BRIDGE CONSTRUCTION highway bridges. The conditions under which highway bridges are purchased by township and county commissioners are decidedly unfavorable to material improvements in the character of their details. It is a comparatively rare occurrence that the commissioners employ a bridge engineer to look after the interests of the taxpayers by providing suitable specifications, making the design, inspecting the material, and examining the construction of the bridge to see that it conforms to all the imposed requirements. These provisions are only made in some of the cities, and accordingly, one must examine the new bridges in cities to learn what progress is making in highway bridge building. The lack of proper supervision in the rural districts and many of the smaller cities results in the continued use of short trusses with slender members built up of thin plates and shapes, whose comparatively light weight causes excessive vibration and consequent wear, as well as deterioration from rust. Under better administration plate girders would be substituted for such light trusses, making both a stiffer structure and one more easily protected by paint. The general lack of inspection and the consequent failure to protect highway bridges by regular repainting will materially shorten their life and thereby increase the financial burden to replace them by new structures. Some progress has been made by adopting riveted trusses for the shorter spans for which pin-connected trusses were formerly used, but the extent of this change is by no means as extensive as it should be, nor equal to the corresponding advance in railroad bridges. The channel span of the cantilever bridge over the Mississippi River at Memphis, Tenn., was for some years the longest one of any bridge of this class in America. It measures 7902 feet between centers of supports. This bridge was finished in 1892, or only two years after the close of the seven-year period of construction and erection_of_the_mammoth cantilever bridge over the Firth of Forth in Scotland. See BRIDGE (Cantilever Bridges). The cantilever bridge erected in 1903 over the Monongahela River in Pittsburg has a span a little longer than that of the Memphis bridge. It is on the new extension of the Wabash R.R. system, and the distance between pier centres is 812 feet. There is another one being built which will not only have a longer span than any other cantilever bridge in this country, but longer than that of any other bridge in the world. It is located near Quebec, Canada, and its channel span over the St. Lawrence River is to have the unprecedented length of 1,800 feet, or nearly 100 feet longer than that of the Forth cantilever bridge and 200 feet longer than the Brooklyn suspension bridge. The towers will have a height of 360 feet above high tide. It will accommodate a double-track railroad, besides two electric railway tracks and highways. In the piers the courses of masonry are four feet high and individual stones weigh about 15 tons each. The character of its design and the simplicity of its details will permit its construction with unusual rapidity and economy for a bridge of this magnitude. The Brooklyn bridge, completed in 1883, is still the largest suspension bridge in the world, its span being 1,595 feet. More people cross this bridge than any other in any country. The Williamsburg Bridge (q.v.), completed in 1904, has a span of 1,600 feet, and its capacity will be very much greater than that of the Brooklyn Bridge. Each of its four cables has a safe strength of over 10,000,000 pounds in tension. See BRIDGE (Suspension Bridges). One of the most interesting developments relating to the subject under consideration is the construction of a considerable number of metallic arch bridges in recent years and the promise of their still greater use in the future. On account of their form they constitute one of the handsomest classes of bridges. The first important steel bridge in the world was completed in 1874. It is the arch bridge which in three spans crosses the Mississippi River at St. Louis. Its arches are without hinges and their ends are firmly fixed to the piers. This is one of the most famous bridges in existence. For a long time after its construction no metallic arches were erected in this country, although many were built in Europe. In 1888, however, the highway bridge across the Mississippi River at Minneapolis was erected, consisting of two spans of 456 feet each and which still remains the longest span of any three-hinged arch. The following year the Washington bridge over the Harlem River in New York was completed. It consists of two spans of 510 feet in the clear and has the largest two-hinged arch ribs with solid web plates. See BRIDGE. These were followed by a number of arches of various types, the most noted of which are the two arch bridges over the Niagara River. The first one is a spandrel-braced, two-hinged arch with a span of 550 feet, and replaced the Roebling suspension bridge in 1897. It accommodates the two tracks of the Grand Trunk R.R. on the upper deck and a highway on the lower deck. The other bridge has arched trusses with parallel chords and two hinges. It replaced the Niagara and Clifton highway suspension bridge in 1898, and as its span is 840 feet, it is the largest arch of any type in the world. The manner in which this arch was erected furnishes an illustration of the effort which is made by engineers to conform the actual conditions so far as possible to the theoretic ones involved in the computation of the stresses. Since the stresses in an arch having less than three hinges are statically indeterminate, stresses of considerable magnitude may be introduced into the trusses if the workmanship be imperfect, the supports not located with sufficient precision, and the arch closed without the proper means and care. The Niagara and Clifton arch was first closed as a three-hinged arch and then transformed into a two-hinged arch by inserting the final member under the sum of the computed stress due to the weight of the truss, and that due to the difference between the temperature at which the closure was made and that assumed as standard in the stress computations. This stress was secured in the member by inserting it when the hydraulic jack which forced apart the adjacent ends of the shortened chords registered the required amount of pressure. The arch had been erected as a pair of cantilevers from each side extending 420 feet out beyond the supports, and when the closure was made the two arms came together BRIDGE CONSTRUCTION within a quarter of an inch of the computed value. Such a result involving the "accuracy of the calculation and design of the entire steel work, the exactness with which the bearing shoes or skewbacks were placed, and the perfection of the shopwork" has been truly characterized as phenomenal. In order to reduce secondary stresses to a minimum the members were bolted up during the cantilever erection and the bolts replaced by rivets after the closure of the arch rib. design of several new types. At South Halstead Street a direct-lift bridge was built in 1893. over the Chicago River, in which a simple span 130 feet long and 50 feet wide is lifted vertically 1422 feet by means of cables to which counterweights are attached. Formerly, only very small bridges of this kind were used, as those, for instance, over the Erie Canal. In 1895 a rolling-lift bridge over the Chicago River was completed. In this new design as each leaf of the bridge rotates to a vertical posiThe past decade witnessed the introduction tion it rolls backward at one end. When closed and extensive development of arches of con- the two leaves are locked at the centre, but crete and of concrete-steel construction. In they are supported as cantilevers. This form the latter kind a small amount of steel is em- has been found to have so many advantages for bedded in the concrete in order to resist any the crossings of relatively narrow streams, tensile stresses that may be developed. During where an unobstructed waterway is required and this period more than 150 concrete-steel bridges the adjacent shores are needed for dock room, have been built in this country. In the same that a score of important structures of this year in which the largest metallic arch was class have been built in different cities. The completed, the five concrete-steel arches of the largest span that has been designed is 275 feet bridge at Topeka, Kansas, were finished. The between centres of supports, while the widest largest one has a span of 125 feet and still re- one is to accommodate eight railroad tracks mains the largest span of this type in America, crossing the Chicago Main Drainage Canal. although it has been exceeded in Europe. Con- About the same time and under similar siderably larger spans are included in the ac- conditions another type of bascule bridge was cepted design for the proposed Memorial bridge built at Sixteenth Street, Milwaukee, in which, at Washington. as each leaf moves toward the shore, one end rises and the other falls, so that its centre of gravity moves horizontally, thus requiring a very small expenditure of power to operate the bridge. It is the smaller steel structures which are destined more and more to be replaced by arches of this material. The steel bridges require repainting at frequent intervals, constant inspection, occasional repairs, and finally replacing by a new structure after a relatively short life, on account of rust and wear, unless it is required even sooner on account of a considerable increase in the live load. The concrete arch requires practically no attention except at very long intervals. The safety of operating the traffic makes it desirable to have as few breaks as possible in the regular track construction of a railroad, and this constitutes an additional reason why concrete or stone arches are being substituted for the smaller openings. The decreasing cost of concrete tends to an extension of this practice to openings of increasing size. In 1901, however, a bridge was completed which marks a decided departure from previous practice. The Pennsylvania R.R. built a stone bridge, consisting of 48 segmental arches of 70 feet span, at the crossing of the Susquehanna River at Rockville, Pa. It is 52 feet wide, accommodates four tracks and cost $1,000,000. This bridge has not only the advantage of almost entirely eliminating the cost of maintenance, but it also has sufficient mass to withstand the floods which occasionally wreck the other bridges on that river. In 1903 the same railroad built a similar bridge over the Raritan River at New Brunswick, N. J. Of movable bridges the largest swing span existing was erected in 1893 at Omaha over the Missouri River. Two years later a fourtrack railroad swing bridge was built by the New York C. R.R. over the Harlem River in New York, which is only 389 feet long between centres of end pins, but which weighs about 2,500 tons, and is accordingly the heaviest drawbridge of any class in the world. During the past decade a remarkable development was made in drawbridge construction by the modification and improvement of some of the older types of lift bridges and the Several improved forms of hinged-lift bridges have also been designed and built in Chicago and elsewhere. In a small bridge erected in 1896 on the Erie R.R. in the Hackensack meadows there is only a single leaf hinged at one end and lifted by a cable attached to the other end. The counterweight rolls on a curved track so designed as to make the counterbalance equally effective in all stages of opening and closing the bridge. A novel bridge was built in 1902 over the ship canal at Duluth which is different from any other type in this country. The general scheme is similar to that of a design made by a French engineer who built three of the structures in different countries. It consists of a simple truss bridge 393 feet 9 inches long, supported on towers at a clear height of 135 feet above high water. Instead of supporting the usual floor of a highway bridge it supports the track of a suspended car which is properly stiffened against wind pressure and lateral vibration, the floor of the car being on a level with the docks. This ferry is operated by electricity. The loaded car, its hangers, trucks, and machinery weigh 120 tons. In the French design a suspension bridge was used instead of the simple truss bridge. A bridge across the Charles River between Boston and Cambridge deserves especial mention and marks a decided advance in the growing recognition on the part of municipal authorities of the importance of æsthetic considerations in the design of public works. It consists of II spans of steel arches whose lengths range from 1011⁄2 to 1882 feet. Its width is 105 feet between railings. It is claimed that this bridge will be not only one of the finest structures of its kind in this country, but will be a rival of any in the Old World." Its length between abutments is 1,7671⁄2 feet, and it is estimated to cost about $2,500,000. BRIDGE OF ALLAN-BRIDGEPORT The problems incident to the replacing and strengthening of old bridges frequently tax the resources of the engineer and demonstrate his ability to overcome difficulties. Only a few examples will be cited to indicate the character of this work. In 1900 the Niagara cantilever bridge had its capacity increased about 75 per cent by the insertion of a middle truss without interfering with traffic. In 1897 the entire floor of the Cincinnati and Covington suspension bridge was raised four feet while the traffic was using it. It may be of interest to state that the two new cables, 101⁄2 inches in diameter, which were added to increase the capacity of the bridge, have just about three times the strength of the two old ones, 123 inches in diameter, and which were made a little over 30 years before. In the same year the old tubular bridge across the St. Lawrence River was replaced by simple truss spans without the use of false works under the bridge and without interfering with traffic. On 25 May 1902 the Pennsylvania R.R. bridge over the Raritan River and canal at New Brunswick, N. J., was moved sidewise a distance of 142 feet. Five simple spans 150 long and a drawbridge of the same length, weighing in all 2,057 tons, were moved to the new position and aligned in 2 minutes and 50 seconds. The actual times that the two tracks were out of service were respectively 15 and 28 minutes. On 17 October 1897, on the same railroad near Girard Avenue, Philadelphia, an old span was moved away, and a new one, 235 feet 7 inches long, put in exactly the same place in 2 minutes and 28 seconds. No train was delayed in either case. HENRY S. JACOBY, College of Civil Engineering, Cornell. Bridge of Allan, a town of Scotland, in Stirlingshire, on the border of Perthshire, on the banks of the Allan, about three miles north of Stirling, with which it is connected by the Caledonian Railway and a line of tramway cars. Owing to the mildness of its climate and the beauty of its situation, Bridge of Allan is a favorite spring and autumn resort for invalids. It is built partly on a plateau of considerable height and partly on low ground on the banks of the river, and is finely laid out with trees and public walks. It is frequented partly on account of its mineral wells. Bridge of Sighs, a bridge in Venice, dating back to 1597. It spans the Rio della Paglia, connects the ducal palace with the prisons, and forms a graceful structure 32 feet above the water, enclosed at the sides and arched overhead. It contains two passages, through which prisoners were led for trial, judgment, or punish ment. Bridgeport, Conn., a city, port of entry, and county-seat of Fairfield County, on Long Island Sound, and on the New York, N. H. & H. R.R.; 18 miles southwest of New Haven, and 58 miles northeast of New York. It is the third city in the State. The city and town are conterminous, and about 15 square miles in area. Pop. (1900) 70,996. Bridgeport harbor is the estuary of a small tidal river, the Pequonnock, and a tidal inlet called Pembroke Lake, with a peninsula between them on which East Bridgeport, the chief manufacturing section, is built. Below the junction the harbor is about two miles long to the Sound, and a mile wide. The main city lies along the west side to its mouth, the business centre opposite the peninsula; the shore is a plain, rising on the west to an elevation of about 70 feet called Golden Hill, the finest residence section, and commanding a beautiful view of the Sound and city. Down the Sound about three miles is the suburb of Black Rock, a favorite summer resort, with its island-guarded harbor, a great yachting rendezvous. There are three handsome parks, about 250 acres in all: Beardsley, left in great measure to its wild state, and the more charming therefor; Washington, and Seaside, of some 75 acres, on the shore west of the harbor, with a sea-wall and a two-mile drive, and monuments to the soldiers and sailors of the War, and to Elias Howe and P. T. Barnum, the sewing-machine of the one and the business enterprise of the other having largely developed the city. Of the cemeteries, the finest is Mountain Grove, of some 75 acres, near the western boundary; others are Lakeview, in east Bridgeport, and Park, in North Bridgeport. There are also two Roman Catholic cemeteries. The most notable public buildings are the government building, with the post-office and customhouse; the county court-house; the Barnum Memorial Institute, bequeathed to the Historical Society and the Scientific Society in common; the Young Men's Christian Association and the Burroughs Library buildings. Very interesting is the Barnum & Bailey circus in its winter home. Bridgeport, like all the cities of western Connecticut, is primarily an immense gathering of the manufacturing departments of New York salesrooms; but many of them started as local Bridgeport institutions, and have remained identified with the city. These establishments are multiplying with great rapidity. By the census of 1900, there were 832, employing 20,462 workmen and supervisors, using $33,066,890 capital, paying $10,622,558 in wages and salaries, $9,133,236 for materials, and turning out $37,883,721 in products. Of these, no figures are given for the two most noted products, sewing-machines and ammunition, from their being concentrated in two or three establishments, mainly the Wheeler & Wilson Company with its 10 acres of works, and the Union Metallic Cartridge Company; but $4,147,452 was brass castings, $3,224,198 coructs, $1,007,244 hardware, cutlery and edge tools, sets, $2,412,796 foundry and machine-shop prod$33,577 rubber goods, $366,585 carriages and wagons. Other products are steel bridges, ordnance and firearms, monumental bronze, bicycles, wire, velvet and plush, silk, belting, undertakers' goods, ice, fur goods, lamps, leather goods, musical instruments and graphophones, britannia, pottery, sporting goods, typewriters, carriages, and many more. The city's transportation facilities aid this growth. The railroad trackage is ample; a new station is to be built and the tracks raised, at an expense of some $3,000,000. The harbor is safe and admits quite large vessels; there are daily steamer lines to New York, and much barge and coasting business. The river, creeks, and Pembroke Lake are spanned by two or three dozen strong bridges. Electric railway service is thoroughly developed, continuous lines running to New Haven and New York. Education, Churches, Charities, etc.- The public school system has 22 grammar schools and |