Chap. XXIII.
TIMBER BRIDGES.
1367
Fig. 2223.
PALLADIO S BRIDGE.
straining-pieces, and other portions of the trusses on each side are out of timber 12 incliesby 9 ; these trusses form the parapets, and are secured by iron straps and bolts at theextremities; they are capable of supporting a considerable weight without any sensibledeflection; their height is about 16 feet, and the principle -seems to have been that ofa series of triangles, which are not liable to any change of form. The abutmentsare of stone, and theheight. of the para-pets or trusses are
sj)an.
Fig. 2223. is an-other of Palladio’sbridges, supportedlike the precedingby a truss on eachside; its length isdivided into eightbays by transversegirders, suspended
by stirrup-irons and bolts from the posts framed into the head and sill of the truss ; theseposts are braced strongly, but their strength is not equal to those of fig. 2222,; the centrebays are liable to drop, as the bearing is too great: in this construction the first baysfrom the abutments should have four depths of sill, the second three, and the third two,leaving the two centre bays with only one ; thus the ends of the bridge would appearto rest on corbels gathered over from the abutments, or these additional timbers mightbe laid side by side; this would require the transverse girders to vary in length, that theymight comprise and tie them together. The height of the trusses is about one-tenth of thespan.
Fig. 2223. is a portion of a polygon of five sides, with four king-posts and braces,like a St. Andrew ’s cross ; the outer heads are doubled, and struts arc added to throw theweight of the first bay on the abutments : the transverse girders are carried by the king-posts, to which strong stirrup-irons and bolts are attached : the side trusses of this design,which form the parapets of the bridge, are about one-ninth of the span in height.
The parapets in fig. 2224. may be considered as consisting of eleven voussoirs, eachformed of posts, strongly braced by diagonal timbers like a St. Andrew ’s cross : thetransverse girders are held up by iron straps, as in the other examples, and the heightof the framing of the parapets is not more than one-twelfth of the span.
Of the four designs left us by Palladio the first combines the best arrangement of thetimbers with the greatest solidity : in the last there is an appearance of great strength, butwhen we take into accountthe shrinking to which thetimber is liable, it is evidentthat the voussoirs wouldbe subject to great changefrom the compression theymust undergo when heavyloads passed over : motiondestroys in the course oftime the tightness firstgiven to the framing, andsubsequently the nice ba-lance of such a timber construction : that design braced like a St. Andrew’s cross, andwhich partakes of the arc of a circle, would not so readily alter its form, and wouldpreserve its firmness and solidity for a length of time.
Bridges of Palladio’s construction could not be adopted where a greater width ofcarriage-way than 14 or 15 feet was required ; the transverse girders would requireupholding by additional longitudinal trusses, if the -width were increased.
Bridge at Bassano , at the foot of the Alps, over the Brentar, was also erected byPalladio : the river is 180 feet in width, and there were five bays of equal widths.
Palladio gives the following directions for the construction of the bridge fig. 2241. :
“ The banks must be first fortified with pilasters; then one of the beams that forms thebreadth of the bridge must be placed at some distance from them, and the beams that formthe sides be disposed upon it, which, with one of their heads, must lie \ipon the bank, andbe fastened to it; upon these, directly plumb with the beams for the breadth, the colonelliare fixed, fastened with cramps of iron, and supported by braces secured to the heads ofthe bridge; the other beams are then placed, leaving as much space between each as thereis between the first and the bank : in the middle of the breadth of the river there must be
4 s 4
Fig. 2224.
PALLADIO’S BBIDGB.