In practice, smaller braces than 12 × 12 would answer, because the four braces in a set may be fastened together, making a post of four pieces 8 × 12, or in all a built post of 44 × 12 inches; twelve being the depth, whence
_W_ = (2240 × 44 × 1728)/841 = 202,511 lbs.;
the forty-four inches being made by blocking the braces four inches apart. The second set of braces are to be treated in the same manner, the weight to be supported being only the rectangle included by those braces; i. e. the whole bridge and load less the two end panels.
As the centre of the span is approached, the pressure on the braces becomes very small; and the scantling of the braces will be reduced to about 6 × 7 inches.
The weight upon the first set of rods is the same as that upon the end sets of braces; in the present case 800000 ÷ 2 = 400000 on each side truss, and 400000 ÷ 2 = 200000 on each end; and if there are five rods in each set, each rod bears 40,000 lbs. Referring to the table on p. 146, opposite to 41,415 lbs., is the diameter 1⅞ inches; whence the first set must contain five rods, of 1⅞ inches diameter. The second set decrease in size as the weight is lessened by the two end panels. The nut and washer for the rod are also found in the same table.
180. When a load is placed on the point C′, fig. 64, the truss tends to sink at that point, and a corresponding rise takes place at C. This motion changes the figure A B C E, from a rectangle to an oblique angled figure; the diagonal E B being shortened, and A C lengthened. This motion is easily checked by the introduction of the counter brace E B.
The action which this timber is called upon to resist, being caused by the moving or variable load on one panel, the brace must resist the load coming thereon, (say fifteen feet,) and is thus the same size as the brace at the centre of the span.
The counter braces may be so confined between the braces, at the intersection, as not to move laterally or vertically, but must not be fastened to the braces; because the action of the separate timbers is thus trammelled.
The manner of adjusting the braces and counter braces to the chord is shown in fig. 67 C. It was formerly the custom to abut the braces against a block on one side of the chord, and to screw the rod against a block on the opposite side; the whole strain acting to crush the chord crosswise. This has been remedied by the arrangement shown in the figure, the two blocks being cast in one piece and connected by a small hollow cylinder passing between the chord sticks.