Fig. 246 shows how the joint at the center of the tie-beam of a king-post truss, or any joint between two struts, may be formed.
The tie-beam is shown at A, and B are the struts. The blocks C, set between the struts, receive the thrust from them. They should be notched into the tie-beam A deep enough to take care of any inequality between the thrusts from the two struts, which have a tendency to balance each other. The block is often made of cast iron. It may be omitted altogether, in which case the struts will come close together and bear against each other. The rod D is the king-post which supports the tie-beam A at this point. It is often made of wood and sometimes the struts B are framed into it instead of being framed into the tie-beam A.
Fig. 247 shows a form of connection for the peak of a truss, where the two top chords or principal rafters come together. The plate A acts as a tic to keep these members in place, as does the bent plate B, also. The plate B, moreover, prevents the crushing of the timber by the nut of the king-post tie rod. The purlin supports the rafters and is hollowed out at the bottom to admit the nut D. The two principal rafters bear against each other and must be cut so that the bearing area between them will be sufficient to prevent the crushing of the timber. In light trusses the king-post E is often
made of wood and is carried up between the principal rafters so that these members bear against it on each side. If this construction is adopted it must be remembered that the post is a tension member, and is held up by the principal rafters, and these pieces must be mortised into it in such a way as to accomplish this result.
There are a great many different ways of arranging the details for wood trusses, each case usually requiring details peculiar to itself and unlike those for any other case. There are, therefore, no hard and fast rules which can be laid down to govern the design of these connections. A perfect understanding of the action of each piece and its relation to all of the other pieces is necessary in order to insure an economical and appropriate design. The aim should always be to arrange the details so that there will be as little cutting of the pieces as possible, and so that the stressesmay pass from one piece to another without overstraining any part of the truss.