1354

THEORY AND PRACTICE OF ENGINEERING

Book II.

cross is constructed, _

upon which the timbersthat carry the road are ^

laid. Another form forbridges of considerablespan is that in which a

bkuyerb’s system.

I'ig. 2191.

BRUYERE'S SYSTEM.

Fig. 2190.

series of polygonal timbers cross over the junctions of the ends of each other, and arethen bound together by the pendant or inclined binding-pieces.

In the system adopted by Bruyere the pendant binding-pieces are placed parallel withthe sides of a square, the diagonal of the square, in bridges of small span, being a per-pendicular let fall from the centre of the bay, (fig. 2190.) When the distance between thepiers is increased,

the timbers that sup- - 1 -

port the platformhave an hexagonalform, as in fig. 2191.,the struts whichspring from theheads of the piersbeing placed paral-lel with the sidesof the square: theseveral thrusts andpressures are all ad-mirably provided forin these designs.

When the streamis tranquil, and notsubject to floods,piers may be intro-duced, so as not tointerfere with thenavigation; andtheir number maybe lessened, where

long pieces of fir timber can be obtained; where oak is used the pieces should be as shortand of as small a scantling as possible, to avoid expense : but whenever piers of masonry canbe adopted, they should bepreferred, as the ice inits descent cannot injurethem, if properly con-structed and protected.

The system of poly-gonal timbers offers con-siderable facility for theemployment of shortlengths, and is adoptedfor small bridges, whereeconomy is an object; buta number of joints in-creases the liability ofmovement in the parts,and it is necessary to in-

resa fri*a

SQ CO

x --Q2 oa

Fig. 2192.

SYSTEM OF POLYGONAL TIMBERS.

Fig. 2193.

ANOTHER SYSTEM WITH POLYGONAL TIMBERS.