Le Britannica encyclopdia ; un dictionnaire des sciences, des arts, de la littérature et de l'information générale . n de 340 ft. Certains d thesetimber les ponts sont dit d'avoir duré quatre-vingt-dix ans avec ordinaryrepairs, mais ils n'étaient pas des ponts routiers lourdement chargé. À partir de1840, fermes, principalement de bois mais avec des lits en fer forgé et des tiges de tension-chaussures en fonte, ont été adoptés en Amérique. Le Howetruss de 1830 et le treillis Pratt de 1844 sont des exemples. TheHowe avait des cordes et bois treillis un treillis de bois,avec struts liens fer vertical : Dans le treillis Pratt le framework Struts et werevertical liens l'incliné. Jusqu'à 1850, br

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The encyclopdia britannica; a dictionary of arts, sciences, literature and general information . n of 340 ft. Some of thesetimber bridges are said to have lasted ninety years with ordinaryrepairs, but they were road bridges not heavily loaded. From1840, trusses, chiefly of timber but with wrought-iron tension-rods and cast-iron shoes, were adopted in America. The Howetruss of 1830 and the Pratt truss of 1844 are examples. TheHowe truss had timber chords and a lattice of timber struts, with vertical iron ties: In the Pratt truss the struts werevertical and the ties inclined. Down to 1850 such bridges-weregenerally Umited to 150 ft. span. The timber was white pine.As railway loads increased and greater spans were demanded, the Howe truss was stiffened by tiinber arches on each side ofeach girder. Such a composite structure is, however, funda-mentally defective, the distribution of loading to the twoindependent systems bding indeterminate. Remarkably hightimber piers were built. The Genesee viaduct^ 800 ft. in length, buUt in 1851—1852 in 10 spans, had timber trestle piers igo ft. in. From J. R.Green^ A Short History of the-English -Peitple, bypermission of Macmillan& Co., Ltd! Fig. 6.—Old London Bridge, ^.i?. 1600. From a Ojra^ii^ in the Pepysian Library, Magdalene College, Cambridge. the building of the first stone bridge commonly called OldLondon- Bridge: About the year 1176, the stone bridge wasbegun to be founded by Peter of Coleciurch, near unto thebridge of timber, but more towards the west. It carriedtimber houses (fig. 6) which were frequently burned dOwn, yetthe main stWctureexistedtiU thebeginning of the 19th century.The span of the arches ranged from 10 to 33 ft., and the totalwaterway was only 337 ft.., Thewaterwayipf, fjie present LondpnBridge is 690 ft., and the remp^ral of the obstriUftionf caused bytj^eold bridge caused a lowering of the low^water level by 5 ft..

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