(2) Roof-trusses with SusEhendea' suspension of a beam— that is, its support from above somewhere in the course of its length—may be either throughout its entire length, if there be no other support inside the building from below, or only upon a more than usually large free-lying part of the beam. The suspension itself may be either single, double, or coin pound.
SlISPellS1071-iniSS. —The roof-truss in Figure 27 (pl. 3) repre sents an instance of single suspension, the beam at its middle being fast ened to a suspended post which, in turn, is held by the oblique timbers directed toward the supported ends of the suspended beam. The suspended post (king-post) takes the place of the standing post, and thus appears only in the main couple. The rest of the ceiling joists are held by girders fast ened above or below the tie beam; if not, as assumed in this diagram, the joists, as they run lengthwise of the building, are fastened directly to the tie beam, which seryes as a girder. The ceiling joists are hnng to the tie beam by screw-bolts. If the rafters are to be again supported at any point in the course of their length, the necessary purlins, as the Figure shows, are to be laid on a horizontal double straining--piece, which is bolted to the inclined struts and to the post. The simple king-post roof is suited for depths of buildings that do not exceed 32 feet.
Double 2S shows a roof with two suspended posts (queen-post roof) and " sunken joists." The support of the ceiling joists is effected by g-,irders lying upon the tie beam at the foot-end of the queen-post. Besides the inclined struts, we have here also the straining beam, which serves to resist the pressure made in the inclined pieces against the posts by the suspension. In other respects the queen-post roof is treated in a manner corresponding to the double standing-post roof, and, like the latter, is suitable for depths of buildings of from 32 to 46 feet.
Combination 29 gives an example of three suspended posts (combination of king- and queen-post roof) built over the apartments, meeting at right angles, a point in the construction being that, instead of a full-joist course, tie beams only, are present; so that the girders above and below are not necessary. The illustration represents the con
struction of the roof-truss in the basilica which Vitruvins, who lived in the time of the Roman emperor Augustns, projected for the Julian colony at Fano, and described in his work De It has been restored by Viollet-le-Duc, partly from such ruins of the basilica as could be depended upon as guides, and partly from the brief descriptions of Vitruvius. The span is 58Y2 feet. The triple-truss frame is suited for depths of buildings from 46 to 65Y2 feet, and is formed, as the illustration shows, by combina tion of the single with the double truss, it being necessary here to construct the truss-posts of the latter of two timbers laid together and bolted.
Truss-frames over different portions of the beams will not require any particular description or illustration after what has already been said; it may be stated, however, that in some cases truss-frames are also arranged lengthwise with the roof.
(3) wilhoul Joists. —When a closed attic is not required and the view of the roof construction does not interfere with the manner in which the room is utilized, the roof beams proper may be omitted entirely, or, as Figure 3o (j5/. 3) shows, the tie beam alone may be used, the former being resorted to chiefly in those cases where the interior is to be kept, as far as possible, open, like a hall. The interior here is required not only to uphold the roofing material and its support, but also to resist the thrust of the roof upon the enclosing walls to the greatest practicable degree. This is effected either by the suitable connection of long timbers, or by the aid of iron rods or cables, or, finally, by plank rafters.
Figure 33 shows the roof-truss constructed by Von Moller over the rid ing-school at Wiesbaden. Purlins instead of rafters are used in this case for supporting the roof material. They are secured at certain places to a sort of rafter. The latter are, in turn, so supported by various struts, which cross one another and are partly doubled, and also by a horizontal double tie, that the thrust upon the enclosing walls is almost overcome.