Figure 1-39.—Typical built-up girders. which are, in turn, supported by columns.

The
horizontal members or beams that connect the exterior columns are called spandrel
beams. If you add additional rows of columns and beams, there is no limitation to the
area of floor and roof that can be supported using skeleton construction. One
limitation on using skeleton construction, however, is the distance between
columns. Oftentimes, large structures, such as aircraft hangars, may require
greater distances between supports than can be spanned by the standard
structural steel shapes. In this case, one of several methods of long-span steel
construction is used. One method uses built-up girders to span the distances
between supports. Two types of built-up girders are shown in figure 1-39. As seen in
this figure, the built-up girder consists of steel plates and shapes that are combined
together to meet the necessary strength. The individual parts of these girders are
connected by welding or riveting. Another method, which is usually more
economical, is to use a truss to span large distances. As you learned in the EA3
TRAMAN, a truss is a frame- work of structural members consisting of a top
chord, bottom chord, and diagonal web members that are usually placed in a
triangular arrangement. (See figs. 1-40 and 1-41.) As shown in figure 1-40, trusses can
be fabricated to conform to the shape of nearly any roof system. A third long-span
method, although not as versatile as trusses, is the use of bar joists. Bar joists are
much lighter than trusses and are fabricated in several different types. One type is
shown in figure 1-42. Prefabricated bar joists, designed to conform to specific
load requirements, are obtainable from commercial companies. Other long-
span construction methods involve several different types of framing systems,
which include steel arches, cable-hung frames, and other types of systems. These
methods are beyond the 30. Trusses For Pitch Roofs

For ordinary conditions and for spans under 100 feet some one of the types shown by Figs. 64 to
75, will generally meet the requirements of strength and economy.

For a narrow shed or shop the shape of truss shown by Fig. 64 is the most economical, the truss
proper being that portion enclosed within the points A, B, C. This truss is practically the same as
that shown by Fig. 65.
For spans of from 24 to 48 feet, and with an inclination not exceeding 6" to the foot, types 66 and
67 are the most suitable.

The truss type represented by these two figures has received the name of "Fan truss." The truss
shown by Fig. 65 is known as a "simple Fink truss." The truss shown by Fig. 67 differs from that
in 66, principally in the inclination of the braces. The braces A, B, in Fig. 67 being inserted to
brace the truss from the column to prevent racking under wind pressure. Fig. 67 should be used
when the truss is supported by columns, and Fig. 66 when the truss rests on brick walls. When
the roof construction demands three purlins on each side of the truss, one of the forms shown by
Figs. 68, 69, 70 or 71 should be used.

Fig. 66. - Simple Fink Truss; Span, 20 to 36 ft.

Fig. 66. - Simple Fan Truss.

Fig. 67. - Fan Truss; Span, 40 to 50 ft.

The names given to these trusses are often confounded by different writers; many engineers
class the French and Fan trusses with the Fink truss. The term "French" appears to be generally
given to those trusses in which the tie-beam is raised in the centre. The truss shown by Fig. 71,
appears to have no generally recognized name. One writer refers to it as an "English" truss. This
truss is not as economical as the Fink truss, except when the inclination of the rafter is less than
one-fourth pitch, on account of the great length of the inner struts.

Fig. 68. - Compound Fink Truss.

Fig. 69. - Eight-Panel French Truss.

Although Fig. 71 somewhat resembles the Queen truss, Fig. 12, it will be seen that the diagonals
run in the opposite direction, the diagonals in Fig. 71 being in tension, and the verticals in
compression, the reverse of the Queen truss.

Much of the economy of Fink and Fan trusses lies in the fact that most of the members are in
tension and the struts are short. Comparing Figs. 70 and 71, it will be noticed that the inner strut
in the former is only 1/3 as long as the strut in the latter. Another advantage of these trusses is
that a partial load, as, for instance, a wind or snow load on one side of the truss never causes
stresses in excess of those produced by a uniform load of the same intensity over the whole
truss. As a general rule, the struts in Fink trusses are placed at right angles to the rafters, as in
Figs. 68, 72 and 73, but if there are trussed purlins it is desirable to have vertical members to
receive the ends of the purlins. Vertical struts are generally required in hip trusses.

Fig. 70. - Fink Truss with Vertical Struts.

Continue to:

scope of this TRAMAN. Figure 1-40.—Typical steel trusses. 1-20