■

1

ROO

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386

ROO

in construction, and from 20 to 100,000 lbs. for the severalmetals.

The bow, a d g, by its resistance to compression, serves toprevent the chain, a k g, from contracting or drawing togetherthe extremities a and g. The lower bow ak g, acting as achain, prevents the upper bow, a d g, from pushing or press-ing out at the points of support. The vertical and diagonalstruts unite in a firm manner the two bows, and the twoforces thus neutralized form a complete whole, that sustainsitself, and can neither thrust out nor draw in.

It must be observed, first, that the strength of the chains,which act in a similar manner to that of suspension bridges,depends upon the depth of the versed sine, and that themore they deflect below the horizontal line, or chord of thearc, the stronger they would be; secondly, that the upperbow, owing to the elasticity of the material, must absolutelyhave the convex form -, so that, when any great sudden weightis thrown on it, causing the lengthening of the chains bytension, and the shortening of the upper bow by compression,the upper bow may not be reduced to a horizontal line, a n g,beneath which it could no longer serve by resistance to thestatical equilibrium of the construction; thirdly, that themethod of combining the extremities, a and g, of the bows,must depend upon the materials employed : for instance, ina wooden bridge, the notching and scarfing at the joints ofthe different pieces of wood ought to be calculated and exe-cuted to the force which they have the power of resisting.The rules and forms most applicable to these joints have alsobeen proved by the experiments of the inventor.

This system of construction is applicable to roofs andbridges of every denomination, such as draw and swingbridges, but especially for suspension bridges, where thelocality on either side, or on both, does not admit ofsecure fastenings for the chains. For covering large rooms,riding-schools, and other openings of large space, it is par-ticularly useful; also the erection of scaffolding, and laddersof large dimensions, and to the stiffening of beams, masts,and supports in general.

The application of this system to roofs and floors of largespan, is extremely economical and useful, and by simplemodifications serves for the covering of large spaces, with-out any intermediate points of support, and also presents thisfarther advantage, that from its vertical pressure, it requiresno other support than walls of moderate thickness. Whenapplied to floors, bridging-joists will remedy the inequalityof the surface in the beam itself. In roofs of large span, theposts may be continued upwards, so as to receive the purlins,and when continued downwards, serve to hold up the ceiling,whether flat, vaulted, or mixed.

The open Gothic roofs of the middle ages, differing as theydo in essential matters of construction from those at presentin use, ought not to be passed over in silence. In many, ifnot the majority of such roofs, tie-beams were altogetherdispensed with ; the only tie between the principals, to pre-vent their spreading, consisting of a collar-beam placed highup in the roof. So much care, however, to prevent spread-ing, in these roofs, was not required ; for the principals werelaid at a high pitch, and therefore had not so much tendencyto thrust outwards, as a roof of modern construction ; besideswhich, the walls were built of very great thicknesses, andwere often strengthened by massive buttresses. Sometimes,in lieu of a collar-beam, two cross-braces were used springingat about half way up the principals, and intersecting eachother in their course upward to the opposite principal, andoccasionally these and the collar-beam were employed in thesame roof; not unfrequently we find the collar strutted upfrom the rafters. Sometimes again, the principals rest on

ied down

arched beams or planks, of which some are carrieu u ^ d> ,swalls, and rest on corbels projecting from thenm gainmeans, the thrust is carried down lower, and yo uadvantage of the weight of the walls above, to res® ^ ] fl rg eSometimes tie-beams are used, which are usual y^ (0 tl‘ escantling, and have the upper surface sloped up' va 0 tb e ^centre, so that the depth there is greater than at a . g ' place®part. Occasionally, upon the centre of the tie-bea® ^ c0 llat"a post, from which spring branches to prop U P i®*

beam, and sometimes the rafters also. This P oS fori*' 9be confounded with the king-post of modern roo s, tie"not used to hang up the tie-beams, but rests up° n

earl ief

- " P

beams to support the upper part of the roof.

Such were the most usual form of roofs in 0 f tl>®Gothic examples; but at a later time, when the fa o{(ie tiiSi eS ’roof was lowered, other methods were adopted. k g jde>merely a horizontal beam was thrown across the u Pr _j,ea® 9being formed into two inclined planes similar to tPe ugeC j \vid‘mentioned above ; at others, inclined rafters vf ? l6 t the"' al13 ’their ends resting upon upright posts, placed again® t ] )6 topand resting on projecting corbels at a distance ii® cU rv® dof the walls, the rafters being connected with it >}_ c e0'strut, so as to prevent deflection of the rafters ne<u ^ fli®tre. The upright posts are termed pendant P os ra j n o' sol ,dstruts are often cut out of thick planking, so i° r °spandrels. _ . e r0 of °°

Frequently a horizontal beam is laid across ^ t,efo ,e ’top of the pendant posts, supported by curved sti u tl®

and above this, rafters are introduced, strutte , g g0 ® e '

centre from the horizontal beam. Where the P’~ t j, e c«>;what higher than usual, inclined struts are adde f\)t3

ofthe k-S>^

,cen-

tral strut, so as to divide the bearingkind of roof is very similar in appearance tobut this is not the case; that which has the appthe king-post, not being employed to suspendbut to support the rafters. _ .. Q f th 0

The hammer-beam roofs so common in ha 3 t0 an/and 16th centuries, are of a different descrip 1 ^ 0 f c°

the above. They are usually of a high pit® ‘ -ea t sk>siderable span, and often bear evidence of very g^ pu^construction : a simple roof of the kind, cons® s^ ^ eI1 j],cipals connected by a collar-beam, and resting °.^ gJ0 the^fa.two hammer-beams, which project horizonta y ^ t l, e c« j g0and carry two queen-posts. The queen-posts ‘ ag a re ‘ ,,beam are usually connected by a curved ' ,raa g ’ tg> Of 6 ,jsometimes the hammer-beams with pendant p ^ q 0u rtroofs, the most remarkable are those of Ilamp . 0 f

Westminster Hall. c 0llS, 4 t0 a

The roof of the great hall, Hampton ge c>i r . i, is

principals which fall short of the apex, an whfa^,

collar or straining-beam at their upper extre j Q>v er c °supported by three vertical posts resting _° n t li®

beam, at about halfway down the P® ncl P a t ], e top ° unt oflower

Walls , is a hamufo.Pf 1110 ^ 8 * 8 ’ a,1 d resting onthe entire space and" ^ pro J ectin ff out about a ' b yflcurved brace wl’,;‘ i Su f > P orfed at its inner extrenii | J faupon a corbel * ^ ests Upon 11 pendant post, nod t " ‘° aCBcarries two unriilT f ’ rom tlle wall. The hamm e fa ipll l

rafters, and the o£r’ t n" e , ° f wiiich sa PP?‘\ t3 f^ thetsupported it er * e i°wer collar, which is sti ,pc®

the extremity'of C +) nti f ^ a ^ved brace, which res fa. fa

rafters spriWnn- r l,amme r-beam. A couple of 1 g{ a n

apex over the 5 r ° m tile fo P of tl,e principals, m p 0 rt^

by a strut rl 6 ° f the ™>f, where they are Bt>pP°

admired: and tn ., estminst0 r Hall has always jo® 1' aDd t0 ,£s great height and extensive ***”