Engineering and Construction

Trusses are now infrequently used for road bridges in the UK because of high fabrication and maintenance costs. However, the recent award-winning Brinnington railway bridge (Fig. 19.2) demonstrates that they can still be used to create efficient and attractive railway structures. In many parts of the world, particularly in developing countries where labour costs are low and material costs are high, trusses are often adopted for their economy in steel. Their structural form also lends itself to transportation in small components and piece-small erection,which may be suitable for remote locations.

Fig. 19.2 Brinnington Railway Bridge

Some of the most commonly used trusses suitable for both road and rail bridges are illustrated in Fig. 19.3. Pratt, Howe and Warren trusses, Fig. 19.3(a), (b) and (c), which are discussed in section 19.2.1, are more suitable for short to medium spans.

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 crossbeams, 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.

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