Use of Trusses in Buildings.

The most common use of trusses in buildings is to provide support to roofs, floors and such internal loading as services and suspended ceilings. There are many types and forms of trusses; some of the most widely used are shown in Fig. 19.1.The type of truss adopted in design is governed by architectural and client requirements, varied in detail by dimensional and economic factors.

The Pratt truss, Fig. 19.1(a) and (e), has diagonals in tension under normal verti- cal loading so that the shorter vertical web members are in compression and the longer diagonal web members are in tension. This advantage is partially offset by the fact that the compression chord is more heavily loaded than the tension chord at mid-span under normal vertical loading. It should be noted, however, that for a light-pitched Pratt roof truss wind loads may cause a reversal of load thus putting the longer web members into compression.

The converse of the Pratt truss is the Howe truss (or English truss), Fig. 19.1(b).

The Howe truss can be advantageous for very lightly loaded roofs in which reversal of load due to wind will occur. In addition the tension chord is more heavily loaded than the compression chord at mid-span under normal vertical loading.The Fink truss, Fig. 19.1(c), offers greater economy in terms of steel weight for long-span high-pitched roofs as the members are subdivided into shorter elements. There are many ways of arranging and subdividing the chords and web members under the control of the designer.

The mansard truss, Fig. 19.1(d), is a variation of the Fink truss which has the advantage of reducing unusable roof space and so reducing the running costs of the building. The main disadvantage of the mansard truss is that the forces in the top and bottom chords are increased due to the smaller span-to-depth ratio.

However, it must not escape the designer’s mind that any savings achieved in steel weight by introducing a greater number of smaller members may, as is often the case, substantially increase fabrication and maintenance costs.

The Warren truss, Fig. 19.1(f), has equal length compression and tension web members, resulting in a net saving in steel weight for smaller spans. The added advantage of the Warren truss is that it avoids the use of web members of differing length and thus reduces fabrication costs.For larger spans the modified Warren truss, Fig. 19.1(g), may be adopted where additional restraint to the chords is required (this also reduces secondary stresses). The modified Warren truss requires more material than the parallel-chord Pratt truss, but this is offset by its symmetry and pleasing appearance. The saw-tooth or butterfly truss, Fig. 19.1(h), is just one of many examples of trusses used in multi-bay buildings, although the other types described above are equally suitable.

Common types of roof trusses: (a) Pratt – pitched, (b) Howe, (c) Fink, (d) mansard, (e) Pratt – flat, (f) Warren, (g) modified Warren, (h) saw-tooth
Fig. 19.1 Common types of roof trusses: (a) Pratt – pitched, (b) Howe, (c) Fink, (d) mansard,
(e) Pratt – flat, (f) Warren, (g) modified Warren, (h) saw-tooth

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