The resistance to compression being one thousand pounds per inch, renders necessary four hundred inches of section to each chord; four pieces 8 × 12 give in all three hundred and eighty-four inches of section, which requires no reduction, as the whole chord pressing together and being properly framed is not weakened by splicing. The splicing blocks in the upper are merely plain pieces, inserted one half inch, the only duty being to keep the sticks at the proper horizontal distance.
The spaces between the pieces should be large enough to allow the rods to pass without cutting the chords; (two inches answers every purpose). The bolts for splicing, have no very great strain to bear. In small spans from ½ to ⅝, and in large bridges from ⅝ to an inch is enough.
The object in framing a built beam for a bridge chord, is to make a stick which shall be uniformly strong. This is done by cutting the pieces in the centre of the panel, and by having no two joints in either chord in one panel; though in long spans this cannot always be done. Figs. 67 D and 67 E (page [153])
BRACES.
| The whole load being | 800,000 pounds, |
| Each truss supports | 400,000 pounds, |
| Each set of braces | 200,000 pounds, |
| Each brace (there being 4) | 50,000 pounds, |
which must be increased for inclination as follows: The length of diagonal is twenty-nine feet, (the height being twenty-five and length 15,) whence
25 to 29 as 50,000 to 58,000 lbs.;
which would need fifty-eight square inches, or 7 × 8 for compression; which, however, is quite too small for flexure. 12 × 12 placed in the formula gives
W = 2240 × bd3
L2,
or