There needs but little to be said of the joints at a joggle worked out of solid timber; they are not near so difficult as the last. When the size of a log will allow the joggle to receive the whole breadth of the abutting brace, it ought certainly to be made with dosquare shoulder ; or, which is still better, an arch of a circle, having the other end of the brace for its centre. (See Note EE.) Indeed this in neral will not sensibly differ from a straight line perpendicular to the brace. By this circular form, the settling of the roof makes no change in the abut ment ; but when there is not sufficient stuff for this, we must avoid bevel joints at the shoulders, because these always tend to•make the brace slide off. The brace in .fig. 36. -No. 1. must not be joined as at 6, but air at a, or some equivalent manner. Observe the joints at the head of the main posts of Drury Lane Theatre, Fig. 41. Plate Lill.
When the very oblique action of one side of a frame of carpentry does not extend but compress the piece on which it abuts (as in fig. 1i.), there is no difficulty in the joint. Indeed a joining is un necessary, and it is enough that the pieces abut on each other ; and we have only to take care that the mutual pressure be equally borne by all the parts, and that it do not produce lateral pressures, which May cause one of the pieces to slide on the butting joint. A very slight mortise and tenon is sufficient at the joggle of a king post with a rafter or strain ing beam. It is best, in general, to make the but ting plain, bisecting the angle formed by the sides, or else perpendicular to one of the pieces. In fig. 36. No. 2. where the straining beam ab cannot slip away from the pressure, the joint a is preferable to b, or indeed to any uneven joint, which never fails to produce very unequal pressures on the different parts, by which some are crippled, others are splin tered off, &c.
When it is necessary to employ iron straps for strengthening a joint, a considerable attention is ne cessary, that we may place them properly. The first thing to be determined is the direction of the strain. This is learned by the observations in the beginning of this article. We must then resolve this strain into a strain parallel to each piece, and another perpendicular to it. Then the strap which is to be made fast to any of the pieces, must be so fixed, that it shall resist in the direction parallel to the piece. Frequently this cannot be done ; but we must come as near to it as we can. In such cases we must suppose that the assemblage yields a little to.the pressures which act on it. We must ex
amine what change of shape a small yielding will produce. We must now see how. affect the iron strap which-we have- already supposed attached to the joint in some manner that we thought suita ble. This settling will perhaps draw the pieces away from it, leaving it loose and unserviceable (this fre quently happens to the plates which are put to cure the obtuse angles of butting timbers, when their bolts are at some distance from the angles, especial ly when these plates are laid on the inside of the an gles) ; or it may cause it to compress the pieces harder than before ; in which case it is answering our intention. But it may be producing cross strains, which may break them, or it may be crippling them. We can hardly give any general rules ; but the reader will do well to read what is written in No. 86, and 41, of the article ROOF, (EnCyd.) In No. 36, he will see the nature of the strap or stirrup, by which the king post carries the tie beam. The strap that we observe most generally ill-placed is that which connects the foot of the rafter with the beam. It only binds down the rafter, but does not act against its horizontal thrust. It should be placed farther back on the beam, with a bolt through it, which will, allow it to turn round. It should embrace the rafter almost horizontally near the foot,' and should be notched square with the back k the rafter. Such a construction is represented in fig. 37. By moving round the eyelolt, it folio** iheittfter, and'cannot pinch and cripple it, which it 'elstays does In its or.• dinary form. We are of opinion that straps which have eye-bolts in the very angles, and allow all 'mo tion round them, are of all the Mat perfect. A branched strap, such as may'rit once bind the 'king post and the two braces which bott On its foot, will be more serviceable if it have a joint. When a roof warps, those branched steeps break the tenons, by affording a fulertun in one of theirtolts. An attentive and judicious ettilit will cohsider how the beams will act on such oddagiorts, and will avoid giving rise to these great Muftis 'by levere.—A skil ful carpenter never eMploys Mary straps, 'consider.' iitg them as auxiliaries 'to 'his art, md'subject to imperfections in workniasithip which he 'cannot discern nor amend. We lutist 'refer reader 'to Nicholson's Carpenter and Joiner's Asdistant for a more particular account of the Various forms of stir rups, screwed rods, and otherison work for 'carrying tie.bearns, &c.