Bridge

Bridge (A.S. brycg, Ger. brucke), a structure traversing a roadway, river, or other impediment, mainly for the purpose of providing a convenient passage across from one side to the other. An account of the more important bridges, taken in the order of their construction, will show the history of their development from the simplest types to the more highly differentiated forms of the present day, though it should be noted that this development has been much more rapid of recent years, since the introduction of railways, than ever before. Leaving the simple expedient of laying a beam of some sort across the gap that has to be traversed, we find that the cantilever principle, recently adopted on a gigantic scale at the Forth bridge, was known and adopted many centuries ago. Beams of timber were fixed in each bank of a stream, and made to project bracket-wise towards each other. A centre beam resting on their two ends effected the span. Built on this principle there exists an ancient bridge across the Sutlej of 200 ft. span.

The arch was probably first introduced by the Romans, whose bridges generally consisted of semicircular arches supporting horizontal roadways, existing examples of which are afforded by the Ponte de Rotto, built 2,000 years ago, and the Pont du Gard at Nimes. This latter is very remarkable both for its design and clever workmanship. It is a combined aqueduct and viaduct. It consists first of a six-arch bridge, 465 ft. long, over the river Gardon. Then this supports a second series of eleven arches continued to the sides of the valley, and this, again, carries a third series of thirty-five arches, supporting a canal 850 ft. in length and 190 ft. above the river. It is built of large stones correctly cut to the required form, and fixed together by iron cramps.

The dynamics of the masonry arch are much more intricate than that of the cantilever, consisting as the former does of a large number of small elements that have to be built up together so as to be mutually supporting. Each stone in the arch is acted on by its neighbours and by the weight it sustains. These forces must balance each other for every stone, and must remain in equilibrium when the load on the arch is varied. The compression due to the lateral forces on the stone must not exceed a certain limit, or the stone will crush. Also the resultant compressive force on any side face must act on the middle third of that face, or there will be a tendency to heave at parts in tension. Speaking generally, if the crown or topmost portion of the arch be too light the deadweight at the haunches or those parts springing from the piers will lift the crown, and the whole arch be reduced to ruin. And if the crown be too heavy the haunches will open up, the crown will sink, and the arch collapse. The lateral forces involved are larger when the arch is flatter, i.e. when it is semi-elliptical or a small segment of a large circle, than when it is semi-circular, with the same span.

The centering (q.v.) or arrangement of scaffolding upon which the arch is built requires careful designing. It must be sufficiently strong to support the unfinished work, it should be easily removable, and its total removal should cause no change of shape of the arch.

The largest stone arch span in the world is in the Washington aqueduct. It was built by Meigs, and is of 220 ft. The second largest is that of the Grosvenor bridge, built by Hartley in 1832 over the Dee at Chester. It consists of a single segmental arch of 200 ft. span, with a rise of 42 ft., and is built of granite and sandstone. Another good example of single-arch bridge is that over the Taff at Pontypridd in South Wales. It was built by William Edwards in 1750, with a span of 140 ft. and a rise of 35 ft. The deadweight at the haunches, which in a bridge built previously by Edwards had been so great as to lift the crown up and ruin the bridge, is diminished by filling the internal spaces with charcoal and by having each side perforated by three cylindrical openings.

Elliptical arches were introduced by Rennie, whose engineering skill has its permanent record in his magnificent bridges over the Thames. Waterloo Bridge, a finely-built structure of granite, has nine equal semi-elliptical arches of 120 ft. span, with a rise of 32 ft. The width of the bridge is 42 ft. and its length 1,380 ft., with 1,100 ft. of approaches. Cofferdams (q.v.) were employed in the building of the piers, with steam engines to pump out the water. London Bridge consists of five semi-elliptical arches, the centre one of 152-1/2ft. span, the two next of 140 ft., and the end two of 130 ft., thus giving a clear waterway of 692-1/2 ft. The width of the roadway is 52 ft., the rise of the centre arch 37-1/2 ft., and the full length of the bridge 1,005 ft. The river has a soft alluvial bottom about 30 ft. deep at low water. The piers and abutments are supported on cofferdams, the floors of which rest on piles about 20 ft. long. The restricted waterway due to the older bridge still remaining, 180 ft. lower down, while the new one was being built, tidal action and other causes supplied many practical difficulties, which, however, were all satisfactorily overcome, and the bridge was opened in 1831, having taken seven and a half years to build.

Arched bridges of cast-iron and of wood have been built. Southwark Bridge, over the Thames, like the previous two, designed and built by Rennie, is a fine instance of the cast-iron arch bridge. This was opened in 1824. There are three arches, each consisting of eight cast-iron ribs, the central arch of 240 ft. span, with a rise of 24 ft., the two side arches of 210 ft. span, and rising 19 ft. Each rib is 2-1/8in. thick, and is built up in lengths of 13 ft., which are bolted together. The ribs are connected by transverse plates. The weight of metal in the central arch is 1,600 tons, in each of the side arches 1,460 tons.

The Newcastle-upon-Tyne high level railway bridge is composite in character, having arched ribs of cast-iron strengthened with ties of wrought-iron. It is, in fact, a form intermediate between the arch and the girder, to which latter type the chief railway bridges since that time have tended. Girders are more fully discussed separately, but it should be stated here that they are simply beams of wood, cast-iron, wrought-iron or steel, of such a section as to be best able to resist fracture due to bending or to shearing. The former of these two causes chiefly influences the shape and size of the top and bottom flanges or booms of the girder, the top boom being usually required to resist compression, the bottom boom to resist tension. The latter cause determines the nature of the web or bracing joining the two booms. If these are joined by cross-bars forming a lattice, the girder is called a lattice-girder. The girder may have two webs connecting the booms, one each side, and in this case it becomes a long box of rectangular section, the top and bottom parts of which are more substantially built than the sides. This form is known as the box-girder, a type of great interest historically. For the first wrought-iron girder bridge of large span the Britannia tubular bridge over the Menai Straits employed box-girders of special design successfully. This bridge was designed and built by Robert Stephenson, and opened for traffic in March, 1850. The girders in this case were made large enough for a line of railway to be laid inside each, thus rendering the bridge simply two long rectangular wrought-iron tubes laid side by side, and supported by masonry towers and abutments. Each tube is 14 ft. 8 in. wide, its height increasing from 22 ft. 9 in. at the abutments to 30 ft. at the centre, outside measurements being given in each case. The roof and floor of each tube is cellular, to increase its strength and stiffness. The bridge has four spans, two of 460 ft. over the straits and two of 230 ft. over land to the abutments. The tubes are supported by three masonry towers, and these end abutments at a height of 100 ft. above high-water level, cast-iron frames taking up their weight at the supports. The central tower rises to the height of 230 ft., and is built on the Britannia rock in the middle of the channel. The whole length of each tube is 1,510ft. Each of the longer spans weighs 1,587 tons, the shorter 630 tons, thus making up 4,680 tons as the total weight of each tube. They are fixed to the central tower, but have roller supports on the side towers and abutments so as to admit of free expansion and contraction due to changes of temperature. Similar tubular bridges have been built on the Conway river, where the span is 400 ft., and on the St. Lawrence at Montreal, where the greatest span is 330 ft. The latter is a railway bridge nearly two miles long, and has its piers specially adapted to resist and break the ice that comes down the river in spring. Coming next to the lattice-girder bridges which are nowadays in such extensive use, we may instance the Charing Cross (South-Eastern Railway) bridge, recently doubled in width to suit the increase in traffic. This is 1,365 ft. long, and is built with nine spans, six of 154 ft. and three of 100 ft. Two lattice girders 50 ft. apart are supported parallel to each other on piers of cast-iron or brickwork. The booms of these main girders are 14 ft. apart, and are built of plate-iron; they are held together by vertical bars and by diagonal bracing. Transverse girders are fixed across below the lower booms, and carry four lines of rails between the main girders. They also project outwards beyond each main girder, the projecting parts carrying a footpath. A type of bridge very early employed is the suspension bridge. Piers are built each side of the obstacle to be crossed, and chains firmly fixed at each end pass over these piers and carry a roadway by means of hanging rods. The chain takes up a definite curvature, parabolic if the roadway is of uniform weight all along, but altering when any extra weight comes on. The stress in the chain is greatest at its lowest part, and is much increased if the chain be pulled out flatter across the same span. Oscillations produced in the structure by a comparatively light rolling load may by gradually increasing in magnitude become very dangerous. Hence the use of stiffened suspension bridges, in which the roadway is rendered more rigid by bracing, the result being to distribute the effect of the rolling load over a greater length of chain.

The Menai suspension bridge, close to the Britannia tubular bridge, designed and built by Telford, and opened in 1825, is a fine example of this type. Here the points of suspension are 580 ft. apart; two carriage-ways and a central footway are supported by four cables, each consisting of four chains, the composite links of which are built of flat iron bars 10 ft. long. The dip of the chain is 57 ft., the total length of the bridge is 1,710 ft., and the roadway is 100 ft. above high-water level. The largest simple suspension bridge in the world crosses the Sarine valley at Freiburg, in Switzerland. Its span is 870 ft., and the roadway is 167 ft. above the river. Clifton bridge, over the Avon, built by Brunel, has a span of 702 ft., and is at a height of 250 ft. above the Avon. This bridge is stiffened by longitudinal girders and by braced handrailing. Many stiffened suspension bridges now exist, by far the largest being the Brooklyn bridge, uniting New York with Brooklyn. The central span is of 1,600 ft., and there are two side spans over land, each of 930 ft. The towers are 276 ft. high, founded by caissons 80 ft. below the high-water mark. There are four suspending cables, 15-3/4 in. in diameter, each built up of 5,000 steel wires. The roadway is 80 ft. wide, and is in five parts, two for ordinary vehicles, two for cars, and a central one for foot passengers; the weight of the structure hanging between the towers is 7,000 tons.

The cantilever principle has recently been introduced in the building of girder-bridges of large span, by the successful erection of the Forth Bridge on the North British Railway at Queensferry. The engineers were Sir John Fowler and Mr. Benjamin Baker. At this place the estuary of the Forth is 1-1/2 miles wide, and in parts as much as 200 ft. deep, much too deep to allow piers to be built there. This led to the adoption of two large spans of 1,700 ft. each, effected by three cantilevers. The shore ends of each of these give spans of 675 ft., and the remainder of the bridge consists of fifteen small spans of 168 ft. each. The centre of each big span is 152 ft. above high-water level, and the highest part of the cantilevers 361 ft. The piers upon which the big cantilevers are built consist each of four cylindrical masonry columns 36 ft. high, tapering from 55 ft. diameter at the bottom to 49 ft. at the top. They were founded by means of coffer-dams for the shallow parts and large caissons 70 ft. diameter for the deeper parts, sunk about 40 ft. below the river bed, and resting on rock or boulder-clay. The general view of the arrangement of each cantilever is shown in the plate; it may be said to consist of two enormous steel composite brackets placed back to back so as to balance each other, and forming a gigantic lattice girder one-third of a mile long, tapering each way from the middle outwards. The structure somewhat resembles the open beam of a chemical balance, each arm of which is over 600 ft. in length. The cantilevers also taper in plan so as to resist wind pressure more effectively, the width diminishing from 120 ft. at the piers to 32 ft. at the extremities. The main columns from which the cantilevers spring are steel tubes 12 ft. in diameter, and all the compression and tension members in the structure are proportionately large. The work to be done was so unique in its great magnitude that special tools were in many cases designed for it. There are 45,000 tons of steel employed in the bridge. Its cost was £1,600,000.

In many cases it is desirable to have the bridge movable, entirely or in part, as in the neighbourhood of clocks, canals, etc. The chief kinds of bridges designed for such purposes are drawbridges, swing-bridges, traversing-bridges, and pontoons. In the first case the bridge is able to open by having part capable of turning upwards about a horizontal axis. Such drawbridges or bascules were in use centuries ago across the moats of old castles. Swing-bridges open by turning about vertical pivots; traversing-bridges open by sliding backwards along one of the abutments. Pontoons are floating bridges built along a series of flat-bottomed boats of iron anchored firmly in position. The Tower bridge across the Thames is the largest bascular bridge in the world. Two masonry towers divide the water-way into three parts. The central part contains the double-bascule, and gives an opening 200 ft. wide and 135 ft. high when the bascule is up. The side spans are of 270 ft. each, and are half-suspension in design. The cost was over £1,100,000. It was completed in 1894.