MEC

159

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of tl le fl T ;

fil atn nDr _ es m the intervals is able to bring the uppermostt| le ® nts into full action, the form A is stronger than b, intt°^ 0rt '°n °i' tb e greater distance of the upper filamentsle ax >s of the fracture : this may be greater than theIf t . nce °f the thickness, if the wood is very compressible.an ( } t'u S u dge©n he in the middle, the effect, both of the jogglesOpp er e . sca *'fings, is considerably diminished ; and if it is on thesi t J. Sl( i e > the scarfings act in a very different way. In this° n ’ t * le loads on the arms are also applied to the5 0iir/ tbe joggled beam is still more superior to theit}];,,,- 01 le - This will be best understood by resolving it in

tl] U ® lnat ‘ 011 into a trussed frame. But when a gudgeon is

str,i

str a

mi,

°n that side of the lever which grows convex by thenet-., 11 1® usual to connect it with the rest by a powerfulto ’ w uich embraces the beam, and causes the opposite point•nt etl < ? me _ tbe resisting point. This greatly changes theit j nto ac fions of the filaments, and, in some measure, bringsty e ,. e ■ t ie Sat ne state as the first, with the gudgeon below.t° (w 1 Possible to have the gudgeon on the upper side, and- ln g the whole ’ ■ -

3tfi

, ot all ; because, in general, the resistance to com-J°gg] e( j 18 greater than to extension. In every situation thec ut e( j. J? a, n bas tlie advantage ; and it is the easiest exe-Stlen gth h Iria / frequently gain a considerable accession ofPart tyi. tins building up of a beam; especially if thePart of 18 sketched by the strain be of oak, and the other•• Fir being so much superior to oak as a pillar (if

into action without a strap, it would be the'

a SSchoj-v . . , © — -- —r- --— ” i- \“

•OiieU n „ 0l0e ks experiments may be confided in), and oak so'‘dv-an, 1 era *’le as a tie, this construction seems to unite bothP<Hv ei .f '? e . 8, -But we shall see much better methods of makingObs(. r evers > girders, &c. by trussing.

° a tlt e j. 3 tb at the efficacy of both methods depends entirely^'de ! i Cuit y °f causing the piece between the cross joints* be n J °. U ® tbe timber to which it adheres. Therefore, ifSoon a ° j rate » is wrong to make the notches deep ; for,l «at to t , S , le y are so deep that their ends have a force suffi-^ a ' a ed v Ua 1 tb e slice along the line of junction, nothing isfc!t P e ndi t , ma ^' n g them deeper ; and this requires a greaterSc:n.e re timber.

are frequently made oblique, as in Figure 28, but) Point Ue i* lat tb * 8 ’ 8 a ba< J P ract ice. It begins to yield atbi'njj, ] ler<i the wood is crippled and splintered off, or at(/ Sc piee z ’ out a little: as the pressure increases, this part,

ln © broader, causes the solid parts to rise a littlePifei r Us > a nd - ■ - F

a nt .S' ve s them some tendency, not only to pushi' “ littl ° 0nists along the base, but even to . tear theme - similar reasons, we disapprove of the

t scn^-n Ctice of many artists, to make the anglest ' v o . ai *i n gs acute, as in Figure 29. This often causesj^ys ij e e ° es to tear each other up. The abutments shouldto the directions of the pressures.f'» * I1 jun c t' * 58 ^ or g°tten in its proper place, we may extend6|’ l ?' e > and 10n a ' so to the abutments of different pieces of aMarina- ,u- re , ComrQen d it to the artist even to attend to thea, c lUely «»e' he en.) y ’ the

timbers by drying. When two timbers abut® a d ; J°int should be most full at the obtuse angle oftlie bea n ? aUse ’ b y drying, that angle grows more obtuse,J t “ ;i, iglr} w °uld then be in danger of splintering off at the

to,11 pl^' n S up’a'j^, tbe tt'oest work is indispensably necessary

tai

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Per

>ei y> * ~ The parts must abut on each other

e ®tcaey t i le . smallest play, or void, takes away

the

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*hO(j to on e At 18 us ual to give the butting joints a smallSerr te ^ows' r ° f tbe bean ‘> so that they may requireS '^e >tly c i 0s 01 a mau ' to force them in, and the joints maythe ' vben the external surfaces are even on eacht* 1. But we must not exceed in the least

degree; for a very taper wedge has great force; and if wehave driven the pieces together by very heavy blows, we leavethe whole in a state of violent strain, and the abutments areperhaps ready to splinter off by a small addition of pressure.

The most general reason for piecing a beam is to increaseits length. This is frequently necessary, in order to procuretie-beams for very wide roofs. Two pieces must be scarfedtogether. Numberless are the modes of doing this; andalmost every master carpenter has his favourite nostrum.Some of them are very ingenious : but here, as in other cases,the most simple are commonly the strongest. We do notimagine that any, the most ingenious, is equally strong witha tie consisting of two pieces of the same scantling laid overeach other for a certain length, and firmly bolted together.We acknowledge that this will appear an artless and clumsytie-beam ; but we only say that it will be stronger than anythat is more artificially made up of the same thickness oftimber. This, we imagine, will appear sufficiently certain.The simplest and most obvious scarfing (after the one nowmentioned) is that represented in Figure 30, No. 1 and 2.If considered merely as two pieces of wood joined, it is plainthat, as a tie, it has but half the strength of an entire piece,supposing that the bolts (which are the only connections) arefast in their holes. No. 2 requires a bolt in the middle ofthe scarf, to give it that strength; and, in every other part, isweaker on one side or the other.

But the bolts are very apt to bend by the violent strain,and require to be strengthened by uniting their ends by ironplates ; in which case it is no longer a wooden tie. The formof No. 1 is better adapted to the office of a pillar than No. 2 ;especially if its ends be formed in the manner shown in theelevation, No. 3. By the sally given to the ends, tbe scarfresists an effort to bend it in that direction. Besides, the formof No. 2 is unsuitable for a post; because the pieces, by slidingon each other by the pressure, are apt to splinter off the tonguewhich confines their extremity.

Figures 31 and 32, exhibit the most approved form of ascarf, whether for a tie or a post. The key represented inthe middle is not essentially necessary : the two pieces mightsimply meet square there. This form, without a key, needsno bolts (although they strengthen it greatly) ; hut if workedvery true and close, and with square abutments, will holdtogether, and will resist bending in any direction. But thekey is an ingenious and a very great improvement, and willforce the parts together with perfect tightness. The sameprecaution must be observed that we mentioned on anotheroccasion, not to produce a constant internal strain on theparts by over-driving the key. The form of Figure 31 is byfar the best; because the triangle of 32 is much easier splin-tered off by the strain, or by the key, than the square woodof 31. It is far preferable for a post, for the reason givenwhen speaking of Figure 30, No. 1 and 2. Both may beformed with a sally at the ends equal to the breadth of thekey. In this shape, Figure 31 is vastly well suited for join-ing the parts of the long corner posts of spires and otherwooden towers. Figure 31, No. 2, differs from No. 1 onlyby having three keys. The principle and longitudinalstrength are the same. The long scarf of No. 2, tightenedby the three keys, enables it to resist a bending muchbetter.

None of these scarfed tie-beams can have more than one-third of the strength of an entire piece, unless with theassistance of iron plates; for if the key be made thinnerthan one-third, it has less than one-third of the fibres topull by.

We are confident, therefore, that when the heads of thebolts are connected by plates, the simple form of Figure 30,