There are several other forms of Bridge, the most notable among which are the Whipple, McCallum's, Post's, Towne's, Haupt's, and Burr's. But enough has been said to give the student an idea of the general arrangement of the different parts of a Truss, arid to enable him to determine the strains to which the various members are subjected. Nothing will be said in regard to Wooden Arches, as our space is too limited.
Pile Bridging. A bridge of this description is useful in crossing marshes, or in shallow water. Fig. 5, Pl. III, gives a good example of this kind of bridge, under 20 feet in height. If on a curve, there must be extra bracing on the convex side.
Trestle Work. This is a combination of posts, caps, and braces; and is used for both temporary and permanent works. Plate IV, Figs. 1, 2, 3 and 4, give some of the best varieties in use. Figs. 1 and 2, may be used up to 15 feet in height; Fig. 4, up to 20 feet; and Fig. 3, to 30 ft. The distance apart of the various bents should not exceed 10 or 12 ft., unless bracing is introduced between them, and the bents should always be raised above the ground a few feet on a solid masonry foundation. Want of space forbids any mention of abutments and piers, which really come more properly under the head of masonry.
Iron Bridging is gradually working its way into favor, and will probably eventually supersede wooden trusses;—but in many cases wood is the only material at hand—and therefore some knowledge of Wooden Bridging is desirable. It is intended to follow this pamphlet with a portfolio of sheets containing working drawings of several kinds of Wooden Bridges, taken from actual measurements of some of the best specimens of the different styles of Truss in use.
PRACTICAL NOTES.
When putting a truss together in its proper position, on the abutments, 'false works' must first be erected to support the parts until they are so joined together as to forma complete self-sustaining truss. The bottom chords are first laid as level as possible on the false works, then the top chords are raised on temporary supports, sustained by those of the lower chord, and are placed a few inches higher at first than their proper position, in order that the web members may be slipped into place. When this is done the top chords are gradually lowered into place. The screws are then gradually tightened, (beginning at the centre and working towards both ends,) to bring the surfaces of the joints into proper contact, and by this method, the camber forms itself, and lifts the lower chords clear of the false works, leaving the truss resting only upon its proper supports. The subjoined Table will be found useful in estimating the strains on a truss when proportioning a bridge for any moving load.
Table of weights per running foot of a bridge, (either of wood or iron,) including weights of floor, lateral bracing, &c., complete, for a single track.
| Clear Span. | Weight of Bridge. | Clear Span. | Weight of Bridge. | Clear Span. | Weight of Bridge. | Clear Span. | Weight of Bridge. | ||||
| Tons. | lbs. | Tons. | lbs. | Tons. | lbs. | Tons. | lbs. | ||||
| 25 | .266 | 596 | 70 | .404 | 905 | 140 | .614 | 1375 | 200 | .792 | 1774 |
| 30 | .281 | 629 | 80 | .434 | 972 | 150 | .643 | 1440 | 225 | .867 | 1942 |
| 40 | .313 | 701 | 90 | .464 | 1039 | 160 | .673 | 1507 | 250 | .940 | 2105 |
| 50 | .343 | 768 | 100 | .494 | 1106 | 170 | .703 | 1575 | 275 | 1.013 | 2269 |
| 60 | .374 | 838 | 120 | .554 | 1241 | 180 | .733 | 1642 | 300 | 1.087 | 2435 |
The weight of a single track railway bridge may be taken as equal to that of a double track highway bridge,—and the trusses that will be large enough for one will be large enough for the other.