Chap. XXIII.
1353
upright posts, the head and sill into which they areframed being bolted well together by iron rods: thedepth or height of the framing is proportioned to thespan ; the string pieces are formed of three timbers,tiie posts and braces of two, and the counter beamsof one.
The twelfth principle is that of suspension, andis admirably adapted to spans of almost any re-quired extent: the platform is supported by hang-ing up the under transverse timbers by iron rods,which pass over a saddle, also of iron, placed at thetop of two long piles, driven in with the requisiteinclination to support them. The chains, which aresuspended from pier to pier, are sometimes placedbeneath the platform, as in fig. 2189., upon the prin-ciple now generally used for trussing longitudinaltimbers, which system is far more economical.
A combination of these principles is sometimesfound adapted to bridges of different span : Bruyerehas given us the method by which we may formfoot and other timber bridges in a firm and solid
r
\
no
manner.
Kraft, in his work on carpentry, has suggestedfor foot-bridges the hexagonal struts and binding-pieces, as shown in fig. 2190, 2191. : and the same -author prefers for bridges, over which loaded *~ [3Fjf~ 7 } r ¥i 'carriages are to pass, that the timbers should be • ■
arranged polygonaliy, and held together at theirjoints by binding-pieces: over the piles a St. Andrew’s Fig. 2187. section op long’s American
J ° BRIDGE.
-Ch
- 3 -
Fig. 2188.
SUSPENSION bridge.
Fig. 2189.
SUSPENSION bridge.