Chap. XXIIT.
1381
carried the road were varied in their curve ; the lowest, having the greatest, was formedof three courses of timber, from 13 to 14 inches in thickness, and about 45 feet in length:these timbers were bent by screws made for the purpose, and when curved sufficientlywere bolted together.
Fig. 2277.
FREYSt.NGEN BRIDGE.
The upper arch or rib was formed of two courses of timber about 15 inches square each ;sixty-eight piles, from 30 to 40 feet in length, and 15 inches square, were driven by a ram,the force of which was equal to 1450 pounds, 17 or 18 feet into the ground, to form eachof the abutments. The middle pier consisted of nine vertical and two inclined piles, about17 inches square, and 45 feet in length; on the heads of these was laid a horizontalcapping, which sustained the vertical posts, against which the ends of the ribs abutted ;the posts were lined with strong oak planking, and the spandrills filled in with beton.
The exterior was cased with planks, and by jointing made to resemble a stone bridge ;to preserve the timbers every mortise and tenon was well saturated with hot oil previousto uniting, and small channels for the water were cut in the curved ribs and inclined strutsand braces; pitch and tar were applied in abundance to all the principal timbers. Afterthe bridge was completed, one arch settled about 3.^, and the other nearly 12 inches, whichprobably arose from the difference of workmanship ; this bridge was destroyed about1809, since which it has been re-constructed on the same plan.
Bridge of Bamberg , over the Rcgnitz , constructed by the same engineer, in the year 1809,has one arch, the span of which is 206 feet, and the versed sine 16 feet 6 inches, with awidth of roadway of 32 feet: the radius of curvature is 422 feet. The ribs and joistsare all of fir timber, and the cross ties and plates of oak: there are three sets ofmain ribs, one placed in the middle of the width, and the other on each exterior side ; thatin the centre has three ribs side by side, the middle having at the abutments five beamsin depth, and at the crown three; the two other or outer main ribs have but three courses
BRIDGE OF BAMBERG.
of timber throughout their whole extent: the dimensions of the timbers which form thesecourses vary from 13^ to 15^ inches square; as in the bridge of Freysingen, cross ties areintroduced between the main ribs, and the roadway is similarly constructed.
This celebrated engineer and bridge-builder erected in Bavaria many others, uponthe same principle to those already described : that at Sharding has a width of roadwav of25 feet, the span of its arches 194 feet, with a rise of 19 feet; another at Augsbourg hasits span 114 feet, and the rise of its arch 10^ inches, the radius of curvature being 158 feet;that at Ettringen, over the "Wertach , has a .span of 139 feet, with a rise of 8 feet, and theradius of curvature 305 feet; that at Irsingen, over the Wertach , 126 feet span, with a riseof 7 feet, and a radius of 285 feet; that of Oettingen , over the Inn, 103 feet span. 6 feet9 inches rise, and a radius of curvature of about 200 feet; that at Yilshoven, 179 feet span,and 11 feet rise, the radius of curvature being 378 feet; that at Altenmarkt , on the Alz,140 feet span, 13 feet rise, and 203 feet radius of curvature.
M. Wiebeking has usually given 25 feet for the ordinary width of his bridges, which issometimes increased to 32, and the rise of roadway is 1 in 24, which is by no meansinconvenient; he found that timber bridges would settle, when constructed upon hisprinciple, about 1 foot in 72, so that, to have a rise of 1 in 24 when finished, it must beframed for a rise of 1 in 18. The proportions he gives for the rise of different spans -are valuable, as they are entirely the result of his practical observations, unmixed with anytheory ; he states generally, that a tenth rise is the best proportion, but that for convenienceit is better to keep them lower : from 100 to 155 feet span, he makes the rise ^; for 200feet, -j'g ; 300 feet, ^ ; 400 feet, T * 3 ; 500 -j* 3 ; and 600 feet span, T l 5 : the greater the span, thegreater must be the rise.
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