Chap. XXIII.
TIMBER BRIDGES.
1357
Fig.2199.
Bays and Arches . —When the opening of a bay isnot more than 10 or 12 feet, a floor is formed ongirders, bearing on the cappings with which the piersare crowned ; the piles as well as the girders are thenfrom 12 to 13 inches square: when the distance ofthe piers is from 17 to 26 feet, pieces of 13 inchessquare have sufficient strength to carry the weight ofthe floor, as well as that of cai riages ; it is, however,always prudent to support them by inclined braces,without which such bays could not last a long time.When the opening of the bays becomes more con-siderable, the middle of the girders must be sustained
Fig. 2200- FRESINGEN BRIDGE.
by under-girders, supported by braces ; this arrangement may be adopted for bays from 26 to30 feet span, and they may be equally used when they are from 30 to 36 feet, but the girdermust then be composed of two pieces framed in the middle, and another under-girder mustbe placed over the capping, whichcrowns the piers; a great part of n!
the weight of the bay is then 1 -
carried by struts, and care is takento prevent these pieces from yield-ing by means of one and some-times two binding-pieces.
It is rare that the system ofunder-girders and struts is usedfor large bays ; but in countrieswhere timber from 12 to 14inches square is easily procured,and not very expensive, this prin-ciple may be applied to bays of from40 to 52 feet span ; in which caseit is necessary to use two struts,one of which carries the under-girder in the middle of the prin-cipal, and the other the extremityof that placed on the capping ofthe piers: these struts are main-tained by two binding-pieces; theunder-girders are united to theupper, the extremities of whichare screwed together very tightly.
If the opening of the bay were asmuch as 65 or 82 feet, anotherarrangement might be used by making the under-girders and struts of double or triplepieces; but timbers applied in this manner are far from resisting so advantageously asthey might do, the pressures to which they are exposed acting perpendicularly to thelength of the pieces, whilst the system might be so disposed that the weight should bedistributed in the direction of the length : in this kind of construction the principals must