Fig. 82. exhibits a form that is more general, but certainly worse. What part of the thrust that is not borne by the tenon acts obliquely on the joint of the shoulder, and gives the whole a tendency to rise up and slide outward.
The shoulder joint is sometimes formed like the dotted line abcdefg of fig. 32. This is much more agreeable to the true principle, and would be a very perfect method, were it not the intervals bd and df are se short that the little wooden triangles bid, def, will be easily pushed off their bases bd, Fig. 33. No. 1. seems to have the most general approbation. It is the joint recommended by Price, and copied into all books of carpentry as the tree joint for a rafter foot. The visible shoulder-joint is flush with the upper surface of the tie-beam. The angle of the tenon at the tie nearly bisects the obtuse angle formed by the rafter and the beam, and is therefore somewhat oblique to the thrust. The inner shoulder ac is nearly perpendicular to bd. The lower angle of the tenon is cut off horizontally as at ed. Fig. 84. is a section of the beam and rafter foot, showing the different shoulders.
We do not perceive the peculiar merit of this joint. The effect of the three oblique abutments, oh, ac, ed, is undoubtedly to make the whole bear on the outer end of the mortoise, and there is no other part of the tie-beam that makes immediate mite ance. Its only advantage over a tenon extending in the direction of the thrust is, that it will not tear up the wood above it. Had the inner shoulder had the form sci, having its face is perpendicular, it would certainly have acted more powerfully in stretching many filaments of the tie-beam, and would have had much leas tendency to force out the end of the mor. tise. The little bit ci would have prevented the sliding upwards along ec. At any rate, the joint ab being flush with the beam, prevents any sensible abutment on the shoulder ac.
Fig. 88. No. 2. is a simpler, and in our opinion a preferable, joint. We observe it practised by the most eminent carpenters for all oblique thrusts; but it surely ,employs leas of the cohesion of the tie bean] than might be used without weakening it, at least -when it is supported on the other side by the wall plate.
Fig. 33. No. 8. is also much practised by the first carpenters.
is certainly the case ; but it supposes the Idolg rafter to go to the bottom of the socket, and the beam to be thicker than the rafter. Some may think that this will weaken the beam too much, when it is no broader than the rafter is thick ; in which case they think that it requires a deeper socket than Nicholson has given it. Perhaps the advantages of Nicholson's construction may be had by a joint like fig. 85. No. 2.
Whatever is the form of these butting joints, great care should be taken that all parts bear alike, and the artist will attend to the magnitude of the different surfaces. In the general compression, the greater surfaces will be less compressed, and the smaller will therefore change most. When all has settled, every part should be equally close. Because great logs are moved with diffieulty, it is very trou me to try the joint frequently to see how the parts fit ; therefore we must expect less accuracy in the interior parts. This should make us prefer those joints whose efficacy depends 'chiefly on the visible joint.
It appears from all that we have said on this subject, that a very small part of the cohesion of the tie-beam is auffiment for withstanding the horizontal thrust of a roof, even though very low pitched. If therefore no other use is made of the tie-beam, one much slenderer may be used, and blocks may be firmly fixed to the ends, on which the rafters might abut, as they do on the j'oggles on the head and foot of a king-post. Although a tie-beam has common ly floors or ceilings to carry, and sometimes the workshops and store-rooms of a theatre, and there fore requires a great scantling, yet there frequent ly occur in machines and engines very oblique stretchers, which have no other office, and are ge nerally made of dimensions quite inadequate to their situation, often containing ten times the necessary quantity of timber. It is therefore of importance to ascertain the most perfect manner of executing such a joint. We have directed to the prin ciples that are really concerned in the effect. In all hazardous cases, the carpenter calls in the assist ance of iron straps ; and they are frequently neces sary, even in roofs, notwithstanding this superabun dant strength of the tie. beam. But this is generally owing to bad construction of the wooden joint, or to the failure of it by time. Straps will be consider ed in their place.