The Works Of Messrs. Schneider And Co.

THE ALEXANDER III. BRIDGE. 141 an average length of 3.650 metres (11 ft. Ilf in.), and their weight varies between 4 and 6 tons. The general appear- ance of the bridge and of its construction are illustrated by Figs. 401 to 403, Plate LXXXVI. It was necessary to design the bridge as a three-hinged arch, so as to render its stability almost independent of variations in temperature, and to insure the coincidence of the pressure curves in the vicinity of the neutral axis, in Order to maintain the ribs in compression. One of the most striking peculiarities of the arch is its flatness, the rise being only 1 17.12’ wliich is less than that of any other arched bridge built in France. This of course, throws considérable pressure on the abutments (288 tons per metre run of bridge, or approximately 825 tons on each springing). The tests specified for the cast steel employed are the same as those prescribed for rolied steel, namely, a tensile strain of 42 kilogrammes per square millimétré (26.67 tons per square inch), witli an élongation of 22 per cent. As the spécifie weight of the varions steel castings made at Creusot for this bridge gives an average of 7.65, it may be said that perfection is almost attained as regards homogeneity of metal. The superstructure is of rolied steel ; it consista mainly of a series of longitudinal lattice girders, and of transverse girders. Near the abutments the former are supported by uprights, testing on the ribs, and connectée! to tliem by cast-steel slioes. Nearer the centre the longitudinal girders bear direct on the ribs, the extrados of which are cast with special bearing surfaces, planed in order to secure a perfect contact. The arches, spandril uprights, and longitudinal girders are stayed transversely and vertically, near the abut- ments by diagonal bracing, at the centre and under the pavement by stays which run the whole depth of the ribs ; at the top and under the roadway by inelined bars which prevent the ribs from distorting. The longitudinal bracing consists of a complété flooring of flat plates, which covers the whole surface of the bridge ; this flooring is carried under the sidewalks by steel joists that rest on the longitudinals, and under the roadway by girders also attache«! to them and placed between the ribs. The roadway is paved with wood Blocks, so as to reduce the weight. The curbs are of cast steel, and in order to allow free transversal ex- pansion of the roadway, they bear against a series of spiral springs. The base of the railing and balustrade are of cast iron ; the railings and pilaster heads are of brass. The cast-iron ornaments of the face of the outer ribs consist of a frieze fixed to the curved webs of the rib ; of a casing on the spandril uprights ; of a cornice along the whole length of the bridge, and supporting the railing; of garlanded pendants in the spandril ; and of a suitably ornamented carving in the centre of the arch. Foundations.—A complété knowledge of the subsoil allowed the builders to secure the stability of the abut- ments, notwithstanding the great pressure from the ribs. On the right bank, beneath recent sandy clay alluvium, a layer of sand was found, covering a bed of hard limestone. On the left bank, the top alluvium consists of gravelly sand ; hard limestone is also found under a bed of sand and pebbles, but at a higher level than on the right bank. The masonry of each abutment is rectangular in plan, measuring 33.50 metres (110 ft.) by 44 metres (144 ft.). Each mass of masonry has been sunk by compressed air, by means of one caisson, divided into five working chambers, normal with the river. These chambers are filled with cement concrète, the remainder of the masonry being form ed of quarry stones laid in cement. In the rear of each spring course, are four beds of rough-faced granité, set normal to the pressure, which reaches a maximum of 48 kilogrammes per square centimètre (682.7 1b. per square inch) ; that on the ortjinary masonry does not, however, exceed 18 kilogrammes (256 1b.) at the joint with the last layer of granité blocks ; the vertical pressure per square centimetre of bearing on the founda- tion does not exceed 3 kilogrammes (42.67 1b. per square inch). The relation between pressure and weight, under the rnost unfavourable conditions, does not reach 0.50. The mass of masonry is so proportioned as to carry back the strains from the bridge to a point more than 30 metres (98 ft. 6 in.) distant from the face of the abutment. Erection.—The work of érection has been a most interesting operation, owing to the size and novel design of the bridge itself, and to the method followed. On account of the difficulties to navigation between the Con- corde and Invalides bridges, it was necessary to leave in the middle of the river a 50-metre. (164-ft.) channel un- obstructed, and of sufficient height to enable the largest boats to pass. Moreover, the bridge being a single span, the construction of false Works, built on piles driven in the river, was out of the question. It was necessary, therefore, to erect two half temporary bridges, one on each bank, formed of beams well braced together, and to use a roller bridge for supporting the cast-steel arches in the middle part of the river. Two types of roller bridges were available ; in one, the track woukl have been placed on the series of piles driven in the river at the edge of the channel ; this would have given a range of 52 metres (170 ft.) only for the roller bridge. In the other, the track would have been made on the abutments, in whicli case the temporary bridge would have covei'ed the perma- nent one, thus constituting a formidable structure 120 metres (394 ft.) span. The second type, though more costly, was decided on, as it was feared that a settling of the piles would take place under the heavy loads to be carried, and also as ice drifts and wreckage during the rising of the river might entail failure of part of the sets of piles. In order to reduce as much as possible the chances of such accidents, groupa of protecting piles were driven on each side of the channel ; these end in a fixed signal up- 2f