Civil Engineering Reference
In-Depth Information
Chapter
4
/
Analysis of Pin-jointed
Trusses
In Chapter 1 we discussed various structural forms and saw that for moderately large
spans, simple beams become uneconomical and may be replaced by trusses. These
structures comprise members connected at their ends and are constructed in a variety
of arrangements. In general, trusses are lighter, stronger and stiffer than solid beams
of the same span; they do, however, take up more room and are more expensive to
fabricate.
Initially in this chapter we shall discuss types of truss, their function and the idealiza-
tion of a truss into a form amenable to analysis. Subsequently, we shall investigate the
criterion which indicates the degree of their statical determinacy, examine the action
of the members of a truss in supporting loads and, finally, examine methods of analysis
of both plane and space trusses.
4.1 T
YPES OF
T
RUSS
Generally the form selected for a truss depends upon the purpose for which it is
required. Examples of different types of truss are shown in Fig. 4.1(a)-(f); some are
named after the railway engineers who invented them.
For example, the Pratt, Howe, Warren and K trusses would be used to support bridge
decks and large-span roofing systems (the Howe truss is no longer used for reasons we
shall discuss in Section 4.5) whereas the Fink truss would be used to support gable-
ended roofs. The Bowstring truss is somewhat of a special case in that if the upper
chord members are arranged such that the joints lie on a parabola and the loads, all of
equal magnitude, are applied at the upper joints, the internal members carry no load.
This result derives from arch theory (Chapter 6) but is rarely of practical significance
since, generally, the loads would be applied to the lower chord joints as in the case of
the truss being used to support a bridge deck.
Frequently, plane trusses are connected together to form a three-dimensional struc-
ture. For example, in the overhead crane shown in Fig. 4.2, the tower would usually
81
