Girders, there are three types or classes of bridge: the girder, the arched, and the suspended. Girders may be of various materials; wrought iron, cast iron, and wood are chiefly used. Professor Fleeming Jenkin describes with apt brevity the essential difference between the three classes of bridge. “In all forms of the suspension bridge the supporting structure isextendedby the stress due to the load; in all forms of the arch the supporting structure (i.e. the ring of voussoirs) iscompressedby the stress due to the load; and in all forms of the beam or girder the material is partly extended and partly compressed by the flexure which it undergoes as it bends under the load. Thus when a beam of wood carrying a load bends, the upper side of the beam is thereby shortened and the fibres compressed, while the lower side of the beam is lengthened and the fibres extended.†So, too, in a girder of metal. In some bridges, as in the High Level Bridge at Newcastle, the girder principle is united to bowstring arches of metal, but a true girder is less expensive and lighter,80.
Girders, there are three types or classes of bridge: the girder, the arched, and the suspended. Girders may be of various materials; wrought iron, cast iron, and wood are chiefly used. Professor Fleeming Jenkin describes with apt brevity the essential difference between the three classes of bridge. “In all forms of the suspension bridge the supporting structure isextendedby the stress due to the load; in all forms of the arch the supporting structure (i.e. the ring of voussoirs) iscompressedby the stress due to the load; and in all forms of the beam or girder the material is partly extended and partly compressed by the flexure which it undergoes as it bends under the load. Thus when a beam of wood carrying a load bends, the upper side of the beam is thereby shortened and the fibres compressed, while the lower side of the beam is lengthened and the fibres extended.†So, too, in a girder of metal. In some bridges, as in the High Level Bridge at Newcastle, the girder principle is united to bowstring arches of metal, but a true girder is less expensive and lighter,80.
Hosking has many good remarks on the subject of cramps and joggles. He says (p.208): “It is very desirable that all the archstones of a large and flat arch should be dowel-joggled in the beds; but as the usual dowel-jogglecannot be introduced with the key-course, plugs of proportionate size must be used instead, and the stones may, besides, be cramped together. In arches of small size, or in large ones of quick sweep, joggling may not be so desirable as in those of large size and flat sweep; though it is to be understood that in any case both joggles and cramps should be considered as surplusage, and as precautions merely, to counteract the effect of any imperfections in the work from want of fulness in any of the stones in an arch, or otherwise. In building London Bridge iron bars were let into the back ends or tails of the archstones, and run with lead as cramps or transverse ties in several courses, and they do not appear to have produced any injurious effect, though it may be questioned how far they are of any use. They ought not to be of any use.†Viollet-le-Duc went further than this; he regarded iron cramps in a stone bridge as likely to be injurious.
Hosking has many good remarks on the subject of cramps and joggles. He says (p.208): “It is very desirable that all the archstones of a large and flat arch should be dowel-joggled in the beds; but as the usual dowel-jogglecannot be introduced with the key-course, plugs of proportionate size must be used instead, and the stones may, besides, be cramped together. In arches of small size, or in large ones of quick sweep, joggling may not be so desirable as in those of large size and flat sweep; though it is to be understood that in any case both joggles and cramps should be considered as surplusage, and as precautions merely, to counteract the effect of any imperfections in the work from want of fulness in any of the stones in an arch, or otherwise. In building London Bridge iron bars were let into the back ends or tails of the archstones, and run with lead as cramps or transverse ties in several courses, and they do not appear to have produced any injurious effect, though it may be questioned how far they are of any use. They ought not to be of any use.†Viollet-le-Duc went further than this; he regarded iron cramps in a stone bridge as likely to be injurious.
Kircher, Athanasius, German traveller and philosopher,b.1602—d.1680, his book on China, translated into French by Dalquié,314,345, andfootnote.
Kircher, Athanasius, German traveller and philosopher,b.1602—d.1680, his book on China, translated into French by Dalquié,314,345, andfootnote.
Lacer, Caius Julius, Roman architect, and builder of Trajan’s Bridge over the Tagus at Alcántara,121,184,190,344. He was buried on the left bank quite close to his bridge,184, a romantic circumstance, like the burial of Bénézet and Colechurch in their bridge chapels.
Lacer, Caius Julius, Roman architect, and builder of Trajan’s Bridge over the Tagus at Alcántara,121,184,190,344. He was buried on the left bank quite close to his bridge,184, a romantic circumstance, like the burial of Bénézet and Colechurch in their bridge chapels.
Law, Modern, in Great Britain law prescribes minimum dimensions for the over and under bridges of railways; but it takes no notice at all of the military considerations which can never be wisely disconnected from the circulation of traffic along roads and over bridges. Anover bridgeis one in which a road goes over a railway; anunder bridgeis one in which a road goes under a railway. Both are exceedingly vulnerable, yet the law centres all its attention on details that concern their size, not on details that concern their protection from violence.Over Bridges.—Width: turnpike road, 35 feet; other public carriage road, 25 feet; private road, 12 feet. Span over two lines (narrow gauge), generally about 26 feet; head room, 14 feet 6 inches above outer rail.Under Bridges.—Spans: turnpike road, 35 feet; other public road, 25 feet; private road, 12 feet. Head room: turnpike road, 12 feet at springing of arch, and 16 feet throughout a breadth of 12 feet in the middle; for public road, 12 feet, 15 feet, and 10 feet in the same places; private road, 14 feet for 9 feet in the middle; for exceptions the Acts must be studied.
Law, Modern, in Great Britain law prescribes minimum dimensions for the over and under bridges of railways; but it takes no notice at all of the military considerations which can never be wisely disconnected from the circulation of traffic along roads and over bridges. Anover bridgeis one in which a road goes over a railway; anunder bridgeis one in which a road goes under a railway. Both are exceedingly vulnerable, yet the law centres all its attention on details that concern their size, not on details that concern their protection from violence.Over Bridges.—Width: turnpike road, 35 feet; other public carriage road, 25 feet; private road, 12 feet. Span over two lines (narrow gauge), generally about 26 feet; head room, 14 feet 6 inches above outer rail.Under Bridges.—Spans: turnpike road, 35 feet; other public road, 25 feet; private road, 12 feet. Head room: turnpike road, 12 feet at springing of arch, and 16 feet throughout a breadth of 12 feet in the middle; for public road, 12 feet, 15 feet, and 10 feet in the same places; private road, 14 feet for 9 feet in the middle; for exceptions the Acts must be studied.