1

I

i !'

CAE

97

CAR

j en ® r ® nt result; as timber is considerably weakened by itsThe rule is therefore not only erroneous, but defec-

tlVe also.

respect to Mr. Price’s table, we have only to observeoak 85 tllere are no details of experiments on the strength ofa °d dr, when employed as posts, we cannot decide in thisare er ‘ It must, however, be observed, that the fibres of firis whilst those of oak are very crooked ; whence it

\) e t t easona kl e to conclude that a body with straight fibres isare ^ a<la P tef l to resist compression than one whose fibresf , ->°ked; and this supposition is strengthened, if not con-tjj a t j’ I*y the experiments of Muchenbreuk, who asserts,■vyill tll °ugh oak will suspend half as much again as fir, itn °h as a pillar, support two-thirds of that load. Now ifb e Cai \ P ut any dependence on these experiments, fir should8j c e<1 m cases of compression, as in story-posts, partitions,i’,’ oak in cases of tension, as ties, truss-posts, &c.fg e t le distances of principal posts are generally about tentfi e ’ an d of the quarters about fourteen inches; but whente f g ‘ re to he lathed on both sides, the distances of the quar-laths lou ^d he such as will be agreeable to the lengths of thethick 0t ' lerw * se there will be a great waste in the laths. Thetty 0 • 6sses °I ground-plates and risings are generally fromto ffc .*r nches and a half to four inches, and are scarfed

^ res pect to lintels, bond-timbers, and naked flooring,■i p rv es as follows:

gird er ° r ..the better disposing of the weight imposed onhaitib ’ ilnte ^ s should always be firmly bedded on a sufficientgarK ®hort pieces of oak, laid across the walls, vul-« j adsd templets, which are of excellent use.it .S'rders be laid in piers, or in lintels over windows ;arcb es m 150111 tlles e cases be commendable to turn smalldec a y e , 0ver their ends, that in case their ends are firstitig a ’ they may be renewed at pleasure, without disturb-atl °int tlf art o1 ' the brickwork ; and for their preservation,fou r> o p leir ends with melted pitch and grease^ viz., of pitchWi t ], S r ease one; and, indeed, were lintels to be coveredt° thf.;„ V 1 an< I grease also, it would contribute very greatly<i fifration.

e koui^ i, tlle carrying up the several walls of buildings, itPiets, a ' 6 caref ully observed, to lay in bond-timbers on tem-C °Sged rt aloresa id, at every six or seven feet in height,CVgfy °wn and braced together with diagonal pieces atla ° st su a r g,e > w hich will bind the whole together in thes ettl etn sta ntial manner, and prevent fractures by unequal

“ Th nt " •

tcet, an< j ^stances of girders should never exceed twelve*beir l eno . h eir scantlings must be proportioned according toll j n „K. 13 ,’ as by experience it is known that a scantling

lri lenotv!* le ? hy 8 inches is sufficient for a fir-girder of 10 feet

to

find tb lf>e area °f whose end is 88 inches, it is very easy^Ppose 2n P , roper scantling for a girder of any greater length,i st teet, by this rule: As 10 feet, the length of the

| e,l gtb 0 j. ea ’ 18 to 88, the area of its end, so is 20 feet, the“ R 0vv ae second girder, to 176, the area of its end.

0t her, shall° lta scantlings, that, being multiplied into each* t * Ust be J- P ro<luce 176 inches, the area found, one of themPeople tk en, . v ' Z- > either the depth or thickness. In this•Jy. hy 19 ®’’ ven depth shall be 12 inches, therefore dividehich j 8 t ,’ an d the quotient is 14 inches and two-thirds,; this p other scantling, or breadth required.”

6 t>s tanc e> j exatn ple, the length is regarded; but in the firstWhig]. 11 the dimensions of the given piece, he does not■°t’ced, a 3 ?[ them is the depth. This should have been^CQaio,, j. e strength of a piece of wood with its greater■sposed vertically, is to the strength of the same2 c

X

x y = 176: w : : 11

8 x = 11 y

From the first equation we have

and from the second,

176

y

ii y

Therefore,

8

11 y, 176

consequently,

8 ~ y

11 if = 1408y* = 128

y - ( 128 )

piece with its less dimension in the same position, as thegreater dimension is to the less. Another uncertainty willarise from the proportion ; for if the scantlings are not in thesame ratio, the strength will be more or less in the one, asthe vertical dimension may be greater in proportion to thehorizontal than those of the other piece. Mr. Langley shouldhave noticed this also. He has assumed 12 inches as tl»edepth, then finds the breadth to be 14 inches and two-thirds,by dividing 176 by 12; but this is only guessing at the pro-portion, which might be properly stated by the rules ofalgebra, as follows: What two numbers are those, whose pro-duct is 176, and whose proportion is in the ratio of 11 to 8?Take x for the greater, and y for the less of the two num-bers ; then we have

and by dividing by 11.3 inches, the depth, we should thenhave the breadth. We have here taken it for granted, thatthe former part of his proposition is true, but, indeed, nothingcan be more erroneous; for their lengths are not in the ratioof the areas of their sections, when the pieces are of equalstrength, but their lengths are in the ratio of the breadthmultiplied into the square of the depth ; or if their sectionsbe similar figures, the lengths will be in the same ratio asthe cubes of their vertical dimensions. By this method, iftwo pieces of timber were of the same thickness, and of equalstrength, the lengths would be as the depths; whereas theyare as the square of the depths. So that, besides the ambi-guity of his rule, allowing the data to be properly fixed, theresults would give the dimensions of the section much toogreat, in calculating from a given beam of less length to oneof a greater; and much too small in calculating from agreater length to a smaller one. In calculating the strengthof beams, it is material to recollect the loss of strength inlarge beams, occasioned by their weight, as the strength ofbeams is not in the same ratio with the stress occasioned bytheir weights, but in a much less degree: but as we shallhereafter discuss this subject, under the article Strength or'Materials, we shall for the present take leave of the sub-ject, and return to our author.

“ To prevent the sagging of short girders, it is usual tocut them camber ; that is, to cut them with an angle in themidst of their lengths, so that their middles shall rise abovethe level of their ends, as many half inches as the girdercontains times ten feet. And, indeed, girders of the greatestlength, although trussed, should be cut camber in the samemanner.”

It may be proper here to notice, that the cambering of gir-ders does not prevent them from sagging, though perhaps itmay obviate their becoming concave on the upper side. Withregard to trussing girders, the flitches should not be cut to acamber, but brought into this state in the act of trussing.