This is a form of truss designed by Ithiel Towne for bridges long before iron was used in this country for such work. Several railroad bridges were built on this principle and the truss has proved very efficient in supporting loads. The truss is well adapted to the support of flat roofs in localities where large timbers and iron rods are expensive or difficult to obtain.
The general shape of the truss, as used for supporting roofs is shown in Fig. 25. The truss is composed of top and bottom chords, united by a lattice of planks and by vertical pieces at the ends. The inclination of the braces or lattice should be the same in both directions and as near 45 degrees as an even division of the span will permit. In the original truss the planks forming the lattice were secured to each other at their crossings, and to the chords and end pieces by wooden pins called "treenails." In the modern truss iron bolts are used for this purpose, although dry oak pins might be used at the intersection of the braces.
The construction is very simple and can be made by any carpenter, and the materials are such as may be easily obtained in almost any village. There is no difficulty in making the truss strong enough to carry any roof load for spans up to 80 ft., but owing to the fact that it requires a large amount of lumber and cannot be tightened up, it is not as desirable a truss to use, where rods can be readily obtained, as the Howe truss.
Proportions and Construction. The height of a lattice truss, measured between the centre lines of the chords, should be from one-eighth to one-sixth of the span and the braces should be placed at an angle of about 45 degrees. When laying out a lattice truss, the first step should be to determine the height, and then the number of spaces between the joints in the top and bottom chords.
Fig. 25. - Lattice Truss.
To find the number of spaces, multiply the span by two, and divide by the height, using the nearest whole number. Thus if the 2 x 60 span is 60 ft. and the height 8 ft. there should be = 15 spaces. If the height is 10 ft. there should be 12 spaces. The truss shown in Fig. 25 has 16 spaces.
Having determined the height and number of spaces, fix the centre of the end joints, and divide the distance between into the number of spaces determined upon, thus fixing the position of the braces. The chords should be built of four thicknesses of plank, two on each side of the truss, and breaking joint opposite their centres, using as long planks for the tie-beam as can be obtained. At the ends, vertical planks should be cut between the chords, on each side of the bracing, to act as posts. The braces should be bolted to the chords and end posts, and also to each other, where they cross. A goodly number of spikes should also be used in the joints, as indicated in Fig. 27.
Fig. 26 - Vertical Section.
Fig. 27. - Detail of Joint 5, of Fig. 25.
The bottom chord should also be bolted every two feet between the joints, as this member is in tension. The top chord, being in compression, will be tied sufficiently by the bolts at the joints, and by a short bolt on each side of each butt joint. The strain on the joints near the ends of the truss will be much greater than on the centre joints.
The first three joints at each end, should have as many, and as large bolts, as is given in the last column of Table II. The bolts in the next three joints may be slightly reduced in size, and those in the centre joints still more.
It should be kept in mind that the strength of a lattice truss is usually measured by the strength of the joints.