OF SPANDRELS AND WING WALLS.
When the arches have been completed, and the cen tres removed, the points of the piers are brought up, and are either finished at some distance above high wa ter mark, by sloping them back to the face of the span drel, in either a triangular or circular form, or they are disposed, so as to receive columns, pilasters, circular, or semi-octagonal turrets ; in all of which shapes very ex cellent works have been constructed. The two latter modes afford greatest stability to the superstructure in acting as buttresses, and on this account they agree more correctly with edifices of this kind. It is needless to observe, that in whatever way the points of the piers are terminated, it is absolutely necessary that it he with stones of good quality, firmly connected with the spandrels. The spandrels of arches have been finished in a variety of ways : in many of the old bridges they were filled up with earth, or gravel; and in small bridges the masonry should be brought up to the level of about one fourth of the rise of the arch, and then be sloped up to the top of the back of the archstones, and the remain ing space filled up with gravel or stone rubbish, (not clay). In the large French bridges, they have been filled up entirely with rubble masonry ; but this throws an unnecessary weight upon the arches. To remedy this, arches have been made sometimes quite through, and kept open, and sometimes concealed; and in West minster, and Orleans, vaults have been constructed to lighten the piers which sunk, and those adjacent to them ; but as these arches are easily deranged by any settlement of the main arch, and by that means render ed injurious rather than beneficial, another more simple and effectual way has been devised. This has been practised with perfect success in Tongucland bridge of 118 feet span, and Durkeld of 90, as well as in many other instances, and ought to be generally adopted in all bridges composed of large arches. This mode con sists of building walls longitudinally : they are founded upon the solid rubble masonry, which has already been described ; and increasing in length as they advance in height, they rest upon and abut against the backs of each row of archstones, and act as struts between them. These walls are placed from two to three feet apart front each other, and are made from 18 inches to three feet in thickness, according to their height, and the na ture of the materials of which they are composed. They are kept steady, by laying long stones occasional ly across from one wall to another. The outside span drel walls, running parallel with these, and being con nected with them by long stones, become a part of the general frame. These walls are all carried up to near the level of the top of the archstones, where they are covered with two rows of flat stones where they can be procured, or where that is difficult, the openings are arched over, (Gothic or pointed,) to lessen the lateral pressure. Sometimes the middle openings are arched, and those next the outside covered with flat stones. Small openings are made in those walls, upon the top of the rubble masonry, through which any water that may fall into, or be by any means collected in the spandrels, is conducted to one point, where it issues through a pipe inserted in the arehstones. Small doorways are also made through the walls; and by a hatchway con structed in the top pavement, a person may, at any time, descend, and examine the state of the interior parts of the spandrels. See Plate Cl.
The outside walls are usually made thicker than the interior walls: they are generally in good bridges faced with square masonry, and have a rubble backing. The facing is composed of headers and stretchers, the latter being from 15 to 18 inches in breadth, and the former from 2 feet 6 inches to 3 feet in length; the whole thick ness of square masonry and rubble, making about of its height upon an average. When these walls are very high, a wall is also built along the middle of the piers, and abutments which cross the other, and into which they are tied by bond stones, or pieces of timber laid at about every six feet in height. When these spandrels have been brought up to the level of the top of the archstones, they are dressed into the slope which it is proposed to make the roadway, which we recom mend to be 1 in 24. Here there is usually laid a cor don or facia course, and cornice, which extends along the whole of the arches, spandrels, and wing walls. This is made varying in thickness, height, and shape, according to the fancy of the engineer; but the upper course of it should be of sufficient breadth to allow for the projection, and to pass quite through under the parapet, which will, by standing upon it, keep all the work secure; the upper side of the projecting part should have a slope or weathering, to throw off the wa ter which will fall upon it, and the upright joints should be set and pointed with British cement.
When arches are so constructed, that each will stand independent of the others, and which appears the trne principle of bridge building, the abutments are merely piers, placed next the shore of the rivers ; and we have accordingly hitherto considered them as such. The French engineers, from the flatness of their arches, and the great weight of the voussoirs and masonry with which they have loaded them, have considered that the greatest push should be against the abutments, and have made them much thicker than the piers. In Neuilly, when the piers were 13 feet, Perronet has made the abutments 30 feet 3 inches. This appears to be a mis taken mode of proceeding, because the abutments, be sides their own thickness, have generally wing walls behind them, upon which, by making the back of the abutments in a circular form, they will abut and be pre vented from moving backwards. The wing walls have sometimes their foundations laid at the same depth as the abutments, and are similarly secured by piles and platlorms. If the ground is firm, they are founded by steps rising up as they retreat. This, when practicable, saves much masonry. Their thickness is made from about -1 to s part of their height upon an average; but as the space between is filled up with earth, they should be formed in curved lines horizontally and vertically, and also have a batter of from to.. of their height, and this should be provided for in setting out their founda tions. When the wing walls are very long and high, there should be a cross wall built, reaching between them, into which they should be tied; and sometimes vaults may be constructed between them, and convert ed to useful purposes.
At the terminations of the wing walls, there should be newells or pilasters ; and these are made round, square, or octagonal, as is most suitable to the general design. The masonry of the wing walls is similar to what has been described for the outside walls of the spandrels.