Fig. 68 B.
The depth of insertion of the block into the chord, depends upon the horizontal strain upon it. In a span of one hundred and fifty feet, with the rods at an angle of 50°, two inches have been found ample at the end of the truss, and one half inch at centre.
From D, perpendicular to m m, lay off D E; equal to H, also from E, at right angles to n n, make E E′ = y. From E′ draw the vertical E′ L.
The strain upon the rod o m, being represented by o m; and that upon o n, by o n, the resultant is shown, both in direction and amount, by o V. It is not necessary that this should pass through the centre of the post, as the excess of tension on o m, over that on o n, is absorbed by the lower chord.
Note.—Screwing up truss bridges, is a more scientific operation than is generally supposed. Many builders commence at each end, and lift the bridge from the scaffolding. By this method the greater part of the load is often borne by a few of the end sets of rods. The better method is to begin at the centre and work both ways towards the ends, being sure that each set of rods does its duty before the next is touched. The lift to be made by each set of rods, should first be calculated, and tested while screwing up, with the level.
LATTICE BRIDGES.
184. Town’s lattice, consists of a simple lattice-work of plank, 3 × 12 inches, treenailed together at an angle of forty, forty-five, or fifty degrees. It possesses great stiffness, without by any means having the material disposed in the best manner. Such bridges might well be made by the mile, and cut off to order according to the span.
The improved lattice, by Hermann Haupt, Esq., C. E., avoids all of the evils attendant upon the common lattice, and gives a very cheap, strong, and rigid bridge. In this plan the braces are placed in pairs, with vertical tie planks between them; by which the twisting seen in the common lattice, is removed. The braces are also brought to the vertical, as the point of support is reached, by which a good bearing is given to the end sets of timbers.
To vary the size of the braces, as the strain upon them decreases, would be both inconvenient and expensive; but the same effect may be produced by varying the distance between them, making it greater as the centre is approached.