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Figs. 19, 20, and. 21 show an independent girder, a cantilever, and a cantilever and suspended girder bridge. In a three-span bridge continuous girders are lighter than discontinuous ones by about 45 per cent, for the dead load and 15 per cent, for the live load, if no allowance is made for ambiguity due to uncertainty as to the level of the supports. The cantilever and suspended girder Fig. 19. types are as economical and free from uncertainty as to the stresses. In longspan bridges the cantilever system permits erection by building out, which is economical and sometimes necessary. It is, however, unstable unless rigidly fixed at the piers. In the Forth Bridge stability is obtained partly by the great excess of dead over live load, partly by the great width of the river piers. The majority of bridges not of great span have girders with parallel booms. This

Fig. 20. involves the fewest difficulties of workmanship and perhaps permits the closest approximation of actual to theoretical dimensions of the parts. In spans over 200 feet it is economical to have one horizontal boom and one polygonal (approximately parabolic) boom. The hogbacked girder is a compromise between the two types, avoiding some difficulties of construction near the ends of the girder.

24) and Whipple truss (Fig. 25) of queen-post trusses ; the Warren truss of queen-post trusses alternately upright and inverted. A combination bridge is built partly of timber, partly of steel, the compression members being generally of timber and the tension

Fig. 24. members of steel. On the Pacific coast, where excellent timber is obtainable and steel works are distant, combination bridges arc still largely used (Ottewell, Trans. Am. Soc. Civil Eng. xxvii. p. 467). The combination bridge at Roseburgh, Oregon, is a cantilever bridge. The shore arms are 147 ft. span, the river arms 105 ft., and the suspended girder 80 ft., the total distance between anchor piers being 584 ft. The floor beams, floor, and railing are of timber. The compression members are of timber

Fig. 25. except the struts and bottom chord panels next the river piers, which are of steel. The tension members are of iron and the pins of steel. The chord blocks and post shoes are of cast iron. The exceptional local conditions at the site of the Forth Bridge led to the adoption there of the cantilever system, till then little considered. Now it is well understood that in many QantjjeYer positions that system is the simplest, most convenient, . . , _ and most economical method of bridging. The spans are virtually reduced to smaller spans so far as the girders are concerned ; and what is much more important, the cantilevers can be built out from the piers, member by member, and no river staging is required. Mr. Waddell has shown that, in some eases, it is convenient to erect simple independent spans, by building them out as cantilevers and converting them into independent girders after erection. Fig. 26 shows girders erected in this way, the dotted lines being temporary members during erection, which are removed

Fig. 21. Most braced girders may be considered as built up of two simple Fig. 26. forms of truss, the king-post truss (Fig. 22, a), or the queen-post truss (Fig. 22, 5). These may be used either in the upright or the afterwards. The side spans are erected first on staging and anchored inverted position. A multiple truss consists of a number of simple to the piers. From these, by the aid of the temporary members, the centre span is built out from both sides. The principal cantilever bridges are (1) the Forth Bridge, with two central spans of 1710 ft., and two anchor spans of 680 ft.; (2) the railway bridge at Memphis over the Mississippi, span 790 ft. ; (3) the Red Rock Bridge over the Colorado river, with a span of 660 ft. and anchor spans of 165 ft., suspended girder 330 ft. in length; (4) the Poughkeepsie Bridge over the Hudson; (5) the Lansdowne Bridge over the Indus at Sukkur, which, though designed later, was erected earlier than the Forth Bridge ; (6) the Niagara Bridge for a railway—towers 495 ft. high, cantilever arms each 175 ft., suspended girder 120 ft., total length of bridge 910 ft. The Forth Bridge was described in Ency. Brit. vol. xx. p. 234. Further details may be found in The Forth Bridge, by W. Westhofen, reprinted from Engineering; “The Forth Bridge,” by Sir Fig. 22. B. Baker, Exports of British Association, 1884 and 1885 ; Die Briicke, von G. Barkhausen, Berlin, 1889 ; The Forth Bridge, trusses, e.g., Bollman truss (see Bridges, ninth edition). Some Forth timber bridges consist of queen-post trusses in the upright position, by Philip Philips, Edinburgh. In the Sukkur Bridge (Fig. 27), the as shown diagrammatically in Fig. 23, where the circles indicate points at which the flooring girders transmit load to the main

clear span is 790 ft., and the suspended girder 200 ft. in length. The span to the centres of the end uprights is 820 ft. ; width between centres of main uprights at bed-plate 100 ft., and between girders. Compound trusses consist of simple trusses used as centres of main members at end of cantilevers 20 ft. The bridge primary, secondary, and tertiary trusses, the secondary supported is for a single line of railway of 5 ft. 6 in. gauge. The back on the primary, and the tertiary on the secondary. Thus, guys are the most heavily strained part of the structure, the stress the Fink truss consists of king-post trusses ; the Pratt truss (Fig. provided for being 1200 tons. This is due to the half weight of Fig. 23.