1356
THEORY AND PRACTICE OF ENGINEERING.
Book II.
The head of each pile should be well bolted and secured to the timbers with which itis capped, or which holds it to the others in the same row : the bridging-piece, or bearer,is laid transversely upon the capping, as fig. 2196., and should be strapped securely down,and where other perpendicular posts rest upon them they should bear upon a plate, con-tinued through the entire length of the pier : mortises and tenons should be avoided asmuch as possible in the framing of all wooden bridges, and where any junction of thetimbers is to be made, it should be formed by iron bolts or straps.
When the river has a considerable depth, a double row of piles is requisite, for it wouldbe hazardous to depend upon a single one : the lower range is driven about 3 feet apart,or from centre to centre, and a capping piece, extending the whole breadth of the bridge, islaid upon the heads of each row ; an intertie crosses these, and on it is placed a thirdtimber, extending the breadth of the bridge, into which is framed the post, which carriesthe platform; these timbers are all well framed and bolted together. Timber 12 inchessquare has sufficient strength to bear any weight to which an ordinary wooden bridge issubject, though double rows of piles are often used: where many rows are requisite, theymust be protected by sterlings so contrived as to prevent ice or other floating bodies frominjuring them, and the best method of effecting this isto drive two rows of piles tending to a point, in frontof the pier to be protected, and plank them so asto resemble a wedge; upon the top of this, whichis level with the surface of the stream, is laid aninclined piece of timber against the pile, which maybe strutted on to the heads of the piles forming thewedge, and, by putting an iron edge to resist the ice,be made very efficient. The posts composing theupper part of the piers are always terminated witha capping, and if their height be considerable theyshould be supported towards the middle by oneor several courses of horizontal binding-pieces, theeffect of which is to support the posts in a parallel position: to prevent their goingout, there must be inclined or discharging pieces ; one of the best examples of their
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Fig. 2197. FLAN OF T1UULRS.