Idealized Structure

Idealized Structure. Having stated the various ways in which the
connections on a structure can be idealized, we are now ready to discuss
some of the techniques used to represent various structural systems by
idealized models.
As a first example, consider the jib crane and trolley in Fig. 2–5a. For
the structural analysis we can neglect the thickness of the two main
members and will assume that the joint at B is fabricated to be rigid.
Furthermore, the support connection at A can be modeled as a fixed
support and the details of the trolley excluded.Thus, the members of the
idealized structure are represented by two connected lines, and the load
on the hook is represented by a single concentrated force F, Fig. 2–5b.
This idealized structure shown here as a line drawing can now be used
for applying the principles of structural analysis, which will eventually
lead to the design of its two main members.
Beams and girders are often used to support building floors. In
particular, a girder is the main load-carrying element of the floor, whereas
the smaller elements having a shorter span and connected to the girders
are called beams. Often the loads that are applied to a beam or girder are
transmitted to it by the floor that is supported by the beam or girder.
Again, it is important to be able to appropriately idealize the system as a
series of models, which can be used to determine, to a close approximation,
the forces acting in the members. Consider, for example, the
framing used to support a typical floor slab in a building, Fig. 2–6a. Here
the slab is supported by floor joists located at even intervals, and these
in turn are supported by the two side girders AB and CD. For analysis it
is reasonable to assume that the joints are pin and/or roller connected
to the girders and that the girders are pin and/or roller connected to the
columns. The top view of the structural framing plan for this system is
shown in Fig. 2–6b. In this “graphic” scheme, notice that the “lines”
representing the joists do not touch the girders and the lines for the girders
do not touch the columns. This symbolizes pin- and/ or roller-supported
connections. On the other hand, if the framing plan is intended to
represent fixed-connected members, such as those that are welded