(6) Raise, aline, and nail several truss
assemblies into position. Nail temporary 1 x 6 braces
across these assemblies (4, fig. 2-21) and other
assemblies as they are brought into position. Check
rafter spacing at peaks as braces are nailed on.
(7) Braces may be used as a platform
when raising those trusses when there is too little
room to permit rotation.
2-9.
HEAVY TRUSSES
a. Definition. A heavy truss is a framed
or jointed structure composed of straight members
connected only at their intersections in such a way
that if the loads are applied at these intersections the
stress in each member is in the direction of its length.
b. Types. The web members of a truss
divide the truss into a number of triangles. It is
possible to arrange innumerable types of trusses, but
certain types have proved to be more satisfactory
than others, and each of these types has its special
uses. The various types of trusses used in heavy
building construction are illustrated by line diagrams
in figure 2-22. The members indicated by heavy
lines normally carry compressive stresses, and those
indicated by light lines normally carry tensile stresses
for vertical loads. In most cases, the compression
members are the shortest members in the truss, while
Figure 2-21. Erection of rafter trusses.
the tension members are the longest. This results in a
the actual installation. The following procedures
great saving of material, for a compression member
may be used in the actual installation of trusses:
requires a greater sectional area for a given stress
than does a tension member. Sometimes the top
(1) Mark proper positions of all truss
chords of these trusses are slightly sloping in one or
assemblies on top plate. The marks will show the
two directions for roof drainage, but this does not
exact position of a given face of all rafters (south or
change the type of truss. The necessary number of
north, etc.).
subdivisions or panels depends upon the length of the
span and the type of construction.
(2) Rest one end of a truss assembly,
peak down, on an appropriate mark on top plate on
c. Terms
used
in
connection
with
one side of structure (1, fig. 2-21).
trusses.
(3) Rest other end of truss on opposing
(1) Member is the component which
mark on top plate on other side of structure (2, fig. 2-
lies between any two adjacent joints of a truss; it can
21).
be of one or more pieces of structural material.
(4) Rotate assembly into position by
(2) Bottom chord is a member which
means of a pole or rope (3, fig. 2-21).
forms the lower boundary of the truss.
(5) Line up rafter faces flush against
(3) Top chord is a member which
marks and secure.
forms the upper boundary of the truss.
2-16
Previous Page
