The Works Of Messrs. Schneider And Co.

THE ALEXANDER III. BRIDGE. 143 the bridge has to bear these strains only at short intervals, when it is shifted from one series of arches to the following. For this operation a calm day is always chosen, so as to reduce the strain on the horizontal stay girder, the moment of flexure of which, under the action of the wind, is also at a maximum for the 120-metre (394-ft.) spån. It will be seen, therefore, that the roller bridge behaves differently during shifting and during érection. In the first case, it rests solely on the end tresties, mounted on wheels, and is calculated to be self-supporting, while bear- ing the surcharge due to the suspended flooring and érection tackle. In the second case, it is lifted by screw jacks and lowered on supports in a line with the Inter- mediate piles, so that it can be compared to a three-span bridge, i.e., with one 53-metre (174 ft.) central spån and two 33.50-metre (110-ft.) shore spåns. The supports on the piles are, however, maintained slightly lower than those on the abutments, as the weight of the arches that acts on the middle part would cause the bridge to sag in the centre, which would tend to make it rise with regard to the roller tresties. This, however, might have been avoided by curving the girders ; but, on the other hånd, it would have been more difficult to deal with girders so shaped, and it was deemed préférable to let the roller bridge be slightly curved in its central part during érection, which, moreover, facilitâtes placing the vous,soirs. Erecting the Roller Bridge.—It was thought, at first, that the bridge could be erected on one of the banks and floated in place ; owing, however, to its great dimensions, especially its height, 17 metres (55 ft. 9 in.) above the rivei- level, the idea was abandoned. In erecting, great difficulties had to be overcome, as there was not sufficient space available on either bank to erect it completely, nor indeed, enough to erect a section that would cross the 53-metre (174 ft.) wide channel. The bridge was there- fore built in three operations. In order to reduce the strains developed in the bridge framework during overhang in the second operation, one end was fitted with a light tapering head 15 metres (49 ft.) in length (Plate LXXXVIL). During érection the lower frames had to transmit to the lattice bars the reactions from the Hearings, when they were not immediately above the bearings. These bars had, therefore, in each panel consecutively, as it came above the bearings, to bear local strains which inereased the work upon them to a large degree. The framea had been stiffened to resist local strains, the height of the webs having inereased from .450 metre (17üin.) to .550 metre (21f in.) ; in addition, steel stanchions join the central part of these frames to the lattice work, in a line with the middle of each panel. As these arrangements were not thought sufficient, strengthening was completed by the addition of an angle-bar to the top of the frame webs in the panels that were liable to be most distressed. As the river traffic was on no account to be interfered with, a floating row of piles was moored, for the sake of prudence, in the centre of the channel, and was adjusted at such a level that by means of a few wedges it was possible to place it in contact with the underside of the bridge. This was done only as a precautionary measure, and the device was not used at all during the érection of the bridge, the 53-metre (174 ft.) wide channel being crossed without any difficulty. The strain on the metal reached 15 to 16 kilogrammes per square millimétré (9.52 to 10.16 tons per square inch) of section at the end of the operation. Three causes contributed to the lowering of the end of the girder during érection and launching: 1. Deflection of the beam under the action of the dead load ; 2. variations in the direction of the bearing line, owing to the settling of the scaffolding, elastic shortening of the woodwork, &c.; and 3. local deformation produced in the lower framing. The first element was estimated at .15 metre (5J in.), and to compensate for the elements that could not be calculated in advance, the end was raised by .20 metre (7-g- in.). All the operations were carried out without mueh trouble ; each one took a half day, and during the second operation, navigation was stopped from 5 A.M. to 7 A.M. only. The forward travel of the bridge was obtained by means of two winches working on two sets of tackle fixed to the set of piles to the right. The shifting device com- prised beams fitted with two or four rollers, according to the loads to be transferred to the scaffolding. The roller bridge behaved well, as also did the wood scaffolding. During the greatest overhang the support placed above the starting point settled by 16 millimétrés (f in.) ; while that in the rear settled by 5 millimétrés (t3k in.), the end of the bridge having drooped 136 milli- métrés (5f in.). The end, however, met its bearings with- out trouble and without it being necessary to lift it, the complété launching taking place without any stoppage. To add to the précautions already taken, the panel uprights subjected to the greatest strains were strengthened with timber stanchions placed in the centres. Erecting the Arches.—Each arch is erected by start- ing from both ends at once ; therefore two ribs in course of being laid occupy four gangs who meet at the key. The varions operations are shown in the illustrations, Figs. 405 to 437, Plates LXXXVIL to XCI. Two steam crânes take the sections from the dépôt on the banks, and lift them on trucks which are used to carry them under the service bridge, from which they are taken by means of steam winches. These are double-acting by means of cables, and serve both for raising the voussoirs and conveying them to the spot where they are to be fixed ; the roller bridge is fitted with tracks for this purpose, on which run the érection trucks, two in number, for each rib. Each truck has, therefore, to serve a 60-metre (196 ft. 10 in.) run of ribs ; this nécessitâtes a perfect system of conductors both for the endless conveying chain and for the lifting cable ; to this effeet the endless chain runs in a trough made of sheet steel while the cable is supported on specially-shapecl sheaves, arrangée! so as to allow free travel for the trucks. These sheaves are in two parts,