1376
THEORY AND PRACTICE OF ENGINEERING.
Book 11.
Bridge at Berne, over the Kandel , was constructed by Joseph Ritter, in the year1764 ; the distance between the abutments is 166 feet; their height has allowed
BRIDGE AT BF.RNE.
the struts to be placed in a more judicious position, and to be inclined almost parallel
with each other; there are five on eachthe longest are about 75 feet. The lon-gitudinal timbers which carry the floorare highest in the middle, where they arestrengthened with an additional thickness,bolted to the upper one from below : intothe longitudinal timbers are framed trans-verse girders at every 13 feet, which areagain diagonally braced; over them areplaced the joists and timber platform.
Bridge over the Saone at Lyons, designed byM. Gauthey for an opening of 164 feet, wasupon the foregoing principles : it was in-tended that this bridge should have had three
side, of fir timbers about 11 inches square;
Fig. 2261.
Fig. 2202.
sets of longitudinal timbers, one passing through the middle, which would have permittedof two independent thoroughfares. Bridges constructed upon a different plan have now
succeeded to the celebrated examples just described : the art of construction had at onetime attained, it was thought, the greatest possible perfection, and Schaffhausen, "Wittingen ,and others were named as works of carpentry not to be surpassed ; but, renowned asthey were, the binding of timbers into arched forms has been found not only stronger, butmore economical and durable.
In the (i British Carpenter ,” by Price, published in 1765, a method is described forforming curved ribs to support the roadway of a bridge. He proposes that theyshould rise as much as one-sixth of the intended span, and be divided into convenientlengths, suitable to the timber to be employed t for an arch or opening of 36 feet hemakes the ribs of oak in five lengths, and 3 inches in thickness, each rib to consist oftwo thicknesses, and 12 inches deep, the joints alternating in the middle of eachlength, and the two thicknesses keyed together with oak keys, and he states that twoperfect ribs, so formed, would be strong enough to support the roadway.
Chazey on the Ain has a bridge of four arches, each 63 feet 6 inches span, supported bystone piers and abutments, and this is said to be the first erected in France uponthe new principle. The platform or floor is supported by several ribs, each of twothicknesses of timber, united by forming the sides in contact, like the teeth of a saw, oneentering the other; these ribs are the arc of a circle, and are so secured by iron boltsthat they cannot easily spring: pendant binding-pieces, radiating from the centre of thecircle, of which the arc is a portion, are attached to the longitudinal timbers whichcross the stream, and the spandrills of the arches or spaces between the curved and longitu-dinal timbers are each filled in with two additional struts bearing upon the abutments.