For open roofs of wide span arched trusses are generally the most economical, and as a rule give the most pleasing appearance.

The economy of an arched truss lies in the fact that the principal compression members follow approximately the line of greatest strain, so that the bracing can be made very light. Thus, if the frame shown in Fig. 93 is so built that the joints come in the line of a parabola with the lower ends secured by a horizontal tie, and all the joints are uniformly loaded, the frame will remain in equilibrium without bracing. In practice perfect equality of loads cannot be maintained, and the weight of the tie must also be supported from the upper chord, so that to make a practical truss it is necessary to introduce bracing between the arched rib and the tie, either as shown in Fig. 94 or Fig. 95.

Fig. 92.   Elevation of Truss and Plan of Two Trusses Showing Lateral Bracing.

Fig. 92. - Elevation of Truss and Plan of Two Trusses Showing Lateral Bracing.

The braces provide for the inequality of the loads, and the tension members take up the thrust in the braces. The effect of the inequality of the loads, however, is usually such that only very slight strains are brought upon the bracing, so that the amount of material required for a truss of this shape is considerably less than in a truss with a straight rafter. That the framework shown in Fig. 93 shall be in equilibrium, the joints must be in the curve of a parabola, but as bracing is always necessary, the upper chord may have the form of the arc of a circle without greatly increasing the strains in the bracing.

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Fig. 93.

Trusses of the types shown in Figs. 94 and 95 have been more extensively used for bridges than for roofs, as they are not so well adapted to the usual conditions of roof construction. For a segmental or parabolic roof, however, this type makes a very economical truss.

In the truss shown by Fig. 94, the vertical pieces B, C and D are in compression when the load is applied at the top, and the vertical member A has no strain except the weight of the tie-rod. If the loads are applied at the bottom all the members of the truss except the curved rafter will be in tension.

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Fig. 94.

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Fig. 95.

With the bracing arranged as shown in Fig. 95, the diagonals are always in compression under a vertical load; the centre vertical has no strain except such as comes from the tie-beam and its load.

The arrangement of the bracing shown in Fig. 95 is the best for a wooden truss.

The arch form in roof trusses is used principally in the types known as:

A. Bowstring and Crescent Trusses.

B. Segmental Arched Ribs.

C. Braced Arches.