This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
There are several methods of supporting the purlins on wood trusses, but the method illustrated in Fig. 244 is one of the best as well as the most frequently employed. A block of wood A is set up against the lower side of the purlin, and prevents it from turning about the corner B, which it has a tendency to do. The block is set into the chord of the truss to a depth sufficient to keep the purlin from sliding downward as it receives the weight from the rafters E. This figure also shows the most simple method of framing a strut into the chord of a truss. The strut C is set into the chord D far enough to hold the strut in place. If it is perpendicular to the chord, it need not be so set into it, if the pieces are well nailed together, because in this case there is no tendency for the strut to slide along the chord. Care should be taken not to weaken the chord too much in cutting these mortises.
In Fig. 245 are shown the most common methods of forming the joint between the top chord and the tie-beam of a truss. The connection shown at A depends upon the bolts for its strength, while that shown at B depends upon the wrought-iron straps E, which are bent so as to engage notches cut in the tie-beam F. The piece C is very often added beneath the tie-beam, at the bearing, to strengthen it at this point, where the beam is subject to considerable bending stress. The block D is merely for filling and to protect the bolts where they pass between the chord and the tie-beam. It may be omitted in many cases. The plate G is placed between the nuts or bolt heads and the wood to prevent the crushing of the latter. Washers should be used with all bolts for this purpose.
Fig. 244. Method of Supporting the Purlins on Wood Trusses.
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.
Fig. 245. Two Methods of Forming a Joint between Both Chord and Tie-Beam of a Truss.
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.
HALL AND PARTIALLY ENCLOSED STAIRCASE IN LONG HALL, GREYROCKS, ROCKPORT, MASS.
Frank Chouteau Brown, Architect, Boston, Mass. For Plans and Exteriors, See Vol. I, Pages 272, 282, and 299.
HALL AND STAIRCASE IN HOUSE AT WOLLASTON, MASS.
Frank Chouteau Brown, Architect.
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 tie 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 C 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.
Fig. 246. Method of Forming Joint at Center of Tie-Beam in King-Post Truss.
Fig. 247. Construction of the Peak Connection of a Truss.
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 stresses may pass from one piece to another without overstraining any part of the truss.