Civil Engineering Reference
In-Depth Information
F
IGURE
1.17
(a)
Statically
determinate truss
and (b) statically
indeterminate truss
(a)
(b)
the members whereas the truss shown in Fig. 1.17(b) is statically determinate only as
far as the calculation of support reactions is concerned.
Another type of indeterminacy,
kinematic indeterminacy
, is associated with the ability
to deform, or the degrees of freedom, of a structure and is discussed in detail in
Section 16.3. A degree of freedom is a possible displacement of a joint (or node as it
is often called) in a structure. For instance, a joint in a plane truss has three possible
modes of displacement or degrees of freedom, two of translation in two mutually
perpendicular directions and one of rotation, all in the plane of the truss. On the
other hand a joint in a three-dimensional space truss or frame possesses six degrees
of freedom, three of translation in three mutually perpendicular directions and three
of rotation about three mutually perpendicular axes.
1.6 A
NALYSIS AND
D
ESIGN
Some students in the early stages of their studies have only a vague idea of the differ-
ence between an analytical problem and a design problem. We shall examine the var-
ious steps in the design procedure and consider the role of analysis in that procedure.
Initially the structural designer is faced with a requirement for a structure to fulfil a
particular role. This may be a bridge of a specific span, a multistorey building of a
given floor area, a retaining wall having a required height and so on. At this stage
the designer will decide on a possible form for the structure. For example, in the case
of a bridge the designer must decide whether to use beams, trusses, arches or cables
to support the bridge deck. To some extent, as we have seen, the choice is governed
by the span required, although other factors may influence the decision. In Scotland,
the Firth of Tay is crossed by a multispan bridge supported on columns, whereas the
road bridge crossing the Firth of Forth is a suspension bridge. In the latter case a large
height clearance is required to accommodate shipping. In addition it is possible that the
designer may consider different schemes for the same requirement. Further decisions
are required as to the materials to be used: steel, reinforced concrete, timber, etc.
Having decided on a particular system the loads on the structure are calculated. We
have seen in Section 1.2 that these comprise dead and live loads. Some of these loads,
