ME C

162

ME C

direction of the strain. We must then resolve this strain intoa strain parallel to each piece, and another perpendicular toit. Then the strap, which is to be made fast to any of thepieces, must be so fixed, that it shall resist in the directionparallel to the piece. Frequently this cannot be done ; butwe must come as near to it as we can. In such cases, wemust suppose that the assemblage yields a little to the pressureswhich act on it. We must examine what change of shape asmall yielding will produce. We must now see how this willaffect the iron strap, which we have already supposedattached to the joint in some manner that we thought suitable,j This settling will perhaps draw the pieces away from it,

1 leaving it loose and unserviceable ; (this frequently happens| to the plates which are put to secure the obtuse angles of! butting timbers, when their bolts are at some distance fromthe angles, especially when these plates are laid on the insideof the angles); or it may cause it to compress the piecesharder than before ; in which case it is answering ourintention. But it may be producing cross strains, which maybreak them ; or it may be crippling them. We can hardlygive any general rules ; he will see the nature of the strapor stirrup, by which the king-post carries the tie-beam. Thestrap that we observe most generally ill-placed is that whichconnects the foot of the rafter with the beam. It only bindsdown the rafter, but does not act against its horizontal thrust.It should be placed farther back on the beam, with a boltthrough it, which will allow it to turn round. It shouldembrace the rafter almost horizontally near the foot, andshould be notched square with the back of the rafter. Sucha construction is represented in Figure 43. By moving roundthe eye-bolt, it follows the rafter, and cannot pinch and! cripple it, which it always does in its ordinary form. We areJ of opinion that straps which have eye-bolts in the very angles,

' and allow all motion round them, are, of all, the most perfect.A branched strap, such as may at once bind the king-post andthe two braces which button its foot, will be more serviceableif it have a joint. When a roof warps, those branched straps; frequently break the tenons, by affording a fulcrum in one oftheir bolts. An attentive and judicious artist will considerhow the beams will act on such occasions, and will avoidgiving rise to these great strains by levers. A skilful car-penter never employs many straps, considering them asauxiliaries foreign to his art, and subject to imperfections inworkmanship, which he cannot discern nor amend.— Supple-ment to the article Carpentry of the Encyclopaedia Britannica,by Professor Robinson.

The following part was written by Mr. Nicholson for theEdinburgh Cyclopedia, and is here inserted by permissionof the editor.

A circular roof may be executed with timbers disposed invertical planes, whether the ribs or rafters are convex, con-cave, or straight, without any tie between the rafters or ribs,even though the wall were ever so thin ; provided that it beonly sufficient to sustain the weight of the roof pressingvertically, by joining the wall-plate so as to form a completering, and by strutting the rafters in one or more horizontalcourses, without danger of lateral pressure, or of the timbersbeing bent by the weight of the covering. The same cannotbe done with the roof of a rectangular building, for singleparallel rafters would not only obtain a concave curvature,but would thrust the walls outwards : hence the means ofexecuting circular roofs with safety are simple ; but those forstraight-sided buildings are complex, and require much skillin contriving, according to the use that is to be made of thespace between the rafters, which may be found necessary in| rendering more lofty, or more elegant apartments, as in con-! cave, or coved ceilings.

A polygonal roof, with a great number of sides, apP ra_jg very nearly to a circle, is stronger than one ofsides ; the less the number of sides, the weaker will the

ch'fetve r

ing very nearly to a circle, is stronger than one ^

be, and more liable to get out of order. A roof e . xe ^ u cbupon an equilateral and equiangular polygonal plan, 13 y 0 rstronger than one that is elongated. All circular roo >the same reason, are stronger than elliptical ones ; fiiesure in the former case being equally distributed roun ^wall-plate, which is therefore kept in an equal 3tatension.

Trusses are strong frames of carpentry, resolved mt ^jjjor a series of triangles, so as to make the truss act as a gbody, and thereby support certain weights, each at a g gimmoveable point, the truss itself being suspended ^ r0X5 ^, c te<lsuch immoveable points. The trusses of roofs are constr o»generally of a triangular form, and disposed equidistan / t0the wall-plates, in parallel vertical planes, at right ang ofthe walls ; the top of the opposite walls are the two P 01 ^ t j,e

suspension, and the weights supported by the truss

immoveable points are horizontal pieces of timber.

rim

hori*°;

i iiia?

,ntal

transversely to the planes of the trusses : these to

pieces of timber support other equidistant pieces p ara p0 rtthe upper sides of the trusses, and these last timbers s . u tb ethe covering, or the covering and timber-work, to tvlu c . fl acovering is fixed. In a truss, some pieces of timber j, u tstate of tension, and some are in a state of compression^^,

a piece of timber which is neither extended nor comp 1 g;lC liis useless. A quadrilateral frame, so constructed tha ^ e jrtwo adjoining timbers, made moveable round a pom 1 aintersection, may be put into an infinite number

of

01

because the whole frame will be revolvable about the all g

6 ]ei : ; i

but if any one of the angles be immoveable, the whole

fr <

le,

win aiso De so. Two pieces of timber forming an ang ^revolving round a point at their intersection, may b e ]immoveable by fastening each end of a bar to each leg’.

taking any two points in the bar, and fastening each P° ^ ]t e0each leg. Now, if a force be applied at any of 1 ^ t\i'®angular points, the frame will be immoveable, but ot ^ te o‘

legs which form the angle, the one will be intension, and the other in a state of compression, P roV .lj,;n ^the direction of the force applied does not fall (ot c6angle if produced ; but if the line of direction of S !fapplied fall within the angle of the triangle, then b ggsi® 1 ’ 1are either in a state of tension or in a state of con'P .p O o 6according as the force applied is pulling or pressing >of the sides of the triangle be lengthened without t 1 t s adary, and a force be applied transversely to the Ilengthened, this force will bend the side of the fig ur ^ gt co !_is in the straight line with the side to which the ^Jr 1applied: therefore, suppose again a quadrangle; ^lateral, revolvable about the angles, and a bar be fi x ^ g po'? gtwo sides forming an angle, viz., a point in the barin one of the legs, and another point in the bar to r tvf othe other leg, and suppose the two points not to be ^the angles, or one of the points to be in the sm > i)t0distance from either end, the figure will be divide ppo 1- ^,parts, one will always be a triangle ; then, if if ^ e ^leOP^etwo of its angles, and a force be applied to the ano^ ]flt t .

S o* e

at

site, the angle which has the bar fixed to its legs>^ 3 fi*direction of the force thus applied does not tend to \vh apoint, which is the farther extremity of the one 3 j^rthe force is applied to the angular point at the o .

mity, all the sides of this figure will be bent, a g .j e3 . p ^ ethus fixed will occasion transverse strains to the p r a 0 e (

if the bar be fixed to two opposite angles, and » th e ayheld immoveable at one of the angular points tv e %tf e ^is fixed, and also at one of the other angles at t