To determine the size of the lower chords, to resist the above 800,000 pounds of tension, proceed as follows: Each side truss will support one half of the whole load, or 400,000 pounds; which, at 2,000 pounds per inch, will require 200 square inches of section. Four sticks of 8 × 12 inches, give an area of 384 square inches, which must be reduced as follows: Deduct 72 square inches for the area cut out by the splicing blocks, 40 inches for the bolts connecting the pieces, 28 inches for inserting the foot blocks, and 10 inches for inserting the washer, and we have remaining 234 square inches; which exceeds by a little the exact demand. This excess (about one seventh) is a necessary allowance for accidental strain, to which all bridges are subjected.
Fig. 67 A.
The splices used in bridge framing are shown in fig. 67 A and fig. 67 B. For the first, the depth of insertion and length of the block depend upon the tension upon the chord. The following dimensions have been much used and are perfectly reliable:—
| Span of Bridge. | A C | B C | C D |
|---|---|---|---|
| Feet. | Feet. | Inches. | Feet. |
| 50 | 1.00 | 1½ | 1.50 |
| 100 | 1.25 | 2 | 2.00 |
| 150 | 1.75 | 2½ | 2.25 |
| 200 | 2.00 | 3 | 2.75 |
There is no need of cutting more than one notch, as in the figure; the resistance of the triangles is thereby lessened, and the work increased.
Fig. 67 B.
In fig. 67 B, the rods must of course be able to resist the tension upon the one piece which is cut.
Upper Chord.—The upper chords of a bridge suffer compression, to the same amount numerically, as the tension on the lower chord; as whatever tension is thrown by any brace upon the lower chord, reacts as just so much compression upon the upper. In the case at hand, 800,000 lbs. in all, or 400,000 on each chord.