Chap. XXIII.

TIMBER BRIDGES.

1369

Bridge over an arm of the Neckcr, at Wurtemberg, has a width of carriage-way of 14 feet,and a clear span of 53 feet; it is queen-post trussed, and strongly strutted against stoneabutments.

Fig. 2229.

BRIDGE OP NECKBR, AT WCRTEMBERG.

This kind of bridge does not require muchskill in framing the timbers, nor any other ap-plication of mechanical principles beyond thatof strength; the squared timbers are placedacross the stream or canal parallel to each other;on these are laid stout planks or transversepieces to carry the roadway, which is madeeither with earth or gravel, and kept in its placeby stout planking at the sides. The princi-pal timbers are prevented from sagging by theintroduction of the two upright posts in therailing, which hold up that portion of the plat-form most liable to drop by means of struts anda stretching-piece put together in the mannerof a queen-post truss; and if the abutments aremaintained in their position and prevented from

Fig 2230. BRIDGE OF NECKER.

Fig. 2231.

BRIDGB OF LOIRET, NEAR ORLEANS.

sliding, and the ends of the timbers protected from decay, such a bridge will endure for aconsiderable length of time. As timber of a uniform scantling and of the proper quality

Fig. 2232.

SCHONENBERG CANAL.

for bridge-building is difficult to obtain be-yond the length of 50 feet, these constructionsmust be limited to that span.

A horse bridge, 6 feet 6 inches wide, overthe Loiret, near Orleans , with a span of 60feet, has its longitudinal timbers bearing on ^very strong truss.

Over the canal of Schonenberg, near Brussels,is an occupation or horse bridge, 8 feet wideand 70 feet span, formed of pendant binding-pieces which diverge from a centre, againstwhich timbers are strutted polygonally, in threerows at a distance from each other, all held

Fig. 2233.

SCHONENBERG CANAL.