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bridge are so adjusted that they are at rest when opened at an inclination of about 40°, instead of in the horizontal position which they occupy when closed. Thus, as soon as the locks are withdrawn, the leaves will, without the application of any power whatever, roll back and upward and open a channel of sufficient width for the passage of vessels. An interesting record is that of the Rush Street Bridge, at Chicago, said to be the most active movable bridge in the world. During an average season of lake navigation, comprising a little over eight months, this bridge is opened between 10,000 and 11,000 times, or fully 40 times every 24 hours. Yet the power expense for the operation of this bridge by electricity does not exceed 67 cents a day.
Another form of movable bridge is that known as a swing bridge. It consists of a bridge balanced on a pivot or on a circle of rollers situated on a pier in the centre of the river. When open the bridge is swung around until it lies within the central pier and, with it, points up and down instead of athwart stream. This gives a passage on each side of the pier. The swing bridge over the Raritan River, New Jersey, gives two passages, each 216 feet wide. A similar bridge in Kansas City crosses two passages each 160 feet wide. The total moving weight is 303 tons. The bridge is opened by steam power in about one and a half minutes, or by manual power in two minutes. From two thirds to three fourths of the moving weight rests on the central pivot. Another form of swing bridge is made in two leaves which swing in to either bank and meet in the middle.
Traversing or telescope bridges are occasionally employed. They are so constructed as to be capable of being rolled horizontally backward or in an oblique direction. The bridge across the Arun, near Arundel, England, on the South Coast Railway, is 144 feet long. It is traversed on wheels, and acts as a sliding cantilever, the overhanging portion resting on the opposite abutment when in place.
Bascule or draw bridges are raised on horizontal hinges, and are made in one leaf, or in two leaves which meet in the middle. The most ancient form of the bascule was that of a flap of framed timber used to cross the moat of a castle, and capable of being drawn up by means of chains from the inside.
Lift bridges are contrived so as to rise perpendicularly without changing their horizontal position. This motion may be imparted by hydraulic power, by counterbalancing weights, or by means of a winch operated by hand, steam, or electrical power. Bridges of this form are used largely on the Erie Canal. One in Chicago has a clear lift of 155 feet above the water. A flying bridge is a boat or raft anchored by a long cable up stream, and carried across by the action of the current acting obliquely against its side, which should be kept at an angle of about 55° with the current.
Pontoon Bridges. These are a development from the ancient bridge of boats, which consisted of boats moored so as to form a continuous line, across which a roadway of planks was laid. The longest floating bridge in the world, probably, is the pontoon bridge across the Hooghly, at Calcutta, designed and constructed by Sir Bradford Leslie. The bridge is 1,530 feet long between the abutments, and is carried on 14 pairs of pontoons, which are held in position
by means of chain cables one and three quarters inches thick, and anchors weighing three tons each, laid on the up stream and down stream sides, 900 feet asunder. By their great length the cables afford the necessary spring to allow for the ordinary rise and fall of the river, the stress on each cable varying from 5 to 25 tons, according to the state of the weather and the stage of the tide, the maximum velocity of which is six miles an hour. The pontoons are rectangular iron boxes having rounded bilges and wedgeshaped ends. They are each 160 feet long, made of such considerable length in order to obviate pitching motion in rough weather, with a beam of 10 feet, and depth of from 8 to 11 feet, presenting a side of from 32 to 4 feet above the water, according to the state of the traffic. For perfect safety each pontoon is divided by bulkheads into II compartments. They are made of iron plates one fourth of an inch and five sixteenths of an inch in thickness, riveted together.. The platform of the bridge is supported by trestle-work on the pontoons at a clear height of 27 feet above the water, a convenient height for boat navigation. The roadway platform is of 3-inch teak planks, and is 48 feet wide, with a footpath at each side 7 feet wide. An opening 200 feet wide, for the passage of ships, is made by removing, when occasion requires, four of the pontoons with their superstructure, and sheering them clear of the opening. The portion so removed is in two divisions, which are separately secured, right and left, and, when in place, are connected by drawbridges with the fixed portions of the bridge. Before launching, the pontoons were ballasted sufficiently to make them float upright; and were afterward coupled in pairs by the sills of the main trusses, when the ballast was removed. The floating bridge is connected with the shore at each end by adjusting ways hinged to the shore. The ordinary time taken to open the bridge is 15 minutes, and to close it, 20 minutes. It is opened only twice a week. An excellent instance of pontoon structure, though not a bridge, is the Great Landing Stage at Liverpool, England.
Military Bridges are temporary constructions. to facilitate the passage of rivers by troops, to restore a broken arch, or cross a chasm of no very great width. Those over a river are either floating or fixed. The former are made of pontoons, boats, casks, rafts of timber, cotton-bales, or anything that will give sufficient buoyancy, and the latter of piles, trestles, or other timber work. Spars, ropes, and planks are used in a variety of ways for spanning narrow chasms. The pontoon bridge is the only one which is carried with an army. Enough material for 100 yards of length accompanies each army corps. All military bridges have their roadway formed in the following manner: Five to nine roadbearers of stout timber support chesses or flat planks 10 feet long, held in position, so as to form a level surface, by two ribands placed above them and over the outer road-bearers, to which they are fastened by rack lashings. The road-bearers are supported by the pontoons, casks, boats, trestles, or piles, which form the piers, usually 10 to 15 feet apart, or by transoms on the ropes in the case of suspension bridges. To prevent injury to the boats, balks of timber are built up along the keel of each for the road-bearers to rest upon. A saddle on pontoons and gunnels on casks answer the same pur
BRIDGE-BUILDING BROTHERHOOD — BRIDGE CONSTRUCTION
pose, and in the latter case keep the casks together by being lashed to them. The maximum loads such bridges are usually calculated to bear are, for infantry, 500 pounds per lineal foot; for cavalry, 200 pounds; for field artillery, with two horses per gun only, 450 pounds. Heavy guns are better warped across on specially constructed rafts.
Natural Bridges. Of the rock formations called natural bridges, the most remarkable is the natural bridge over Cedar Creek, Virginia, 125 miles west of Richmond. The mass of siliceous limestone through which the little river passes is presumably all that remains of a once extensive stratum. The cavern or arch is 200 feet high and 60 feet wide. The solid rock walls are nearly perpendicular, and the crown of the arch is 40 feet thick. JOHN STERLING DEANS, Chief Engineer the Phoenix Bridge Co., Phanixville, Pa. Bridge-Building Brotherhood, a fraternal religious order formed in the 12th century in southern France. Its object was the building of bridges and the keeping of ferries. Tradition connects its origin with St. Bénezét, through whose efforts a bridge across the Rhone at Avig non was begun in 1117. After the completion of this bridge in 1185 the order received the sanction of Clement III. The order was dissolved by Pius II.
Bridge Construction, American. The application of scientific principles to the construction of bridges is more complete to-day than ever before. This statement applies to the specified requirements which the finished structure must fulfill, the design of every detail to carry the stresses due to the various loads imposed, the manufacture of the material composing the bridge, the construction of every member in it, and finally the erection of the bridge in the place where it is to do its duty as an instrument of transportation.
A close study of the economic problems of transportation in the United States and the experimental application of its results led the railroad managers to the definite conviction that, in order to increase the net earnings while the freight rates were slowly but steadily moving downward, it was necessary to change the method of loading by using larger cars drawn by heavier locomotives, so as to reduce the cost of transportation per train mile. While these studies had been in progress for a number of years and there was a gradual increase in the weight of locomotives, it is only within the past five years that the test was made, under favorable conditions and on an adequate scale, to demonstrate the value of a decided advance in the capacity of freight cars and in the weight of locomotives for the transportation of through freight. The test was made on the Pittsburg, B. & L. E. R.R., which was built and equipped for the transportation of iron ore from Lake Erie to Pittsburg, and of coal in the opposite direction.
When the economic proposition was fairly established, it was wonderful to see how railroad managers and capitalists met the situation, by investing additional capital for the newer type of equipment, and for the changes in road bed and location necessarily involved by that in the rolling stock. Curves were taken out or dimin
ished, grades were reduced, heavier rails were laid, and new bridges built, so that practically some lines were almost rebuilt. The process is still going on and money by the hundred millions is involved in the transformation and equipment of the railroads. Some impression of the magnitude of the change in equipment may be gained from the single fact, that one of the leading railroads has within a few years expended more than $20,000,000 for new freight cars alone, all of which have a capacity of 100.000 pounds.
The form of loading for bridges almost universally specified by the railroads of this country consists of two consolidation locomotives followed by a uniform train load. These loads are frequently chosen somewhat larger than those that are likely to be actually used for some years in advance, but sometimes the heaviest type of locomotives in use is adopted as the standard loading. The extent to which the specified loadings have changed in eight years may be seen from the following statement based on statistics compiled by Ward Baldwin and published in the Railroad Gazette for 2 May
miles, located in the United States, Canada, and Of the railroads whose lengths exceed 100 Mexico, only 2 out of 77 specified uniform train tracks in 1893, while in 1901, only 13 out of 103 loads exceeding 4.000 pounds per linear foot of railroads specified similar loads less than 4,000 pounds. In 1893, 37 railroads specified loads of 3,000 pounds and 29 of 4,000 pounds, while in 1901, 4,000 pounds was specified by 50, 4,500 pounds by 14, and 5,000 pounds by 17 railroads. The maximum uniform load rose from 4,200 in 1893 to 7,000 pounds in 1902.
In a similar manner in 1893 only I railroad in 75 specified a load on each driving wheel axle exceeding 40,000 pounds, while in 1901 only 13 railroads out of 92 specified less than this load. In 1893 only 21 of the 77 railroads specified similar loads exceeding 30,000 pounds. The maximum load on each driving wheel axle rose from 44,000 pounds in 1893 to 60,000 pounds in 1901.
The unusual amount of new bridge construction required caused a general revision of the standard specifications for bridges, the effect of which was to include the results of recent studies and experiment, and to eliminate some of the minor and unessential items formerly prescribed.
Meanwhile another movement was in progress. Experience having shown the great advantage of more uniformity in various details and standards relating to the manufacture of bridges both in reducing the cost and the time required for the shop work, an effort was begun to secure more uniformity in the requirements for the production and tests of steel, which is the metal now exclusively employed in bridges.
With greater uniformity in the physical, chemical and other requirements for steel, as determined by standard tests, the unit stresses to be prescribed for the design of bridges will naturally approach to a corresponding uniformity. To what extent this is desirable may be inferred from the fact that the application of several of the leading specifications to the design of a railroad bridge under a given live road yields results which may vary by an amount
ranging from zero to 25 per cent of the total weight.
In the revision of specifications a decided tendency is observed to simplify the design by making an allowance for impact, vibration, etc., by adding certain percentages to the live load according to some well-defined system. It needs but relatively little experience in making comparative designs of bridges under the same loading, to show the advantage of this method over that in which the allowance is made in the unit stresses according to any of the systems usually adopted in such a case. Not only are the necessary computations greatly simplified, but the same degree of security is obtained in every detail of the connections as in the principal members which compose the structure.
Experiments on a large scale are very much needed to determine the proper percentage of the live load to be allowed for the effect of impact, SO as to secure the necessary strength with the least sacrifice of true economy. An investigation might also be advantageously made to determine the proper ratio of the thickness of cover plates in chord members which are subject to compression, to the transverse distance between the connecting lines of rivets. The same need exists in regard to the stiffening of the webs of plate girders, concerning which there is a wide variation in the requirements of different specifications.
A movement which has done much good during the past decade and promises more for the future is that of the organization of bridge departments by the railroad companies. The great economy of making one design rather than to ask a number of bridge companies to make an equal number of designs, of which all but one are wasted, is the first advantage; but another of even greater significance in the development of bridge construction is that which arises from the designs being made by those who observe the bridges in the conditions of service and who will naturally devote closer study to every detail than is possible under the former usual conditions. The larger number of responsible designers also leads to the introduction of more new details to be submitted to the test of service, which will indicate those worthy of adoption in later designs. In order to save time and labor and secure greater uniformity in the design of the smaller bridges, some of the railroads prepare standard plans for spans varying by small distances. For the most important structures consulting bridge engineers are more frequently employed than formerly, when so much dependence was placed upon competitive designs made by the bridge companies.
An investigation was made by a committee of the Railway Engineering and Maintenance of Way Association in regard to the present practice respecting the degree of completeness of the plans and specifications furnished by the railroads. It was found that of the 72 railroads replying definitely to the inquiry, 33 per cent prepare "plans of more or less detail, but sufficiently full and precise to allow the bidder to figure the weight correctly and if awarded the contract to at once list the mill orders for material"; 18 per cent prepare "general outline drawings showing the composition of members, but no details of joints and connections"; while 49 per cent prepare "full specifications with survey plan only, leaving the bidder to submit a
design with his bid." If, however, the comparison be made on the basis of mileage represented by these 72 railroads, the corresponding percentages are 48, 24, and 28 respectively. The total mileage represented was 117,245 miles. A large majority of the engineers and bridge companies that responded were in favor of making detail plans.
The shop drawings, which show the form of the bridge, the character and relations of all its parts, give the section and length of every member, and the size and position of every detail whether it be a reinforcing plate, a pin, a bolt, a rivet, or a lacing bar. All dimensions on the drawings are checked independently so as to avoid any chance for errors. The systematic manner in which the drawings are made and checked, and the thorough organization of every department of the shops, make it possible to manufacture the largest bridge, to ship the pieces to a distant site, and find on erecting the structure in place that all the parts fit together, although they had not been assembled at the works.
The constant improvement in the equipment of the bridge shops, and the increasing experience of the manufacturers who devote their entire time and attention to the study of better methods of transforming plates, bars, shapes, rivets, and pins into bridges, constitute important factors in the development of bridge construction.
As the length of span for the different classes of bridges gives a general indication of the progress in the science and art of bridge building, the following references are made to the longest existing span for each class, together with the increase in span which has been effected approximately during the past decade.
In plate girder bridges the girders, as their name implies, have solid webs composed of steel plates. A dozen years ago but few plate girders were built whose span exceeded 100 feet, the maximum span being but a few feet longer than this. To-day such large girders are very frequently constructed. The longest plate girder span was erected on the Mahoning division of the Erie R.R. in 1902 and measures 128 feet 4 inches between centres of bearings. The longest ones in a highway bridge are those of the viaduct on the Riverside Drive in New York, erected in 1900, the span being 126 feet. The heaviest plate girder is the middle one of a four-track bridge on the New York C. R.R. erected in 1901 near Lyons, N. Y. Its weight is 103 tons, its span 107 feet 8 inches; and its depth out to out 12 feet 2 inches.
The large amount of new construction and the corresponding increase in the weight of rolling stock have combined to secure a more extensive adoption of plate girders and the designs of many new details for them. These affect chiefly the composition of the flanges, the web splices, the expansion bearings and the solid floor system. Although solid metal floors built up of special shapes were first introduced into this country 15 years ago, their general adoption has taken place largely within the past decade on account of their special adaptation to the requirements of the elevation of tracks in cities. Solid floors may not only be made much shallower than the ordinary open type, thereby reducing the total cost of the track elevation, but they also permit the ordinary track con