In panel No. 1, it will be 25 feet. In panel No. 2, it will be 18 feet. In panel No. 3, it will be 17 feet. In panel No. 4, it will be 16 feet. In panel No. 5, it will be 16 feet.
we get, in round numbers, the following dimensions, the braces being bolted and blocked together:—
For the 1st panel, 25 feet long, 8 × 10 For the 2d panel, 37 feet long, 8 × 10 For the 3d panel, 49 feet long, 8 × 10 For the 4th panel, 64 feet long, 8 × 10 For the 5th panel, 78 feet long, 8 × 10.
For the lattice-work, a double course on each side of each truss, in long spans, (150 to 200 feet); and a single course in shorter spans, of 3 × 6 plank, treenailed at intersections, is ample.
GENERAL TABLE OF DIMENSIONS FOR ARCH BRACE TRUSS.
┌──────────┬──────────┬──────────┬──────────┬──────────┬──────────────┐ │ Span. │ Rise. │ Chords. │ Ties. │ Braces. │ Lattice. │ ├──────────┼──────────┼──────────┼──────────┼──────────┼──────────────┤ │ 50│ 10│ 2–8 × 10│ 1–8 × 10│ 2–6 × 6│2 × 9 or 3 × 6│ │ 75│ 12│ 2–8 × 10│ 1–8 × 10│ 2–6 × 6│2 × 9 or 3 × 6│ │ 100│ 15│ 3–8 × 10│ 2–8 × 10│ 3–6 × 6│2 × 9 or 3 × 6│ │ 150│ 20│ 4–8 × 12│ 3–8 × 10│ 4–6 × 8│2 × 9 or 3 × 6│ │ 200│ 25│ 4–8 × 16│ 3–8 × 10│ 4–6 × 9│2 × 9 or 3 × 6│ └──────────┴──────────┴──────────┴──────────┴──────────┴──────────────┘
Fig. 69 A, shows the method of bringing the arch braces to the chord. To find the dimensions of the cast-iron block, make a complete drawing of all of the braces, at their proper angles, and then draw in the block around the feet, as shown in fig. 69 A.
NOTE.—The centre of pressure of the braces in fig. 69 A, is not, as might seem, at C; because the vertical components of the forces, coming down the brace, are much less in the braces at small angles than in those at the end of the span. The load applied to each brace being the same, and the inclines being found, we find the centre of pressure, or the centre of bridge seat as follows:—
The length of the brace is to the vertical height, as the applied load to the vertical pressure. In fig. 69 A, we have the following lengths of braces: _a_, 25; _b_, 37; _c_, 49; _d_, 64; _e_, 78; _f_, 92; and _g_, 106; and the weights corresponding thus,
_a_, 25 : 20 :: 6000 : 4800. _b_, 37 : 20 :: 6000 : 3243. _c_, 49 : 20 :: 6000 : 2450. _d_, 64 : 20 :: 6000 : 1870. _e_, 78 : 20 :: 6000 : 1540. _f_, 92 : 20 :: 6000 : 1304. _g_, 106 : 20 :: 6000 : 1132.