ME C
161
M E C
ME C 1
61 MEC
w * iat we please, by the portion of the beam that wef aft 1 a ° wr1, If the wall plate lie duly before the heel of theit , r ’ there is no risk of straining the tie across, or breakingb eatn °. ause the thrust is made direct to that point where thejog„i Is su PPorted. The action is the same as against thed°Ubt l° n the head or foot of a king-post. We have noev er bu t that this is a very effectual joint. It is not, how-th practised. It is said that the sloping seam at the
8ec fet ^ ^ 0< dS es water ; but the great reason seems to be athe p n °t* on that it weakens the tie-beam. If we considerIf® 01 ' 011 in which it acts as a tie, we must acknowledgeof . 8 form takes the best method for bringing the whole*nto action.
Zr: t ^ W . re 38 exhibits a form that is more general, butby ( '|. ri -f worse. The part of the thrust that is not bornegi Ve3 te non acts obliquely on the joint of the shoulder, andle whole a tendency to rise up and slide outward.a i ® ®houlder joint is sometimes formed like the dotted line,the tr °f -Figure 38. This is much more agreeable to
it no) . e Principle, and would be a very perfect method, werelitt] e *d |at the intervals, b d and df, are so short that thetheir ^° 0( len triangles, bed, de f will be easily pushed off
. *C ses > hd ' d f-
>s 3^ seems to have the most general approbation. It
' ) °°ks'*p lnt recommen( Ied by Price, (p. 7), and copied into allshom i° c . ar _Pentry, as the trpejoint for a rafter foot. The visiblethe ’ s flush with the upper surface of the tie-beam.
ntigf e J=>e of the tenon at the tie nearly bisects the obtuseNl(;w /-ned by the rafter and the beam, and is therefore• 8 ^earl'^ °3fll u e to the thrust. The inner shoulder, a c,
*s cm- f P ei 'pendicular to b d. The lower angle of the tenon
n^Wpan 1 ' 1 - 20 "* 3117 ’ aSaterf< , ,
‘he ( i: ff w is a section of the beam and rafter foot, showing
\y e ® rer >t shoulders.
of° ? 0t P erce ’ ve the peculiar merit of this joint. Thee, lly t tlie three oblique abutments, ab,ae, e d, is undoubt-ed tlier* 14 - 6 ^ le bear on the outer end of the mortise,
. Me r ? 18 no other part of the tie-beam that makes imme-the ( i- ISI:a ^ ce - Its only advantage over a tenon extendingl' ,Qo ^ ab reCt -° n ^ ie thrust, is, that it will not tear up the
lav in» - J . Ve tt. Had the inner shoulder had the form e c i,'hhed® S A lC(l > i c, perpendicular, it certainly would havele -be atll le Powerfully in stretching many filaments of the0|, t the ’ an A would have had much less tendency to forcethe mortise. The little bit, c i, would have^'it, a j * *® sliding upwards along e c. At any rate, thebein g flush with the beam, prevents any sensible
39 111 ” ' l,onlder ' “«•
c Joint ’ a 0 ' ’ S a s ’ ra P^ er ’ an( I ' n our opinion a prefer-of^nter' t observe it practised by the most eminent
cob* • a A oblique thrusts ; but it surely employs lessb 6a keni no . e ? 10n °i > the tie-beam than might be used without^ Ale at least when it is supported on the other side
' all 'P la te.
r j>en tp > -No. 3, is also much practised by the first
to ,;/">•(! o
p *igur e oq 13 Proposed by Mr. Nicholson, as preferable8u n is v 9, No. 3, because the abutment of the innerJjPoge* er supported. Thi^ is certainly the case ; but ithi be * e W ^°i e rafter to go to the bottom of the socket,
\'q/ 0 ' 3e dicker than the rafter. Some may thinkhi a ^ e r tli a ' vv ®aken the beam too much, when it is noP^ 'tr e J 1 ^e rafter is thick : in which case they thinkliMl a Ps thg^j a fl ee P el ‘ socket than Nicholson has given it.hr 7 a ini aavan tages of Nicholson’s construction may beFigure 41 > No ‘ 2 -
ti, b ® Ae form of these butting joints, great care\\ 2 x
should be taken that all parts bear alike, and the artist willattend to the magnitude of the different surfaces. In thegeneral compression, the greater surfaces will be less com-pressed, and the smaller will therefore change most. Whenall has settled, every part should be equally close. Becausegreat logs are moved with difficulty, it is very troublesome totry the joint frequently, to see how the parts fit ; thereforewe must expect less accuracy in the interior parts. Thisshould make us prefer those joints whose efficacy dependschiefly on the visible joint.
It appears, from all that we have said on this subject, thata very small part of the cohesion of the tie-beams is sufficientfor withstanding the horizontal thrust of a roof, even thoughvery low pitched. If therefore no other use be made of thetie-beam, one much slenderer^ may be used, and blocks maybe firmly fixed to the ends, on which the rafters might abut,as they do on the joggles on the head and foot of a king-post.Although a tie-beam has commonly floors or ceilings to carry,and sometimes the workshops and store-rooms of a theatre,and therefore requires a great scantling, yet there frequentlyoccur in machines and engines very oblique stretchers, whichhave no other office, and are generally made of dimensionsquite inadequate to their situation, often containing ten timesthe necessary quantity of timber. It is therefore of impor-tance to ascertain the most perfect manner of executing sucha joint. We have directed the attention to the principlesthat are really concerned in the effect. In all hazardous cases,the carpenter calls in the assistance of iron straps ; and theyare frequently necessary, even in roofs, notwithstanding thissuperabundant strength of the tie-beams. But this is gene-rally owing to the bad construction of the wooden joint, orto the failure of it by time. Straps will be considered intheir place.
There need but little to be said, of the joints at a joggleworked out of solid timber ; they are not near so difficult asthe last. When the size of a log will allow the joggle toreceive the whole breadth of the abutting brace, it oughtcertainly to be made with a square shoulder ; or, which isstill better, an arch of a circle, having the other end of thebrace for its centre. Indeed, this in general will not sensiblydiffer from a straight line perpendicular to the brace. Bythis circular form, the settling of the roof makes no change inthe abutment ; but when there is not sufficient stuff for this,we must avoid bevel joints at the shoulders, because thesealways tend to make the brace slide off. The brace, inFigure 42, must not be joined as at a, but as at b, or someequivalent manner.
When the very oblique action of one side of a frame ofcarpentry does not extend, but compress the piece on whichit abuts, as in Figure 21, there is no difficulty in the joint.Indeed, a joining is unnecessary, and it is enough that thepieces abut on each other ; and we have only to take carethat the mutual pressure be equally borne by all the parts,and that it do not produce lateral pressures, which may causeone of the pieces to slide on the butting joint. A very slightmortise and tenon is sufficient at the joggle of a king-post,with a rafter or straining beam. It is best, in general, tomake the butting plain, bisecting the angle formed by thesides, or else perpendicular to one of the pieces. In Figure
42, No. 2, where the straining beam, a b, cannot slip awayfrom the pressure, the joint a is preferable to b, or indeed toany uneven joint, which never fails to produce very unequalpressures on the different parts, by which some are crippled,others are splintered off, &c.
When it is necessary to employ iron straps for strengthen-ing a joint, a considerable attention is necessary, that we mayplace them properly. The first thing to be determined is the