Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/369

Rh SUBSTRUCTURE. BRIDGES 325 easily calculated by tho method of reciprocal figures or otherwise. Occasionally metal [tiers are continuous metal structures, such as cast-iron cylinders. The maximum intensity of stress can then bo calculated by resolving the thrust on tho upper jtart of the [tier into a horizontal and a vertical component, calculating the bending moment produced on each horizontal cross section by the horizontal component, and adding the intensity of stress caused by this bending moment to the mean intensity caused by direct compression. The manner in which any metal-work pier is held by its foundation against a bending moment will require special consideration ; the resultant pressure should always fall well within the base. GO. Practice. In the design of the usual masonry bridge the thickness of the pier is generally determined by practical considerations. In small arches the [tier is made thick enough to allow the two rings of the two abutting arches to spring from the pier without interfering with one another, a clearance of about half a brick being often allowed between the two rings. In larger arches the piers will generally be found to vary in thickness from ^th to yV-u of the span, with a slight batter (i.e., with walls spread ing outward towards the base). In very old bridges piers are sometimes found equal in width to the open ing of the arch. In large bridges, or with very high /tiers, care must be taken that the pressure per square foot on the masonry or foundation does not exceed a safe value. The brickwork in the piers of Charing Cross bridge is subject to a compression of 9 tons per square foot ; four or five tons is a much more usual load. Eight tons per square foot may be considered a maximum for rubble stone-work, and perhaps 20 tons for the best dressed ashlar. Strong concrete may be trusted with Stons; firm rock foundations with 9 tons, soft sandstone with 2 tons, and firm earth with from 1 to li tons. The depth of the first course below the surface (on dry laud) should not be less than 3 feet in sand and 4 feet in clay. When framework, either of wood or iron, is used as a [tier, care must be taken by cross-bracing to provide against the effect of wind and vibration. 67. Site. The site proposed for a pier must be carefully examined by borings ; the ground should be uniform, for if a pier rests partly on one formation and partly on another, unequal settlement will certainly occur, even if the weaker formation be such as would have been amply strong enough to bear the pressure had the pier been wholly founded upon it. Solid rock may be considered the best foundation, but where rock is broken up by cracks or other inequalities it is inferior to such formations as uniform gravel, chalk, and some kinds of sand and clay. These foundation? may be described as incompressible. The worst foundations are afforded by those formations which can be compressed or squeezed out sideways by the imposition of weight. Muddy earth, certain clays, and certain sands are of this nature. Alternate beds of stone and slippery clay are very treacherous. The foundation should be dressed level so that the masonry may everywhere start from the same height, and therefore settle equally. Unavoidable in equalities are better filled up by concrete than by masonry. For foundations in water it is very important that the ground should not be such as can be scoured away by the current or wash of the water ; many bridges have failed by tho undermining of the piers due to this cause. Special precautions, to be presently described, must be taken against the effects of the scour if the soil itself is not of a sufficiently resisting nature. The piers must be so placed and formed that the obstruction to the flow of water may be as small as possible and the effect which the piers will have in altering the level of the stream above and below the bridge must be considered. Data as to the maximum flood waters to be provided for must be examined ; and provision must in some climates be made against ice by suitable cut- waters or fenders, an example of which is given in fig. 93, showing a pier of the Victoria Bridge, Montreal, FiG. 93. Side Elevation and Plan of Pier. G8. Mode of Founding in Water. The chief difficulty met with by an engineer about to erect a large bridge over a deep stream is to secure a sound foundation for the piers. The following are some of the principal methods of building piers in or under water : Cofferdams. Cofferdams are embankments or dams which surround the site so as to exclude the water from it. They are formed in general by driving two rows of piles round the site so as to enclose between them a water-tight wall of clay puddle ; in depths of less than 3 or 4 feet, where there is little current, a simple clay dam may be used. In greater depths, the timber walls consist of guide piles at intervals, with some form of sheet piling between them ; in extreme depths the timber walls may be composed of stout piles driven in side by side all round. The clam must be sufficiently strong to bear the pressure of the water against the outside when the space enclosed has been pumped dry. Kankine states that the common rule for the thickness of a cofferdam is to make it equal to the height above ground if the height does not exceed 10 feet, and for greater heights, to add to ] feet one-third of the excess of the height above 10 feet. The &quot; Cours de Pouts &quot; at the school of the Fonts et Chaussees, states that a coffer dam need never be made of greater thickness than from 4 to feet, as the interior can always be sufficiently stayed inside. This method of founding is now seldom practised ; it is costly and causes great obstruction in the stream. Caissons. Some foundations have been constructed as follows : A level or nearly level bed was prepared in the stream by digging or by driving piles and sawing off the heads at a uniform depth ; a huge timber box, called a caisson, was then filled with masonry, and sunk on the foundation thus roughly prepared. This method is now abandoned. It was peculiarly liable to danger from the scour of the stream. The name caisson is also sometimes