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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) 544 Trusses The economic span range of the Pratt and Howe trusses may be extended by subdividing the diagonals and the deck support chord as shown for the Petit truss, Fig. 19.3(h), although this often gives rise to high secondary stresses for short to medium spans. In the case of the Warren truss the unsupported length of the chords may be too great for the economic span and depth range; in such a case the modified Warren truss, Fig. 19.3(d), is more appropriate. The variable depth type truss such as the Parker, Fig. 19.3(e), offers an aesthetically pleasing structure. With this type of truss its structural function is emphasized and the material is economically distributed at the cost of having expensive fabrication due to the variable length and variable inclination of the web and top chord members. For economy the truss depth is ideally set at a fixed proportion of the span. As the span increases the truss depth and bay width increase accordingly. The bay width is usually fixed by providing truss nodes on the centrelines of the deck cross- (NN/ (VVV (a) (b) /\/\/\/\/\/ (c) (d) (e) (f) I I (g) (h) 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
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) 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.
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