534 



BRIDGE. 



Practice. 



OF SPANDRELS AND WING WALLS. 



Spandrels When the arches have been completed, and the 

 and wing centres removed, the points of the piers are brought 

 np, and are either finished at some distance above high 

 water mark, by sloping them back to the face of the 

 spandrel, 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 excellent works have been constructed. 

 The two latter modes afford greatest stability to the 

 superstructure in acting as buttresses, and on this ac- 

 count 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 abso- 

 lutely necessary that it be with stones of good quality, 

 firmly connected with the spandrels. 1 he 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 remaining space 

 filled up with gravel or stone rubbish, (not clay). 

 In the large French bridges, they have been filled up 

 entirely with rnbble masonry ; but this throws an un- 

 necessary 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 ren- 

 dered injurious rather than beneficial, another more 

 simple and effectual way has been devised. This has 

 been practised >with perfect success in Tongueland 

 . bridge of >18 feetpan, and Dunkeld of 90, as well as 

 in many other instances, -and ought to be generally 

 adopted in all bridges composed of large arches. 

 This mode consists 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 from each other, 

 and are made from 18 inches to three feet in thick- 

 ness, according to their height, and the nature of the 

 materials of which they are composed. They are kept 

 steady, by laying long stones occasionally across from 

 one wall to another. The outside spandrel walls, 

 running parallel with these, and being connected 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 

 (tones. 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 spandrel*, it conducted to one point, where it 



issues through a pipe inserted in the archstones ? Pracii 

 Small doorways are also made through the walls ; and V 

 by a hatchway constructed in the top pavement, a 

 person may, at anytime, descend, and examine the 

 state of the interior parts of the spandrels. See Plate p LATt 

 CI. 



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 back- 

 ing. The facing is composed of headers and stretchers, 

 the latter being from 1.5 to 18 inches in'breadth, and 

 the former from 2 feet 6 inches to 3 feet in length ; 

 the whole thickness 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 arch-stones, 

 they are dressed into the slope which it is proposed 

 to make the roadway, which we recommend to be 1 

 in 24. Here there is usually laid a cordon 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 para- 

 pe', 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 

 water 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 true principle of bridge building, the abutment! 

 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 roussoirs 

 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 mistaken mode of pro- 

 ceeding, because the abutments, besides their own 

 thickness, have generally wing walls behi'id them, 

 upon which, by making the back of the abutments in 

 a,circular form, they will abut and be prevented from 

 moving backwards. The wing walls have sometimes 

 their foundations laid at the same depth as the abut- 

 ments, and are similarly secured by piles and plat- 

 forms. If the ground is firm, they are founded by 

 steps rising upas-they retreat. This, when practicable, 

 saves much masonry. Their thickness is made from 

 about -J to 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 -J- of 

 their height, and this should be provided for in set- 

 ting out their foundations. When the wing walls are 

 very long and high, there should be a cross wall built, 



