Queen-post trusses are used for spans over 30 feet, and contain two perpendiculars to brace up the tie-beam spanning the walls.
Fig. 379 is a "queen-post truss" for a 32-feet span. The same form is suitable up to about 42 feet span, and beyond that size princesses, or intermediate posts and struts, have to be inserted between the "queensn and the heels of the roof, as shown by dotted lines; and it is sometimes necessary to frame a small king-post truss (also shown on the figure by dotted lines) above the straining beam St, to support the ridge. S S is the straining sill, and Q the queens. The other members are known by the same names as in other trusses.
A good rule to ascertain the thickness of queen-post trusses is as follows: Divide the span by 8, and the quotient will be the required thickness (in inches), making up for odd parts by adding another inch for heavy-tiled roofs, and omitting such fraction for slates, as before explained.
Taking the tie-beam for 32-feet span at 11 inches deep, and principal rafters at 6 inches, by adding 1 inch for every 5 feet additional span, we can arrive at their depth for the different roofs.
The struts and middle of the queens are usually square.
Fig. 381 is an illustration of a woodtruss, with an iron king-rod, the joints of which are explained in detail on figs. 382 and 383.
From these it will be seen that a wrought-iron king-bolt, 3/4 inch in diameter, slings up the tie-beam to the apex, where a cast-iron head receives the principal rafters into sockets on either side of the king-rod; which goes through the hollow cast-iron work, and is bolted up from above the apex to the bottom of the tie-beam. The struts are likewise received by cast-iron sockets, screwed on to the top of the tie-beam through which the king-rod is placed.
Fig. 381. 1" Scale.
Fig. 383. 1" scale.
Sometimes, to save expense, a plate is fixed on the underside of the tie-beam and bolted through, to secure the shoe-plate for the struts.
Sometimes the struts butt against each other round the king-rod, as fig. 384; from which it will be seen that there is nothing to keep them in position, laterally, except the spikes which fix them to the beam. This is called a strut and beam joint.
Fig. 364. 1" scale.
Another form of this kind of roof-trussing with wood and iron, is as fig- 385; which is all iron except the principal backs, wrought-iron being used for struts, ties, and king-rod, and cast-iron for heel and head sockets.
The details of joints and construction are: -
Fig. 386 represents joint at A.
Fig. 387 shows the joint at B.
Fig. 388 that at C.
Therefrom the student will see that wood struts could be substituted for the Tee irons, if thought advisable; and the tie-rod could be forged out to a sufficient width to give them a bearing on each side of the king-rod.
Plan looking up Fig 388.
Another form is as fig. 389, the joints of which it is needless to enlarge upon. The student can make them out from others of a similar kind already illustrated in the course of these notes.
The next form of roof to which the student's attention should be drawn is the queen-post roof, used as a flat roof over the straining beam; the only points new to the student being the suspension-rod in the centre, and the beam and strut joints under the straining beam which carries the bearers of the flat to be boarded and covered with lead or zinc (fig. 390).
The Mansard roof (invented by a Frenchman of that name with a view to utilising the roof spaces as much as possible) is s roof similar to fig. 391. The joints are all known to the student, and it only remains to be said that this form of roof can be very much strengthened by the tie-beam being made the head of a bolted and trussed partition below, especially if queen-trussed with intertie built into the enclosing walls, or a partition with no openings in it.
Occasionally cases crop up where trusses are necessary, but cannot be used for various reasons, and the cross-walls are too far apart for ordinary purlins; recourse then has to be made to trussed purlins or ridges, but sometimes the ordinary purlins are flitched to give them additional strength (vide ante, fig. 295), or trussed (as figs. 296 and 297, Chapter VIII (Wood Floors. Naked Floors And Floor-Boards).).
Another way is to build up an ordinary framing, as fig. 392, the joints of which have been already illustrated in different places, or under preceding headings. Here, the wall posts, P, are called punckeons; and the framing may be either of the king or queen-post class, according to circumstances and the bearings between walls. The dimensions of the timbers also follow the ordinary rules.