1386

THEORY AND PRACTICE OF ENGINEERING.

Book II.

which radiated to the centre, to pass, without any mortises being cut to receive them;by this means the air circulated freely round all the timbers, and dry rot was prevented:the main ribs consisted of three double rows of timbers, laid three deep, or one above theother, the whole bound together strongly with wrought-iron. Between the tops of theking-posts straining beams were introduced, which kept the heads from approaching eachother, and in addition two other timbers, placed diagonally, like St. Andrew 's cross, wereinserted in each of the divisions, strutting the king-posts more firmly and preventing thearch from springing. The abutments, against which the timber arch pressed, wereof solid masu:.rv, and carried up considerably higher than the top of the arch. Thefloor of the bridge was upon girders, laid upon shoulders formed in the sides ofthe king-posts, to which they were firmly bolted ; on the tops of the kings, and in thedirection of the transverse girders, were the tie-beams of the roof; these latter notonly served to maintain the roof securely, but also the heads of the kings in their perpen-dicular and proper position. The roof was lightly formed, and the sides of the bridgewere close boarded, so that the timbers, or the principles of their construction, could not beseen.

Bridge across the Tees., with an arch of 150 feet span, and a versed sine of 16 feet, is abold piece of construction: the arch is formed by an upper and lower circular rib, withvertical or rather radiating timbers between them; and the bracing or St. Andrew’s cross,inserted between each pair, gives at first the character of 28 hollow voussoirs: eachcircular rib is formed of two thicknesses of timber 12 inches by 6, bolted togetherwith | round bolts, and the heading joints arc broken every 10 feet in length; verticaltimbers are placed upon these ribs, 12 inches by 10, which support the longitudinal piecesthat carry the road : these are also of a similar scantling to the main ribs, and are halvedand bolted to the transverse bearers upon which the roadway is formed, which has aclear width of 13 feet. ,

The abutments at each side are of red sandstone quarried out of the bed of theriver, and laid in courses with a face tooled, and a cast seating plate separates the stonefrom the timber arch: the latter was placed by means of a stage formed on piles driven 15feet apart; and when the two ribs with their radiating timbers and diagonal braces werefixed, the wedges were drawn, and the weight of the ribs thrown upon the iron seatingat tiie abutments, and upon the head joints of the framing, which were protected by twoplates of lead, 7 pounds to the foot: after this the .piles and stage were removed : in thisbridge there was employed 4240 cubic feet of Memel timber, and the total cost was 1200/.

Swing Bridges of Iron are now so generally adopted at docks and canals that those oftimber are scarcely to be met with; they revolve easily upon a circle, and can be movedwith the greatest facility.

i r

Fig. 2293.

SWING BRIDGE OF IKON.

Swing Bridges over canals are simply framed and loaded at the turning end, where aniron circle and wheel facilitate their motion in either direction : where the bridge

SWING BRIDGE.