OF CENTRES.
Having carried the picrs and abutments up to the height at which the arches are to spring, the next object is to set up timber frames, usually known by the name of centres. To construct and erect these in a judicious manner, is one of the most masterly operations in bridge building ; but as this forms also a principal feature in CARPENTRY, we must refer the reader to that head, under which he shall find the subject fully discussed ; and at present content ourselves with giving drawings, and very general descriptions of centres, which have been used in constructing some of the principal bridges in Britain and France. See Plates XCI X. and C.
In a centre, the principal objects to be kept in view, are to construct and fix such a frame as shall support the weight of the arch-stones, through all the progress of the work, from the springing of the arch, to the fixing of the key-stone, without changing its shape, and to admit of its being removed with safety and ease. From inspecting the Plates, it will he seen that the French engineers brought the timbers of each frame very near together, and kept them very narrow at the footing upon which they rested ; whereas, in England, it is the practiCe to place the timbers more apart, and extend the footing. The comparative merits of these prin ciples will be discussed when treating of CARPENTRY ; but we are warranted in observing, that the English cen tres have succeeded best in practice. At Neuilly, previous to the centre being eased, Perronet states, that the arch (of 120 French feet) had sunk IS inches, and it afterwards sunk 101 inches more, making in all 231 inches. At Mautz, (also 120 French feet span,) before the centre was eased, the sinking was 12 inches, and it afterwards increased to 20,!,-. In Britain, no such imperfections have ever been known. In the largest arches constructed with cut stone, under the direction of experienced persons, the sinking has been scarcely discernible. In Dunkeld bridge of 90, Tongueland of 118, and Aberdeen of ISO feet span, the sinking has never exceeded three inches, consequently no derange ment of general form or of joints took place. In rivers where there is no navigation, or where there is no risk from the floating down of ice or timber, the centres are framed with horizontal tie beams, and supported in sundry places by frames or piles fixed in the bed of the river ; and, in old bridges, corbels of stone were fre quently placed in each abutment, to receive the beams of the centres. But when any of the before mentioned inconveniences are to be guarded against, the frames are trussed so as to leave a large proportion of the space in the middle unobstructed. In this case,
more art is required in framing and placing the centres, so as to afford effectual support, and admit of a perfect mode of lowering and removing them. The offsets of the stone work and platforms, afford the most substantial abutments for the supports; but unless the bed of the river is very soft, great advantage may be derived from also placing frames or piles at a small distance from the body of the pier ; and as in large arches this operation cannot incommode the navigation, it ought to be made use of. The modes of lowering or casing the centres practised in Britain, appears to be much more simple and safe than those described by Perronet, as practised in France. In France, the bridgings across the frames, upon which the archstones were set, were cut away, either beginning at the springing, and proceeding regularly upwards to the key, or otherwise by first cut ting out each other bridging, over the whole centre, and next, each other that was left, and so alternately, until the whole were removed. In Britain, the centres arc set upon beams ; between the centres and the beams, wedges are introduced, sometimes in separate pairs across, under each frame ; sometimes the wedges arc formed or fixed upon one piece of wood, which reaches across the whole width of the soffit, passing between all the centre frames, and the supporting frames or beams ; and, lastly, the wedges arc formed or fixed upon a piece of wood, which is placed longitudinally under the foot of each centre frame, and resting also on the supporting frame. When the centre is to be eased, these wedges arc driven along each other, or the pieces of wood on which the wedges are formed or fixed, are driven hack so that the wedges are moved along each of their inclined planes into larger spaces than they had before occupied. In all these cases, the whole centre is made to descend very gently, and may be suffered to rest at any part of the operation. The mode of striking the wedges is varied according to circumstances. It is, in the smaller arches, usually performed by men with mauls striking each pair of wedges. In the larger arches, such as Westminister and Blackfriars, it was performed with a beam mounted as a battering ram. The frames are placed from four, six, to seven feet apart, from middle to middle, and are secured with cross ties and braces.
When there are three arches, two centres will be required ; and when there are five arches, three centres will be needed. See page 542.