538 



BRIDGE. 



Practice. 



descrip- 

 tion of the 

 wooden 

 bridge at 

 Schauff- 

 hausen. 



PLATE 

 iXXXIX. 



xc. 



Figs. 1,2, 3. 



width across the soffit ; and there are two rows gf 

 frames in each arch, with a space of one foot between 

 them. The piers are of stone, and the spandrels of 

 the arches are covered with boarding, and painted 

 and sanded in imitation of stone, so that the whole 

 has the appearance of three neat stone arches ; but 

 the apparent effect of the carpentry by this means is 

 lost. 



We know of no wooden bridges in Britain so judi- 

 ciously constructed as those executed by Mr Burn. 

 Short piecesof timber maybe employed. Theprincipal 

 pieces abutting endwise, little change can take place 

 from shrinking. The principle will admit of carrying 

 an arch to a very great extent ; and by judicious ar- 

 rangement, the parts may be taken out and renewed 

 separately. 



When it is necessary to keep the roadway low, the 

 trussing is performed chiefly above its level. When 

 the bridge is narrow, the supporting framing is made 

 entirely on each side, and the roadway suspended 

 between. When it is wider than is advisable to have 

 supported entirely by outside frames, another is in- 

 troduced on each side of the carriage way, separating 

 it from the foot-paths ; or there are two carriage 

 ways, with a frame or truss between them. 



The following short description, extracted chiefly 

 from a publication by Mr Taylor of Holborn,to whose 

 judicious exertions the British artists are much indebt- 

 ed, with the annexed Plates, will convey a distinct 

 idea of the manner in which the ingenious Ulric Gru- 

 bcnma>i constructed the celebrated bridge over the 

 Rhine at Schauffhausen, in Switzerland. Fig. 1, 

 Plate LXXXIX., exhibits an elevation of one side, in- 

 cluding the roof, which was covered with shingles. 

 Fig. 2 is a cross section at AAA, shewing the up- 

 rights which are placed on the pier, the framing un- 

 der the level of the roadway, the points from whence 

 the braces proceed, the mortices for the beams which 

 support the roadway, and the interior construction of 

 the roof at these uprights. Fig. 3 is also a cross sec- 

 tion at B, shewing in what manner the aforesaid 

 roadway beams and the braces pass through the 

 other uprights, how the uprights are connected im- 

 mediately below the roof, and also how the two pie- 

 ces of which they are composed are bolted together. 

 Fig. 4 shews the form of the roof at that place. Fig. 

 5 shews the manner in which the roadway beams, and 

 those along the top of the uprights, are united. And 

 Fig. 6 explains the nature of the points at C and D, 

 by which the several pieces which compose the beam 

 are connected together lengthwise. In Plate XC. 

 Fig. 1, is a longitudinal section including the lower 

 part of the roof, and in which the situations of all 

 the uprights, beams, braces, and iron ties, are distinct- 

 ly shewn. Fig. 2 is a plan of the floor, with every 

 part of its framing ; and Fig. 3 is a similar plan of 

 the roof. 



In these Figures every part of the construction 

 is so particularly delineated, as' to render its of- 

 fice evident by inspection. The braces proceeding 

 Jrom each abutment, are continued to the beam which 

 passes along the top of the uprights, and the lowest 

 of these general braces are actually united under that 

 beam, thereby forming a continued arch between the 

 abutments, the chord line of which is 364 feet, and 



the versed sine about 30 feet. These braces are 

 kept in a straight direction by the uprights, which ' 

 are placed 17 feet 5 inches apart. If this bridge had 

 been formed in a straight line between the abutments, 

 we can see no reason why this form of construction 

 should not have supported a roadway of about 18 

 feet in breadth, as well as a slight roof ; because, in 

 that case, all the weight arising from the braces which 

 proceed from the middle pier would have been saved, 

 and the roof might have been made much simpler and 

 lighter ; but tlie general direction being 8 feet out of 

 a straight line, and being loaded with an unnecessa- 

 rily heavy roof, it was certainly advisable to make 

 use of the braces from the middle pier, and thereby 

 composing two distinct arches. 



Although the principles, and even the form of con- 

 structing this bridge, might have been drawn from 

 Gauticr's publication, or even Palladio's designs for 

 wooden bridges, yet from the account of Ulric Gru- 

 benman, being an illiterate man, there is reason to 

 think it was from his own inventive genius that the 

 whole design originated. There is not only a great 

 boldness in the principal members, but also a wonder- 

 ful attention to alltheminutix of the edifice; and from 

 even the roofing being overdone in aiming at excel- 

 lence and security, it is evident this was a first at- 

 tempt, and that there was an anxiety to avoid the 

 possibility of failure, in what he conceived, and what, 

 as far as regards him, was really a totally new project. 



We are informed that John Grubenman construc- 

 ted a bridge upon the same principles, of 2tO feet 

 span, over the Rhine, near Richenau ; also that the 

 two brothers erected one 200 feet span over the river 

 Limmat, near Baden. And that the last work of 

 Ulric was a bridge of 230 feet span at Wittingen. 

 In this last, the form of construction was varied : in- 

 stead of placing the braces diverging from each other, 

 seven beams were built close upon each other, form- 

 ing a catenarian arch between the abutments, of 

 which the rise was 25 feet. These beams were of 

 oak, in lengths of 12 or 14> feet, breaking joint in 

 the manner of masonry. They were not fastened by 

 pins, bolts, or scarfings ; but were kept together by 

 iron straps, placed five feet distant from each other, 

 and fastened by bolts and keys. The roadway inter- 

 sects them about the middle of their rise. 



Over the river Portsmouth, in North America, a 

 Mr Bludget has constructed a wooden bridge '250 

 feet span, nearly in the same form as the last men- 

 tioned of Grubenman ; that is to say, each truss or 

 arch consists of three rows of beams placed parallel 

 with, but at some distance from, each other, and each 

 beam consists of two halves, connected by dovetailed 

 keys passing through them horizontally : and similar 

 keys are also passed vertically through all the three 

 beams. This has a more elegant appearance, than 

 where the beams are laid close together; but we doubt 

 if the frame is equally firm. 



We have now given a succinct account of the dif- 

 ferent modes in which wooden bridges have been 

 constructed : from this, the practical mechanic may 

 judge of their comparative merits, and the propriety 

 of their applications in the various situations occur- 

 ring in practice. Their principles will be discussed un- 

 der the head of CARPENTRY, as connected with roof- 

 3 



