Chap. XXIII.
1365
A bridge for carriages of 30 feet distance between the piers may be constructed uponthis principle; but as greater stability is required, ano ther strong piece of timber may beput under the principal bearers, and strutted as shown : the clear width is 15 feet betweenthe parapets.
Fig. 2213.
WURTSMBBRG.
TRACH
Horse Bridge at Vrach in 'Wurtemberg exhibits a simple form of truss, which answers forthe parapets of the bridge; the distance between the piers is about 35 feet, and the clearwidth between the parapets is 7 feet 6 inches.
There is considerableability displayed in thearrangement of the timbersof the several bridges overthe arm of the Necker,which are so disposed asto form the railing orparapet; a kind of con-struction productive of thegreatest economy, at thesame time allowing theentire space beneath theplatform of the bridge tobe open for the passageof boats, and unimpededby the torrent, which mayoccasionally require theentire opening of thebridge for its passage:where timber is floated down a stream from the forest to a port for its embarkation, thereshould not be the slightest impediment presented, which would be the case if the framingof a bridge were continued down to the water’s edge.
Fig. 2214.
DETAILS OF THE BRIDGE AT VKACH.
Fig. 2215,
NECKER.
Fig.22)6.
That over an arm of the Necker, built by M. Ezel, is of thesame character ; the distance between the piles is not less than38 feet, and the clear width between the parapets is 17 feet.
St. Clair, over the Rhone at Lyons , constructed by M. Morandin the year 1775, shows great skill in the application ofthe timbers: it has 17 bays, varying in width; that in thecentre is 45 feet, and they proportionally decrease on each side, the extreme openings beingonly 33 feet. The first piles that were driven to form the piers were cut off level with lowwater, and on their tops was laid a timber platform, into which the posts which sup-ported the bridge were framed : the bridge was 36 feet in width, and the piers con-sisted of 13 posts in a row, each composed of two pieces of timber strengthened and
4 s 3