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Steel Designers' Manual - 6th Edition (2003)
beams, thus avoiding high local bending stress in the deck chord. For large-span
bridges with an economical spacing for the deck cross-beams, the height of the truss
may be as much as four times the bay width. In such a case a subdivided form of
truss will be required to avoid very long uneconomical compression web members,
or tensile members subjected to load reversal due to moving live loads. The
diamond, Petit and K-trusses, Fig. 19.3(g), (h) and (f), are just three types which can
be used.
The diamond and Petit trusses have the advantage of having shorter diagonals
than the K-truss. The main disadvantage of trusses such as the diamond or Petit,
which have intermediate bracing members connected to the chords away from the
main joints, is that they give rise to high secondary stresses for short to medium
spans due to differential joint deformation caused by moving live loads. The K-truss
is far superior in this respect.
19.2 Guidance on overall concept
19.2.1 Buildings
Guidance on overall concept 545
For pitched-roof trusses such as the Pratt, Howe or Fink, Fig. 19.1(a), (b) and (c),
the most economical span-to-depth ratio (at apex) is between 4 and 5, with a span
range of 6 m to 12 m, the Fink truss being the most economical at the higher end of
the span range. Spans of up to 15 m are possible but the unusable roof space
becomes excessive and increases the running costs of the building. In such circumstances
the span-to-depth ratio may be increased to about 6 to 7, the additional steel
weight (increase in initial capital expenditure) being offset by the long-term savings
in the running costs. For spans of between 15 m and 30 m, the mansard truss, Fig.
19.1(d), reduces the unusable roof space but retains the pitched appearance and
offers an economic structure at span-to-depth ratios of about 7 to 8.
The parallel (or near parallel) chord trusses (also known as lattice girders) such
as the Pratt or Warren, Fig. 19.1(e) and (f), have an economic span range of between
6 m and 50 m, with a span-to-depth ratio of between 15 and 25 depending on the
intensity of the applied loads. For the top end of the span range the bay width should
be such that the web members are inclined at approximately 50° or slightly steeper.
For long, deep trusses the bay widths become too large and are often subdivided
with secondary web members.
For roof trusses the web member intersection points with the chords should
ideally coincide with the secondary transverse roof members (purlins). In practice
this is not often the case for economic truss member arrangements, thus resulting
in the supporting chord being subject to local bending stresses.
The most economical spacing for roof trusses is a function of the span and load
intensity and to a lesser extent the span and spacing of the purlins, but as a general
rule the spacing should be between 1 /4 and 1 /5 of the span, which results in a spacing
of between 4 m and 10 m for the economic range of truss spans.