Trusses - Guidance on overall concept - 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 pro- portions 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 40m and 100m for railway bridges and up to 150m 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.

Common types of bridge trusses: (a) Pratt, (b) Howe, (c) Warren, (d) modified Warren, (e) Parker, (f) K, (g) diamond, (h) Petit
Fig. 19.3 Common types of bridge trusses: (a) Pratt, (b) Howe, (c) Warren, (d) modified
Warren, (e) Parker, (f) K, (g) diamond, (h) Petit

For spans of between 100m and 250m 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 sec- ondary 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 150m variable depth trusses are normally adopted for economy.

The spacing of bridge trusses depends on the width of the carriageway for road bridges and the required number of tracks for railway bridges, in addition to con- siderations regarding lateral strength and rigidity. However, in general the spacing should be limited to between  1/18 and 1/20 of the span, with a minimum of 4m to 5mfor through trusses and approximately 1/15 of the span, with a minimum of 3m to 4m, for underslung trusses.

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