Page:1902 Encyclopædia Britannica - Volume 26 - AUS-CHI.pdf/414

This page needs to be proofread.


372

BRIDGE

OF

AL LAN — BRIDGES the chances are so great against any particular result, aggregate length of 3000 feet of girders fell, this being that at the score of love-all the advantage of getting to one of the greatest disasters which ever occurred to an any particular score has no appreciable effect in determin- engineering structure. Since that time greater attention ing the choice of suit. In the play of the hand, the has been given to the loading forces acting on bridges, to advantage of getting to certain points should be borne the determination of the severest straining actions, and to in mind. The principal points to be aimed at are 6, 18, the best and most economical arrangement of material. and, in a less degree, 22. The 2 points that take the The general use of mild steel in place of wrought iron in score from 4 to 6 are worth 4, or perhaps 5, average bridge-work involves much greater care in the selection points; and the 2 points that take the score from 6 to and testing of the material. The analysis of the secondary 8 are worth 1 point. When approaching game it is an straining actions, bending and twisting due to the local advantage to make a declaration that may just take the conditions in which loads are applied to individual player out, and, in a smaller degree, one that will not members, has been worked out. With more complete exactly take the adversaries out. When the score is 24 knowledge of the straining actions engineers have greater to 22 against the dealer, hearts and clubs are half a trick confidence in design, and bridges have been built with better relatively to diamonds than at the score of love-all. spans approaching the theoretical limit possible for the In the first and second games of the rubber the value of material used. Lastly, with increase of size, more study each point scored for honours is probably about a third of methods of erection has been necessary, and it is reof a point scored for tricks—in a close game rather less, in cognized that in large bridges facility of erection becomes a one-sided game rather more. In the deciding game of a governing consideration in design. In materials other the rubber, on account of the importance of winning the than iron or steel the greatest change has been the game, the value of each point scored for honours sinks extension of the use of cement concrete for foundations to one-fifth of a point scored for tricks. (w. H. w.*) and substructures, and in some cases for the arches of large Bridge Of Allan, a police burgh (1870) and arched bridges. Concrete strengthened by iron bars or a health resort of Stirlingshire, Scotland, on the Allan water, network of steel has also been used, and seems likely to be 3 miles IST. of Stirling by rail and tramway. There are two of considerable value. In metal bridges wrought iron has public halls and two reading-rooms, a hydropathic estab- been replaced by mild steel—a stronger, tougher, and lishment, a mineral spring and well-room, and a fine art better material. Ingot metal or mild steel was sometimes and natural history museum. The industries are bleaching, treacherous when first introduced, and accidents occurred, dyeing, and paper-making. The private schools are noted. the causes of which were obscure. In fact, small differences of composition or variations in thermal treatment Population (1881), 3004; (1901), 3240. during manufacture involve relatively large differences of Bridgend, a market town of South Wales on the quality. How it is understood that care must be taken Ogmore, 20 miles W. by N. of Cardiff by rail, in the in specifying the exact quality and in testing the material southern parliamentary division of the county of Glamorgan. supplied. Structural wrought iron has a tenacity of 20 to There are a town hall, and two lunatic asylums, for the 22^ tons per sq. in. in the direction of rolling, and an county of Glamorgan and the county borough of Swansea, ultimate elongation of 8 or 10 per cent, in 8 inches. erected at a cost of nearly £200,000. Bridgend has a Across the direction of rolling the tenacity is about 18 foundry, joinery works, a tannery, and brickworks. Area tons per square inch, and the elongation 3 per cent, in of urban district (created in 1894), 2448 acres ; population 8 inches. Steel has only a small difference of quality in (1881), 4153; (1901), 6399. different directions. There is still controversy as to what Bridge north, a municipal borough and market degree of hardness, or (which is nearly the same thing) town in the Ludlow parliamentary division (since 1885) of what percentage of carbon, can be permitted with safety Salop, England, on the Severn, 20 miles S.E. of Shrews- in steel for structures. bury by rail. A steep line of rail between the Low and The qualities of steel used may be classified as follows :—(a) Soft the High Town has been opened, and the town hall (1652) steel, having a tenacity of 22-| to 26 tons per sq. in., and an restored. Area, 1168 acres; population (1881), 5317; elongation of 32 to 24 per cent, in 8 in. (6) Medium steel, having a tenacity of 26 to 30 tons per sq. in., and 28 to 25 per cent, elonga(1901), 6049. tion. (c) Moderately hard steel, having a tenacity of 30 to 34 tons Bridgeport, a town and city of Connecticut, per sq. in., and 20 per cent, elongation, (d) Hard steel, having a U.S.A., and capital of Fairfield county, the town and city tenacity of 34 to 40 tons per sq. in., and 10 per cent, elongation. Soft steel is used for rivets always, and sometimes for the whole occupying the same area, situated on Long Island Sound, superstructure of a bridge, but medium steel more generally for at the mouth of Pequanoc river. The Hew York, Hew the plates, angle bars, &c., the weight of the bridge being then Haven, and Hartford railway furnishes communication. reduced by about 7 per cent, for a given factor of safety. Moderately There are extensive manufactures, which in 1890 em- hard steel has been used for the larger members of long span bridges. Hard steel, if used at all, is used only for compression ployed over $13,000,000 of capital, and produced more members, in which there is less risk of flaws extending than in than $22,000,000 of manufactured goods. The principal tension members. With medium or moderately hard steel all among these were corsets, steam - fitting and heating rivet holes should be drilled, or punched inch less in diameter apparatus, and foundry and machine-shop products. In than the rivet and reamed out, so as to remove the ring of material 1900 the assessed valuation of real and personal property strained by the punch. The external forces acting on a bridge may be classified was $62,905,862, the tax rate $13 per $1000, and the net debt $1,270,098. Population (1880), 27,643 ; (1890), as follows: — I. The live or temporary load, for road 48,866; (1900), 70,996, of whom 22,281 were foreign- bridges, is taken to be the weight of a dense crowd (about born, and 1149 were negroes. The death-rate in 1900 120 lb per sq. ft.), or the weight of a heavily loaded waggon (say 10 to 25 tons on four -wheels), while for was 17‘3. railway bridges it is the weight of the heaviest train likely Bridges. —The article in the ninth edition of this to come on the bridge. II. An allowance is sometimes Encyclopaedia ends with a description of the Tay Bridge, for impact, that is, the dynamical action of the live then in course of construction (1876).1 That bridge was made load due to irregularities of the permanent way or yielding overturned in a storm on 28th December 1879, when an of the structure. III. The dead load comprises the 1 Later information will be found, however, in the ninth edition under weight of the main girders, flooring and wind bracing, or other headings, e.g., the Forth Bridge under Railways, xx. 234. total weight of the superstructure exclusive of any part