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

For polishing [see note].

PROBLEM IN TRUSS WORK.

The preceding work is what might be termed joiner work; the carpenter also is called upon to join timbers, and uses to a great extent the same joints that the joiner does, but the joiner’s work is usually where it must bear inspection, whereas the carpenter’s work is generally covered over either by plaster or casings. A single mechanic may be able to perform every kind of work that is required in the construction of a building; thus the two trades are usually spoken of as one, i. e., carpenter work.

Fig. 118.

In [Fig. 118] is shown a method that is sometimes used in the construction of trusses. A truss is that part of a roof which supports the purlines, rafters and sheathing. A roof is the covering or upper enclosure of a building with the frame work by which it is supported. It may be of almost any shape. A light roof is usually of moderate span, without trusses, the rafters being supported by the walls or partitions of the building. A heavy roof is employed for wider spans, and the rafters are then supported by the purlines and trusses. A truss is usually required for spans of more than 20 feet.[A]

The span of a roof is the horizontal distance between the external surfaces of the walls of the building; its rise is a vertical let fall from its ridge to a horizontal line joining the intersections of the external surfaces of the walls and the roof surfaces. The inclination of a roof equals the angles between its surface and a horizontal.

The span of a truss is the horizontal distance between the centers of its end joints, and is usually the same as that between the centers of the walls, which support the truss. Its rise is the vertical connecting its span line and the center of the joint at the apex or highest point of the truss.

A member of a truss is any straight or curved piece which connects two adjacent joints of the truss.

The upper chord is composed of the members which form the upper edge or margin of the truss. Each half of the upper chord of a triangular truss is often called a principal. The lower chord is composed of the members forming the lower edge of the truss. If straight, this is termed the tie-beam or tie-rod; the first being a wooden timber; the second, one or more rods of iron.