To return again to our elementary truss of two struts and a tie, it js evident that if the tie is jointed as at A and B, Fig. 33, and we connect these joints with the apex by means of the rods r r, we can, by screwing up on the rods from the top, bring the truss into the shape shown in Fig. 34, and if the parts have sufficient strength to resist the strain, it is evident that the truss will still be in equilibrium.
This gives us another type of truss with a raised tie-beam, and one that is much used both for wooden and steel construction.
The most common forms of this type of truss, for wooden construction, are shown in Figs. 35, 36 and 38. In these trusses only the rafters or principals a a and the struts s s are in compression, the other members being in tension.
The outward thrust of the rafters is resisted by the tie t, t1, t, which is kept in position by the ties r r. Besides resisting the thrust of the rafters, the portion of the tie t also acts as a belly-rod to sustain the thrust of the struts, which act in the same way as the post in a belly-rod truss. The purlins should always be placed opposite the braces s s.
When built of wood this type of truss is most commonly classed with the scissors truss, but is also sometimes referred to as a "trussed rafter roof," as it really consists of two trussed rafters held in position by the ties t and r, as shown in Fig. 37, and the truss could be built in that way, but it would not be an economical shape, the figure being given merely to illustrate the principle of the truss and not as a model to follow.
When built of steel, the type of truss shown by Figs. 35 and 36, is known as a "French" truss.
As in the trusses shown in Figs. 29 and 30, this type requires much greater resistance in the ties and rafters than a truss with a horizontal tie-beam, but the increase in the strains is not quite as great as in Figs. 29 and 30.
The trusses shown in Figs. 35 and 36, are not very well adapted to wood construction on account of the difficulty in making the joints, but for steel roof trusses in which a raised tie is desired, they are both practical and economical.
Fig. 38 shows the same type of truss, built of wood, but with the tops of the rafters cut off and a straining beam inserted between them.