Fig. 125-Represents a part of a rib of Waterloo Bridge over the Thames. a, a, b, three heavy beams, forming the striking plates, which with the shores h, h, form the, framed support for the struts of the centre. GC, struts abutting against the blocks g, g placed under the joints of the back pieces, J. d, d, bridle or radial pieces in pairs which are confined at top and bottom between the hori zontal ties n, n of the ribs, also in pairs. e, e, cast-iron sockets. m, m, bolsters of the centre resting on the back pieces f. Fig. 126-Represents a frame of the struts which support the back pieces. This combination, and all of a like character, require that the arch should not be constructed more rapidly on one side of the centre than on the other, as any inequality of strain on the two halves of the centre would have a tendency to change the shape of the frame, thrusting it in the direction of the greater strain. 577. Style of Architecture. The design and construction of a bridge should be governed by the same general princi ples as any other architectural composition. As the object of a bridge is to bear heavy loads, and to withstand the effects of one of the most destructive agents with which the engineer has to contend, the general character of its architecture should be that of strength. It should not only be secure, but to the apprehension appear so. It should be equally removed from Egyptian massiveness and Corinthian lightness; while, at the same time, it should conform to the features of the surrounding locality, being more ornate and carefully wrought in its minor details in a city, and near buildings of a sumptuous style, than in more obscure quarters; and assuming every shade of conformity, from that which would be in keeping with the humblest hamlet and tamest landscape to the boldest features presented by Nature and Art. plicity and strength are its natural characteristics; all ornament of detail being rejected which is not of obvious utility, and suitable to the point of view from which it must be seen; as well as all attempts at boldness of general design which might give rise to a feeling of insecurity, however unfounded in reality. The heads of the bridge, the cornice, and the parapet should generally present an unbroken outline; this, however, may be departed from in bridges where it is desirable to place recesses for seats, so as not to interfere with the footpaths; in which case a plain buttress may be built above each starling to support the recess and its seats, the utility of which will be obvious, while it will give an appearance of additional strength when the height of the parapet above the starlings is at all considerable. Sim 578. Construction. The methods of laying the foundations of structures of stone, &c., described under the article of Masonry, are alike applicable to all structures which come under this denomination. 579. Various expedients have been tried to secure the bed of the natural water-way around and between the piers; among the most simple and efficacious of which is that of covering the surface to be protected by a bed of stone broken into fragments of sufficient bulk to resist the velocity of the current in the bays, if the soil is of an ordinary clayey mud; but, if it be of loose sand or gravel, the surface should be first covered by a bed of tenacious clay before the stone be thrown in. The voids between the blocks of stone, in time, become filled with a deposit of mud, which, acting as a cement, gives to the mass a character of great durability. 580. The foundation courses of the piers should be formed of heavy blocks of cut stone bonded in the most careful manner, and carried up in offsets. The faces of the piers should be of cut stone well bonded. They may be built either vertically, or with a slight batter. Their thickness at the impost should be greater than what would be deemed sufficient under ordinary circumstances; as they are exposed to the destructive action of the current, and of shocks from heavy floating bodies; and from the loss of weight of the parts immersed, owing to the buoyant effort of the water, their resistance is decreased. The most successful bridge architects have adopted the practice of making the thickness of the piers at the impost between one sixth and one eighth of the span of the arch. The thickness of the piers of the bridge of Neuilly, near Paris, built by the celebrated Perronet, whose works form an epoch in modern bridge architecture, is only one ninth of the span, its arches also being remarkable for the boldness of their curve. 581. The usual practice is to give to all the piers the same proportional thickness. It has, however, been recommended by some engineers to give sufficient thickness to a few of the piers to resist the horizontal thrust of the arches on either side of them, and thus secure a part of the structure from ruin, should an accident happen to any of the other piers. These masses, to which the name abutment piers has been applied, would be objectionable from the diminution of the natural water-way that would be caused by their bulk, and from the additional cost for their construction, besides impairing the architectural effect of the structure. They present the advantage, in addition to their main object, of permitting the bridge to be constructed by sections, and thus procure an economy in the cost of the wooden centres for the arches. 582. The projection of the starlings beyond the heads of the bridge, their form, and the height given to them above the springing lines, will depend upon local circumstances. As the main objects of the starlings are to form a fender or guard to secure the masonry of the spandrels, &c., from being damaged by floating bodies, and to serve as a cut-water to turn the current aside, and prevent the formation of whirls, and their action on the bed around the foundations, the form given to them should subserve both these purposes. Of the different forms of horizontal section which have been given to starlings (Figs. 127, 128, 129, 130), the semi-ellipse, from experiments carefully made, with these ends in view, appears best to satisfy both objects. The up and down stream starlings, in tidal rivers not subject to freshets and ice, usually receive the same projections, which, when their plan is a semi-ellipse, must be somewhat greater than the semi-width of the pier. Their general vertical outline is columnar, being either straight or swelled (Figs. 131, 132, 133, 134). They should be built as high as the ordi E A Fig. 131-Represents in elevation starlings A, their hoods B, the voussoirs C, the spandrels D, and the combination of their courses and joints with each other in an oval arch of three E, parapet; F, cornice. centres. nary highest water-level. They are finished at top with a coping stone to preserve the masonry from the action of rain, &c. this stone, termed the hood, may receive a conical, a spheroidal, or any other shape which will subserve the object in view, and produce a pleasing architectural effect, in keeping with the locality. E Fig. 132-Represents in elevation the combinations of the same elements as in Fig. 131 for a flat segmental arch. E Fig. 133-Represents in elevation the combinations of the same elements as in Fig. 132, from the bridge of Neuilly, and oval of eleven centres. om, curve of intrados. on, arc of circle traced on the head of the bridge. Fig. 134-Represents a cross section and elevation through the crown of Fig. 132, showing the arrangement also of the roadway, footpaths, parapet, and cornice. |