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.
[Illustration: 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]
[A] Definitions from Ricker’s Trussed Roofs.
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.
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.)
[Illustration: 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.
[Illustration: FIG. 120.]
The line E, D, on the diagram represents a tie-rod, which by the
construction of this truss would naturally tend to stiffen the
structure by supporting the center of the tie-beam.
Wire, nuts, and washers are supplied (where the student makes a whole
model) to make the tie-strut.
The student in writing out notes will make two sketches of trusses he
may have observed on shop visits. The buildings visited almost all have
trussed roofs, either wood or iron.