The fifth claim, is the formation of a stronger chord than by the plan of using a few large sticks. The chord being made of a great number of small pieces, the strength is of course less affected at any one point, by a joint, than when only a few pieces are used.
In the bridges built by the above engineer, are to be seen some of the most perfect built beams in the country. The following conditions being observed, the most uniform, and highest average strength possible is obtained.
First. To cut but one stick in any one panel.
Second. To cut no stick at the centre of the bridge.
Third. To place every joint in the middle of the collateral piece.
The chords are cut by two rows of pins, two inches each; and if the chord be fifteen inches, the cutting at centre, where there is no joint, is but four fifteenths of the whole section. To resist the parting of any two sticks, there is the resistance to shearing of ninety-six pins; and the section of each being three square inches, the whole resistance is two hundred and eighty-eight inches of area. If the intermediate chord has the same number, the whole area to resist shearing, in the lower chord, is five hundred and seventy-six square inches. The bearing surface of each pin in the chord stick is 2 × 3 inches, or six inches; and 96 × 6 = 576: and in both chords 1152 inches.
| Sq. in. | |
|---|---|
| The whole chord timber, (both chords,) is (6 × 3 × 14), | 252 |
| In the intermediate chord, (6 × 3 × 12), | 216 |
| Whole timber section, | 468 |
| Deduct 4 pins for both chords, 468 – (4 × 2 × 3 × 6) or 468 – 144 = | 324 |
| Deduct for joints 3 × 10 + 3 × 8 or 324 – 54, Square inches of available area. | 270 |
Comparing the amount thus cut away with that cut away in other plans, we have the following figures;—
| A. Hall’s Bridge, (actual bridge,) | 30 100 |
| B. Howe’s bridge, (actual bridge,) | 35 100 |
| C. Page 163, (Handbook R. R. Construction,) | 39½ 100 |
| D. McCallum’s, (Susquehanna bridge,) | 58 100 |
The sixth claim, is the peculiarly convenient form for applying an arch,—the superiority consisting in convenience for attachment; in the connections being less affected by shrinkage than when posts are locked into arches; in the timbers not being weakened by cutting. The arch is loaded with the tension timbers, inwardly, and acts as a general arch brace, transferring at once all of the several tensions to the abutment, thus really combining the arch with the truss.