ME C

160

ME C

No. 1, is stronger than those more ingenious scarfings. Itmay be strengthened against lateral bendings by a littletongue, or by a sally ; but it cannot have both.

The strongest of all methods of piecing a tie-beam wouldbe to set the parts end to end, and grasp them between otherpieces on each side, as in Figure 33. This is what the ship-carpenter calls fishing a beam; and is a frequent practice foroccasional repairs. M. Perronet used it for the tie-beams, orstretchers, by which he connected the opposite feet of a centre,which was yielding to its load, and had pushed aside one ofthe piers above four inches. Six of these not only withstooda strain of 1,800 tons, but, by wedging behind them, hebrought the feet of the truss inches nearer. The stretcherswere 14 inches by 11, of sound oak, and could have withstoodthree times that strain. M. Perronet, fearing that the greatlength of the bolts employed to connect the beams of thesestretchers would expose them to the risk of bending, scarfedthe two side pieces into the middle piece. The scarfing wasof the triangular kind (trait de Jupiter), and only an inchdeep, each face being two feet long, and the bolt passedthrough close to the angle.

In piecing the pump rods, and other wooden stretchers ofgreat engines, no dependence is had on scarfing ; and theengineer connects everything by iron straps. But we doubtthe propriety of this, at least in cases where the bulk of thewooden connection is not inconvenient.

These observations must suffice for the methods employedfor connecting the parts of a beam ; and we now proceed toconsider what are more usually called the joints of a piece ofcarpentry.

Where the beams stand square with each other, and thestrains are also square with the beams, and in the plane ofthe frame, the common mortise and tenon is the most perfectjunction. A pin is generally put through both, in order to;keep the pieces united, in opposition to any force which tendsto part them. Every carpenter knows how to bore the holefor this pin, so that it shall draw the tenon tight into themortise, and cause the shoulder to butt close, and make neatwork ; and he knows the risk of tearing out the bit of thetenon beyond the pin, if he draw it too much. We may justobserve, that square holes and pins are much preferable toround ones for this purpose, bringing more of the wood intoaction, with less tendency to split it. The sliip-carpentershave an ingenious method of making long wooden bolts,which do not pass completely through, take a very fast hold,though not nicely fitted to their holes, which they must notbe, lest they should be crippled in driving. They call it fox-tail wedging. They stick into the point of the bolt a verythin wedge of hard wood, so as to project a proper distance :when this reaches the bottom of the hole by driving the bolt,it splits the end of it, and squeezes it hard to the side. Thismay be practised with advantage in carpentry. If the endsof the mortise are widened inwards, and a thin wedge be putinto the head of the tenon, it will have the same effect, andmake the joint equal to a dovetail. But this risks the split-ting of the piece beyond the shoulder of the tenon, whichwould be unsightly. This may be avoided as follows :—Letthe tenon, t, Figure 34, have two very thin wedges, a and c,stuck in near its angles, projecting equally : at a very smalldistance within these, put in two shorter wedges, b, d, andmore within these, if necessary. In driving this tenon, thewedges a and c will take first, and split off a thin slice, whichwill easily bend without breaking. The wedges b, d, willact next, and have a similar effect, and the others in suc-cession. The thickness of all the wedges taken togethermust be equal to the enlargement of the mortise toward thebottom.

The mortise in a girder for receiving the tenon^binding joist of a floor should be as near the upper ® u \ypossible, because the girder becomes concave on that sithe strain. But as this exposes the tenon of the bnjoist to the risk of being torn off, we are obliged to m ^farther down. The form (Figure 35) generally given w ajoint is extremely judicious. The sloping part a b, g 1 ^very firm support to the additional bearing c d, withoutweakening of the girder. This form should be cop ie 'every case where the strain has a similar direction. 0 (

The joint that most of all demands the careful atten ^ 0 fthe artist, is that which connects the ends of beams,which pushes the other very obliquely, putting it into ,lof extension. The most familiar instance of this is 41 -ir itof a rafter pressing on the tie-beam, and thereby drawaway from the other wall. When the direction 19 itoblique (in which case the extending strain is the g re ?**f tl> e iis difficult to give the foot of the rafter such a hold ^ ;tie-beam as to bring many of its fibres into the proper ' j 0 1 1There would be little difficulty if we could allow the 0 f

the tie-beam to project to a small distance beyond the t o ;

the rafter; but, indeed, the dimensions which are S' ^{yntie-beams, for other reasons, are always sufficientenough of abutment when judiciously employed. ^ t3 ofnately, this joint is much exposed to failure by the e . | ieS l>/the weather. It is much exposed, and frequently P erl p (ir ce ' 5rot, or becomes so soft and friable, that a very small ‘ al ji,sufficient, either for pulling the filaments out of the iie ' e i t®or for crushing them together. We are therefore oh ° 9 o>secure it with particular attention, and to avail ours eevery circumstance of construction. j )0 j ( 1 W

One is naturally disposed to give the rafter a deep 0 lt>a long tenon ; but it has been frequently observed)roofs, that such tenons break off. Frequently f j p® 8 * 1observed to tear up the wood that is above them,_ url jj-pf®'their way through the end of the tie-beam. This, > n c0 n>'bability, arises from the first sagging of the roof, by 1pression of the rafters and of the head of the king-p 08 ^^,, |8head of the rafter descends ; the angle, with the tie - @ ti e 'diminished by the rafter revolving round its step in >2beam. By this motion the heel, or inner angle of 44becomes a fulcrum to a very long and powerful 1® ^loaded. The tenon is the other arm, very short, c p ,

ivee"rf

still fresh, it is therefore very powerful. It therefor ^up the wood that is above it, tearing it out from be 4 'cheeks of the mortise, and then pushes it along. ‘ to e 6have therefore given up long tenons, and give to 0 f _the tenon a shape which abuts firmly, in the direct' jufthrust, on the solid bottom of the mortise, which is n h 1

tear

all 4 '

ti> e v

to 1

ported on the under side by the wall-plate,the farther advantage of having no tendency to ^end of the mortise ; and is represented in Fig 1 * 19

tenon has a small portion, a b, cut perpendicular t0 ^ rface of the tie-beain, and the rest, b c, is perpendic"

it

to call in the assistance of the whole rafter.

It is 10

.rts, jt

tribution of the strain among the various abutting P^t.the varieties of joints and their merits chiefly = |,all 0

> Si 1 * 1 '

.rould be endless to describe every nostrum, andtymention a few that are most generally approved o e %W iThe aim, in Figure 37, is to make the abutm g ]y;perpendicular to the thrusts. It does this very of tlie "the shaie which the tenon and the shoulder have

rafter. ,

But if the tenon be not sufficiently strong (» nd . on g C *strong a 3 the rafter, which is thought not to be sti 11 s 1 v

. 1 then the raftei^sg |

out along the surface of the beam. It is therefor®^ tlo 3 ^ :