When the strain is transverse to tbeplane of the two beams, the principles laid down in No. 85, 86, of the article STRENGTH OP MATERIALS, will direct the artist in placing his mortise. Thus the mortise in a girder for receiving the tenon of a binding joist of a floor should be as near the upper side as possible, because the girder becomes concave on that side by the'strain. But as this exposes the tenon of the binding-joist to the risk of being torn off, we are ob liged to mortise farther down. The form (fig. 294 generally given to this joint is extremely jicious. The slop mg part a b gives a very firm support to the additional bearing e d, withciut much weakening of the girder. This form should be copied in every case where the strain has a similar direction.
The joint that most of all demands the careful attention of the artist is, that which connects the ends of beams, One of which pushes the other very obliquely, putting it into a_ of extension. The most familiar instance of this is the foot of, a rafter pressing on the tie-beam, and thereby drawing it away • from the other wall. When the direction is very oblique (in which case the extending strain is the greatest), it is difficult to give the foot of the rafter such a hold of the tie-beam as to bring many of its fibres into the proper action. There would be •little difficulty if we could allow the end of the tie 'beam to project to a small distance beyond the foot of the rafter : but, indeed, the dimensions which are given • to tie•eams, for other reasons, are always sufficient to give enough of abutment when judici ously employed. Unfortunately this joint is much exposed to failure by the effects of the weather. It is much exposed, and frequently perishes by rot, or becomes so soft and friable that a very small force is sufficient, either for pulling the filaments out of the tie-beam, or for crushing them together. We are therefore obliged to secure it with particular atten tion, and to avail ourselves of every circumstance of construction.
One is naturally disposed to give the rafter a deep hold by a long tenon ; but it has been frepent ly observed in old roofs that such tenons break off. Frequently they are observed to tear up the wood that is above them, and push their way through the end of the tie-beam. This in all probability arises from the -first sagging of the roof, by the compression of the raf ters and of the head of the king-post. The head of the rafter descends, the angle with the tie•beam is dimi nished by the rafter revolving roundits step in the tie •eam. By this motion the heel or inner angle of the
rafter becomes a fulcrum to every long and powerful lever much loaded. The tenon isithe other arm, very short, and being still fresh, it is therefore very powerful. It therefore forces •up -the wood that is above it, 'tearing it out from between the cheeks of the mortise, and then pushes it along. Carpenters have therefore given up long tenons, and give to the toe of the tenon a shape which abuts firmly, in the •direction of the threat, on the solid bottom of the mortise,- which is well supported on the under side by • the wall plate. This form has the farther advan tage of having no tendency to tear up the end of the mortise. This form .is represented in fig. SO. The tenon has a small portion ab cut perpendicular to the surface of the tie-beam, and the rest be is perpendicular to the rafter. (See Note CC..) But if the tenon is not sufficiently strong (and it is not so strong as the rafter, which is thought not to be stronger than is necessary), it will be crushed, and then the rafter will shade out along'the surface of the beam. It is therefore necessary to call in the assistance of the whole rafter. It is in this distribu tion of the strain among the various abutting parts that the varieties of joints and their merits chiefly consist. It would be endless to describe every nos trum, and we shall only mention a few that are moat generally approved of.
The aim in fig,. 81. is to make the abutments ex actly perpendicular to the thrusts. (See Note CC.) It does this very precieely ; and the share which the tenon and the shoulder have of the whole may be what we please, by the portion of the beam that we notch down. if the wall plate lie duly before the heel of the rafter, there is no risk of straining the tie across or breaking it, because the thrust is made to direct to that point where the beam -is The action is the same as against the joggle on the head or foot of a king-post. We have no doubt but that this is a very effectual joint. It is not, however, much practised. It is said that the sloping seam at the shoulder lodges water ; but the great reason seems to be a secret notion that it weakens the tie. beam. If we consider the direction in which it as a tie, we must acknowledge that this form takes the best method for bringing the whole of it into ac. titan.