Building Design and Construction Handbook - Merritt - Ventech!

STRUCTURAL STEEL CONSTRUCTION 7.23
Welded girders generally are composed
of three plates (Fig. 7.10c). This
type offers the most opportunity for simple
fabrication, efficient use of material,
and least weight. Top and bottom flange
plates may be of different size (Fig.
7.10d), an arrangement advantageous in
composite construction, which integrates
a concrete floor slab with the
girder flange, to function together.
Heavy girders may use cover-plated
tee sections (Fig. 7.10e). Where lateral
loads are a factor, as in the case of girders
supporting cranes, a channel may be
fastened to the top flange (Fig. 7.10ƒ).
In exceptionally heavy construction, it is
not unusual to use a pair of girders diaphragmed
together to share the load
(Fig. 7.10g).
The availability of high-strength,
FIGURE 7.10 Typical built-up girders. weldable steels resulted in development
of hybrid girders. For example, a highstrength
steel, say A572 Grade 50, whose yield stress is 50 ksi, may be used in a
girder for the most highly stressed flanges, and the lower-priced A36 steel, whose
yield stress is 36 ksi, may be used for lightly stressed flanges and web plate and
detail material. The AISC specification for allowable-stress design requires that the
top and bottom flanges at any cross section have the same cross-sectional area, and
that the steel in these flanges be of the same grade. The allowable bending stress
may be slightly less than that for conventional homogeneous girders of the highstrength
steel, to compensate for possible overstress in the web at the junction with
the flanges. Hybrid girders are efficient and economical for heavy loading and long
spans and, consequently, are frequently employed in bridgework.
Trusses. When depth limits permit, a more economical way of spanning long
distances is with trusses, for both floor and roof construction. Because of their
greater depth, trusses usually provide greater stiffness against deflection when compared
pound for pound with the corresponding rolled beam or plate girder that
otherwise would be required. Six general types of trusses frequently used in building
frames are shown in Fig. 7.11 together with modifications that can be made to
suit particular conditions.
Trusses in Fig. 7.11a to d and k may be used as the principal supporting members
in floor and roof framing. Types e to j serve a similar function in the framing
of symmetrical roofs having a pronounced pitch. As shown, types a to d have a
top chord that is not quite parallel to the bottom chord. Such an arrangement is
used to provide for drainage of flat roofs. Most of the connections of the roof
beams (purlins), which these trusses support, can be identical, which would not be
the case if the top chord were dead level and the elevation of the purlins varied.
When used in floors, truss types a to d have parallel chords.
Properly proportioned, bow string trusses (Fig. 7.11j) have the unique characteristic
that the stress in their web members is relatively small. The top chord,
which usually is formed in the arc of a circle, is stressed in compression, and the
bottom chord is stressed in tension. In spite of the relatively expensive operation