1374
THEORY AND PRACTICE OF ENGINEERING.
Book II.
girders or beams which supported the floor of the bridge were suspended to the per-pendicular binding-pieces, at a point immediately under the lowest longitudinal beams;the top longitudinal beams were composed of five thicknesses throughout the five middledivisions of each set of framing, and four at the two outer. Every precaution was takento prevent the framing from yielding in the centre or middle of the arches ; the whole ofthe timbers were judiciously applied, and perfectly succeeded; the platform or floorwas made with a double thickness of planks, the lowest being nailed to joists, and theothers crossing them at right angles. This bridge was not constructed in a straight line;
Fig. 2254.
BRIDGE OP SCHACFFHACSEN.
the middle pier being placed about 13 feet up the stream, which it was supposed would givethe structure greater strength and stability, but it trembled considerably, even from theweight of one passenger passing over it. It was only once repaired during the 42 yearsthat it stood, though the oak timbers which rested immediately on the abutments andpier, from not being thoroughly seasoned and sufficient air allowed to circulate aroundthem, wont early to decay ; to replace these defective timbers, screw-jacks were employedupon scaffolding supported on piles, to raise this ponderous piece of carpentry in a mass,when others were introduced.
Bridge over the Limmat , near the Abbey of Wittingen , was also erected by the samecarpenter, assisted by his brother John Grubcnmann, and burnt soon after that ofSchaffhausen . It consisted of one opening, 390 feet span, with a rise of 43 feet, and wasa more solid and even superior piece of carpentry, all the timbers being well proportioned
Fig. 2255.
to the great opening. The floor was sustained by two longitudinal beams on each side,put together one over the other en cremaiUere , and supported at each abutment byinclined struts, “the lowest of which were doubled: level with the plate of the roof was asecond longitudinal timber, which formed the principal support to the whole structure;this consisted of a single thickness at the parts nearest the abutments, but increasedin number as it approached the middle, where it was composed of four thicknesses. Strutsof various inclinations were placed between these longitudinal timbers, and the whole