Steel Designers Manual - TheBestFriend.org

This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007
To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/
Steel Designers' Manual - 6th Edition (2003)
546 Trusses
For short-span roof trusses between 6 m and 15 m the minimum spacing should
be limited to 3–4 m.
19.2.2 Bridges
Road and railway truss bridges can either be underslung (deck at top chord level),
through (deck at bottom chord level) or semi-through.
Limits on headroom, navigation height and construction depth will determine
whether an underslung or through truss will be the most appropriate. For large-span
bridges the through type is often adopted as ample headroom will be available to
permit direct lateral restraint to the top compression chord using cross bracing. For
short-span bridges, however, the underslung type is most appropriate provided the
navigation height or construction depth limits are satisfied. Underslung trusses are
usually more economical than either through or semi-through trusses as the deck
structure performs the dual function of directly supporting the traffic loads and providing
lateral restraint to the compression chords. In the case of short-span through
trusses the span-to-depth ratio may be uneconomically low if the top chord is
restrained by cross-beams, as sufficient traffic headroom must be provided. In such
a situation it is more economical to brace the top compression chords by U-frame
action.
A span-to-depth ratio of between 6 and 8 for railway bridges and between 10 and
12 for road bridges offers the most economical design. In general terms the proportions
should be such that the chords and web members have approximately an
equal weight.
The bay widths should be proportioned so that the diagonal members are inclined
at approximately 50° or slightly steeper. For large-span trusses subdivision of the
bays is necessary to avoid having excessively long web members.
The Pratt, Howe and Warren trusses, Fig. 19.3(a)–(c), have an economic span
range of between 40 m and 100 m for railway bridges and up to 150 m for road
bridges. For the shorter spans of the range the Warren truss requires less material
than either the Pratt or the Howe trusses. For medium spans of the range the Pratt
or Howe trusses are both more favourable and by far the most common types. For
large spans the modified Warren truss and subdivided Parker (inclined chord) truss
are the most economical.
For spans of between 100 m and 250 m the depth of the truss may be up to four
times the economic bay width, and in such a case the K-, diamond or Petit (subdivided
Pratt or Howe) trusses are more appropriate. For the shorter spans of the
range the diamond or Petit trusses, by their nature, are subject to very high secondary
stresses. In such a case the K-truss, with primary truss members at all nodes,
is more appropriate as joint deflections are uniform, greatly reducing the secondary
stresses.
For spans greater than 150 m variable depth trusses are normally adopted for
economy.