approach to the bridge over the Harlem Valley, now in construction, are shown on page 19. These are arches of granite, of a span of 60 feet. The illustration shows also the method of supporting the stone work of such arches during construction. Braced timbers form what is called the centre, and support the curved frame of plank upon which the masonry is built, which, of course, cannot be self-supporting until the keystone is in place; then the centre is lowered by a loosening of the wedges which support it, and the stone work of the arch is permitted to assume its final bearing. It is generally considered that where it is practicable to construct masonry arches under railways there is a fair assurance of their permanency, but some engineers of great experience in railway construction advance the theory that the constant jar and tremor produced by passing railway trains is really more destructive to masonry work than has been supposed, and that it may be true that the elements of the best economy will be found in metal structures rather than in masonry. It is true that repairs and renewals of metal bridges are much more easily accomplished than of masonry constructions.

In this country the wooden bridge has been an important, in fact an essential element in the successful building of our railways. At this moment the length of wooden bridges on the railway lines is very much greater than of metal. There have been a number of forms of wooden structure, but the Howe truss is, in many respects, the most per

[graphic]
[graphic][merged small]

fect; its construction is simple, it has the minimum amount of metal, the vertical rods being of iron, the rest of the structure, with the exception of some of the angle blocks, bolts, nuts, etc., being entirely of wood. A bridge built by Mr. Howe in 1840, across the Connecticut River at Springfield, with seven spans of 180 feet each, was one of his first works. It lasted until 1853, when it was replaced by a Howe truss of more modern design, which was in good condition when, in 1874, it was replaced by a double-track iron bridge. This improved form of truss has held its place in public favor, and, where timber is convenient, is an economical bridge.

Timber is also used extensively in railroad construction in the form of trestles; one example of which has been alluded to on page 7. There were also constructed, years ago, some very bold viaducts in wood. One of the most interesting is shown on page 20, being the viaduct at Portage, N. Y. This construction was over 800 feet long, and 234 feet high from the bed of the river to the rail.

The masonry foundations were 30 feet high, the trestles 190 feet, and the truss 14 feet; it contained more than a

million and a half feet, board measure, of timber. The timber piers, which were 50 feet apart, are formed by three trestles, grouped together. It was framed so that defective pieces could be taken out and replaced at any time. This bridge was finished in 1852 and was completely destroyed by fire in 1875. The new metal structure which took its place is shown on page 21, and is an interesting example of the American method of metal viaduct construction, an essential feature of that construction being the concentration of the material into the least possible number of parts. This bridge has ten spans of 50 feet, two of 100 feet, and one of 118 feet. The trusses are of what is called the Pratt pattern, and are supported by wrought-iron columns, two pairs of columns forming a skeleton tower 20 feet wide and 50 feet long on the top. There are six of these towers, one of which has a total height from the masonry to the rail of 203 feet 8 inches. There are over 1,300,000 pounds of iron in this structure.

The fundamental idea of a bridge is a simple beam of wood. If metal is substituted it is still a beam with all superfluous parts cut away. This re

[graphic][subsumed][subsumed][merged small][merged small][subsumed]
« AnkstesnisTęsti »