[83] When Mr. Brunel for architectural effect employed Gothic or pointed arches, he occasionally made the main part of the arch of a form different from the curve visible on the face, but he more frequently made it of the same pointed form throughout. In this case he did not obtain equilibrium by loading the crown, but he kept the line of pressure sufficiently within the thickness of the arch by strengthening the haunches.
[84] Brees’ Railway Practice, pl. 42.
[85] There is an illustration of this bridge in Bourne’s History and Description of the Great Western Railway.
[86] Joggles are small pieces of hard wood or cast-iron of rectangular cross section, placed between two beams, and fitted carefully into notches cut across them. The beams are then bolted firmly together. The object is to stop the slipping which would occur between the two surfaces if the beams were merely laid one on the other and loaded, and so to make two pieces of equal size act as one of double the depth, and therefore of four times the strength of a single piece. In 1841 Mr. Brunel made experiments to satisfy himself of the strength gained by this method; and he afterwards perfected the arrangement by ensuring an exact fit in the notches by tightening up the joggles with wrought-iron wedges.
[87] A drawing of this bridge is appended to the Report of the Commission on the Application of Iron to Railway Structures, 1849.
[88] Mr. Brunel, by taking care in his timber structures to distribute the load uniformly, was frequently able to dispense with costly foundations. On one occasion a timber viaduct 30 feet high was placed upon a broad platform resting on an old embankment upwards of 50 feet in height. A similar arrangement was adopted by him in some cases on slips in embankments which it would have been uncertain and expensive to make up with earthwork.
[89] For a description of this viaduct, see Proceedings Inst. C. E., vol. xiv. for 1854-5, p. 492.
[90] Mr. Brunel about this time introduced a great improvement in the manufacture of wrought-iron bolts for bridges, when these, as is usually the case are screwed at the ends. A screw cannot be made on a bolt without the metal being cut into to the extent of the depth of the thread, and the strength thereby considerably reduced. The section, taken at the bottom of the thread, is much smaller than that of the bolt; and, moreover, owing to the abrupt change at the commencement of the thread, the strength is not so great as that due to the reduced sectional area. The improvement consisted in swelling out the iron of the end of the bolt where the thread was to be made, so that the diameter, at the deepest part of the thread, should be fully equal to that of the bolt. The saving of metal by this improvement, especially in the case of long bolts, is very considerable.
[91] An arrangement was introduced by Mr. Brunel to prevent any disturbance in the permanent way by a settlement of the embankments at the ends of the viaducts. There were no large abutments and wing walls, but the end of the viaduct was formed with a queen truss in the parapet, which rested on a platform on the top of the embankment slope. In any slight settlement of the earth, the end of the viaduct sunk with it, and the permanent way was not disturbed. Wedges were provided to raise the ends of the trusses and readjust their level. This was an important provision, as it applied to about 60 cases on the Cornwall Railway.
[92] The simple arrangement of the timber-work was specially arranged with a view to giving facility for replacing portions of it, should they decay.