Elementary Course in Woodwork / Designed for use in high and technical schools, with one hundred and thirty-four illustrations: First Edition

by George Alexander Ross

The web members connect the joints of one chord with those of the other, and may be radials in case of curved trusses, diagonals, or verticals. They are commonly called struts where they resist compression, ties where they resist tension, and strut-ties where they resist compression and tension.

A joint is the connection of two or more members whose center lines must intersect at a common point if possible, this common point being the center of the joint.

The rafters of light roofs are not trussed, but rest directly on the walls, and support the sheathing and covering of the roof.

Heavy roofs are supported by trusses resting on the side walls.

The sheathing is supported by rafters which rest on the purlines, these being supported by the trusses.

The drawing, [Fig. 118], shows the half of a truss; the members are the upper chord, the lower chord, and a strut.

Although carpenter work is usually of a rough character, the joints of a truss should fit snugly so that there will be no room to give when loaded; so, for the practice, the student will plane the stock either to the sizes given in the drawing or double the sizes, making the whole truss as time and circumstances permit. (This to be determined by the instructor.)

Fig. 119.

[Fig. 119] shows what is termed a truss diagram; the distance from point A, to B, is the distance between the center of the walls, and the angle A, C, D, is the inclination or pitch of the roof. The pitch of the roof is determined by the distance the peak of the roof rises above the walls; thus if a roof has a quarter pitch, the peak would rise above the walls one quarter the width of the building; if half pitch the peak would rise one half the width of the building, etc. For simplicity in laying out this problem we will make the pitch one half. The points A, B, represent the span of the walls; also the lines A, C, and B, C, show the outside margin of the upper chord of the truss. By bisecting A, B, and erecting a perpendicular at D, to C, we divide, the triangle A, B, C, into two triangles, A, D, C, and B, D, C. Now, the line A, C, is the hypotenuse of the right-angled triangle A, D, C. We had one example of finding the length of the hypotenuse of a right-angled triangle in Exercise No. 4. The workman who lays out rafters or trusses rarely takes time to calculate the hypotenuse of the triangle, but uses the steel framing square in the following manner. He obtains the horizontal distance at the bottom of the rafters, and the pitch. Take for example a truss that is 30 feet across from point to point, and a pitch of one half; then the distance the peak would rise would be 15 feet. Take the framing square and lay it on the chord, taking 12 inches on the blade and 12 inches on the tongue and mark off 15 triangles as shown in [Fig. 120], which is half the width of the building. The rise was also 15 feet; so by using the square as shown, we obtain the rise and the run of the rafter. The line on one side of the square gives the angle at which the chord or rafter is to be cut at the peak. The line at the other end of the chord gives the line from which to measure the distance the tenon and shoulders go down into the tie-beam. The strut shown in the drawing, [Fig. 118], has one joint square, and the other at an angle of 45 degrees. Where the pitch is one half, the angles are 45 degrees and right angles.