Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/388

Rh 340 BRIDGES [EXAMPLES. with grey granite from the State of Maine, which cost 10 per cubic yard in situ. The interior of the work is of magnesian lime stone. The massive appearance of the granite rock lacing is very striking. The contract prices, and the total quantities of the steel and iron work required for the bridge, are as follows : 2,500 tons of steel, at 60 per ton ) 500 ,, wrought iron, at 40 per ton... / of 2000 Ibs. IjOOO ,, rolled iron, at 28 per ton ) 200 ,, cast-iron at 16 per ton of 2240 Ibs. The bridge has three spans, each formed with ribbed arches made of cast steel, a novelty in bridge building. The centre span is 520 feet and the side ones 502 feet in the clear. The rise of the centre arch is 47^ feet, that of the side ones I i feet each. These are by far the largest arched spans in the world, anil under the able direction of Colonel Read, Captain Eads s chief assistant, they are now being rapidly erected gradually from each pier and abutment without the aid of centering. Each span is composed of four double ribs of steel (well braced together at their relative distances from each other), and the tubes forming them are jointed butt to butt. They are clasped together by wrought iron couplings (which proved to be much better than steel), furnished with parallel grooves corresponding with similar grooves in the tubes. Steel pins, varying from 41 inches to 7 inches in diameter, pass through the centre of the couplings and the ends of the tubes at every joint. The vertical bracing between the upper and the lower tubular ribs, which are 12 feet apart from centre to centre, convert the two members into a single arch. At the time of the author s visit two of the openings were already spanned by the steel tubes, which are all 18 inches in diameter, and 12 feet to 13 feet long, but of thicknesses varying from I?, inches to 2^ inches. The arches are to carry a double railroad track, and above the track a roadway 54 feet wide for carriages and foot passengers.&quot; The bridge was opened, subsequently to Sir C. Hartley s visit, on the 4th July .1874. 83. Projected bridges. The Tay Bridge is a railway bridge in course of construction (1876), to form a connec tion between the town of Dundee and the North British Hallway system in Fife, and crosses the Firth of Tay about a mile and a half to the west of Dundee. The length of the bridge exceeds two miles. It will therefore be the longest iron bridge in the world. The following descrip tion has been furnished by Mr A. D, Stewart, who assisted the chief engineer, Mr T. Bouch, in the design of the bridge : Carves. Commencing at the south shore, the bridge for the first five spans is on a curve of a quarter of a mile radius. It is then straight for a distance of a mile and a half. At the north shore, between high and low water mark, it describes a curve of a quarter of a mile radius, forming nearly a quarter of a circle towards the town of Dundee. Gradients. The level of the rails at the south end of the bridge is 78 feet above high water. The gradient descends 1 in 100 for the first three spans ; it is level for the next two spans ; it then ascends 1 to 353 towards the centre of ths bridge. Over the navi gable part of the river it is level, and the rails are 92 feet above high water, leaving 88 feet for the passage of shipping above high water of spring tides. From this the gradient falls 1 in 73^ to the end of the bridge. Spans. The portion of the bridge at present being executed extends to 3420 yards. It is intended to add an opening of 120 feet and a number of 27 feet spans at the north end. The magnitude of the several spans in order, commencing at the Fife shore, is as follows: 3 spans of 60 feet, 2 of 80 feet, 10 of 120 feet, 12 of 136 feet, 13 of 230 feet, 1 of 150 feet, 11 of 120 feet, 25 of 60 feet, 1 of 155 feet, and 6 of 27 feet the total number of spans being thus 84. Piers and Foundations. The first fourteen piers are founded upon rock, which was generally covered to the depth of a few feet with clay or other soft material. These piers consist of double solid cylinders of brickwork, built with strong Portland cement mortar, connected by a wall of brickwork from low water to the super structure. Foundations for these piers were obtained by placing caissons or hollow cylinders on their site, excavating within them, and sinking them by forcing out the water by air pressure. These caissons were built on the foreshore on a properly prepared foundation, and lined internally with brickwork to such a height that when fully sunk the brickwork extended above low water. When carried out and placed in position, this brickwork formed port of the permanent pier, and gave weight and stability to the caisson when the water was displaced from the inside. When the whole material above the rock was thus excavated, the working chamber and shaft of the caisson were filled with concrete, put in in a liquid state, and when this solidified the building upwards of the pier was continued. A difficulty arose in keeping the cylin ders vertical during the sinking. This was overcome by combining them into a single caisson. Between the fourteenth and fifteenth pier the rock disappears. At the sites of the next six piers the bed consists of a layer of hard material resting on silt. It is proposed to pile these piers from an outside staging within an oval-shaped wrought iron caisson. After the piles are driven, their heads are to be surrounded with concrete, the water is then to be pumped out of the caisson, and brickwork to be built up to the level of about 5 feet above high water. The upper portions of these, and of all the piers to the north of them, are to consist of cast-iron columns braced together. From the twenty-second pier northwards the bed of the river con sists of sand, with occasional layers of coarse gravel and boulders. It was, however, necessary to modify the designs for the piers, and the method of founding and building them, according to the load each had to carry. For the 120 and 136 feet spans there are eighteen piers. For each pier two wrought iron caissons are prepared, partly cylin drical and partly conical in shape, and having a base of 15 feet. These are built on the fore-shore, and lined with brickwork; they are then floated out by means of pontoons having hydraulic machinery for lowering: when they have been sunk in their proper place until they take a bearing in the sand, the pontoons are removed, and by means of sand-pumps the material from the interior is removed and they sink by their own weight. During the operation of sinking, rings of wrought iron and brickwork aro added to the top of the caisson, and stones are laid round the out side to fill the void caused by the scour and pumping. When a sufficient depth has been obtained the pumps are removed, and the interior is filled with liquid concrete; and when this has solidified, the brickwork is continued to above high water. For the 230 feet spans, large wrought iron caissons of a cylindrical form, 31 feet in diameter, are erected on the fore-shore, one for each pier, and lined with brickwork to the height which it is intended to sink them in the sand. The upper and temporary portion of tin- caisson, of the same diameter as the under, but with no lining of brickwork, is placed on the top of the lower part and bolted securely to it. The compound caisson is floated out and sunk, as above described, by means of sand-pumps, and the permanent portion of the caisson is filled with concrete. The temporary portion is then unbolted by divers, and removed for further use. From the surface of the ground to above low water, the pier consists of a brick oval- shaped hollow cylinder, which is built on the fore-shore on girders ; and when it has thoroughly set, it is also floated out and lowered on the concrete foundation. The interior of this brick pier is then filled with concrete, and the building of the brickwork is continued to above high water as tidal w r ork. For the 60 feet spans towards the north end of the bridge, the piers consist of three braced cast-iron columns placed in a row across the bridge, the western column having a rake or batter of 1 in 3. Every fourth pier is double. Some of these were sunk as screw piles, others were founded in 6 feet cylinders, previously sunk by sand-pumps, and filled with concrete. Superstructure. The superstructure consists wholly of wrought iron girders. With the exception of two spans which have girders of the bowstring form, they have the top and bottom members straight and parallel. The bracing is of the double lattice form, crossing nearly at right angles, and from the point of intersection a vertical support is carried to the member on which the cross-girder or beam rests. The girders for the 230 feet spans, and the bow string girders, have wrought iron cross girders resting on, and rivetted to, the lower member on which the roadway is placed. All the others have timber cross-beams resting on and rivetted to the top flange, and the roadway is above these girders. They are sent to the Tay built in convenient pieces for shipment, and rivetted together on jetties prepared near the shore. They are then floated out and raised to their places by machinery suited to their respec tive weights. They are generally continuous in groups of four consecutive spans. In order to make continuity perfect, the further end of each girder is raised through a certain calculated height before rivctting it to the next. Montreal papers state that a bridge 15,500 feet in length is about to be constructed over the St Lawrence at Montreal, from the designs of Mr Legge. It will have one span of between 500 and 600 feet, and GO smaller spans, with a height of 130 feet above the water at high tide. The estimated cost is 800,000. 84. Statistics. Table XVI., from the 8th edition, gives some statistical information as to the weight, cost, and dimensions of some of the principal cast-iron bridges.