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j ffofu ®' n g not les3 than 76 feet, and the height of the roofIfqgg ae Unders 'de of the tie-beam, 21 feet 6 inches ; the'id o /. are placed at alternate intervals of 2 feet 6 inches,E feet -

*** 8 J h tl ’ uss has the tie-beam, which is scarfed, suspended■ ^Points by queen-posts, the innermost pair being pro-

ot,; v,tl *. a straining-beam, and with auxiliary rafters on! but s er «de of them, each of which is supported by two

resting on the foot of the queen-posts. The two■*t queen-posts on either side are hollow, and aree d from the principal rafters, as are also the inner' nieans of straps. The straining-beam, which is 22?i(ir> 18 suspended from the apex of the principals by aif'g ne ° s , t > i' rom which struts are raised to shorten the bear-Two sloping timbers but against the

f % Of .—

!®et 0 f le Principals.

\ „„ inner queen-posts, from the apex of which, an> n der is ------

~o«a ' - bolted to the tie-beam. The ends of the

i ,^jns an( j p r i nc ip a i s reS (; j n i ron shoes. The scantlings

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P r | earn s, 141 inches by 7-J- inches.

J '-U Jt n' Pal9 ’ ^2 inches by 7^ inches.

lt)j,' lar y rafters, or under principals, 14 inches by7J inches.

_ T'fien-posts, 8^ inches by 7\ inches.

In( e * ditto, 10 inches by 4-1 inches.

*in,^diate ditto, 12 inches by 4£ inches.

Str a , |"P° st > 6 inches by 7Jr inches.lir. lc ,Un n'beam, 14 inches by 7^ inches.tTpjJ 53 ’ inches by 7\ inches.

PurJj 1, ditto, 6 inches by 7^ inches.

C 0rtl as ’ inches by 4 inches.

^i'Kp ° n rafters, 5 inches by 21 inches.

^V a n_ ~Pi®ee, 8 inches by 3^ inches.

^°le-ni tes ’ ^2 inches by 6£ inches.

P ates, 12 inches by 4 inches.

, e f p j Uss over the arched roof of St. George’s Hall,tuition ’ ' s 65 feet in width, is of very simple con-

h 1 ana' ^e lie-beam is sustained at five points by a king-f our queen-posts. “ ’ '

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■ the 4 ueen-posis. The principals have auxiliariestt t *' e in ttl ’ ^ Ut do not meet at an a P ex > reaching only as farJu ini ne r queen-posts, which are kept apart by a double

n * PWoa eam Ut ton. and hv another at. their feet Strnta

ween bet

at top, and by another at their feet. Struts

ween the head of the king-post, and feet of theand thence again to the head of the outer

KJ° sti h

Pt^'U 4 f vin S Ibese descriptions of various kinds of roofs,h>.r ae b e out of place to give some rules for finding theln = s °f Ibe different members, as determined. ° ld ’ and the manner in which each member is

V lr >g, ari ^'~~The king-post is intended to support thek.'jdit uy means of the braces, to support part of thec 1 b 6 „ th e roof. — ' '

ldie^ 0 P 0 ]'li o nal to the span of the roof; therefore, to

scantiin

The weight suspended by the king-post,

.'ii

ting

(if e ®t. -r,. ultiply the length of the post in feet, by the spanV 0 l? multi P 1 7 ibis product by the decimal 0.12 fori ^ill' n S-po t • *"°. r oa k’ which will give the area of section ofV ,8*Ve th ln ' r ! c ' les > and this area divided by the breadth,

; r> ea dth 6 Sickness ; or by the thickness, will give

j % tee n. p

l L'' e d i n os ^.-—Queen-posts and suspending - pieces arejl'U' s oni v 8lDa '*ar manner to king-posts, but the load uponP r °portional to that part of the length of the1 V.^n-p P ende d by each suspending piece or queen-post.

| s > Ibe part suspended by each is generally half

Multiply the length in feet, of the queen-post or

suspending piece, by that part of the length of the tie-beamit supports, also in feet. This product multiplied by thedecimal 0.27 for fir, or by 0.32 for oak, will give the area ofthe section of the first in inches; and this area divided bythe thickness will give the breadth.

Tie-Beams. —A tie-beam is affected by two strains, theone in the direction of the length from the thrust of theprincipal rafters, the other is a cross strain from the weightof the ceiling. In estimating the strength, the thrust of therafters need not be considered, because the beam is alwaysabundantly strong to resist this strain ; and when a beam isstrained in the direction of the length, it rather increasesthe strength to resist a cross strain. Therefore the pressureof the weight supported by the tie-beams will be proportionalto the length of the longest part of it that is unsupported.

To find the scantling of a tie-beam, which has only tosupport a ceiling, the length of the longest unsupported partbeing given:

Rule. —Divide the length of the longest unsupported partby the cube-root of the breadth ; and the quotient multipliedby 1.47, will be the depth required for fir, in inches;-ormultiply by 1.52, which will give the depth for oak in inches.

Principal Rafters. —In estimating the strength of princi-pal rafters, we may suppose them supported by struts, eitherat or very near all the points where the purlins rest upon.The pressure on a principal rafter, is in the direction of itslength, and is in proportion to the magnitude of the roof;but the effect of this pressure does not bear the same propor-tion to the weight, when there is a king-post, as when thereare queen-posts ; therefore the same constant number willnot answer for both cases.

Case 1.—To find the scantling of the principal rafter,when there is a king-post in the middle.

Rule. —Multiply the square of the length of the rafter infeet, by the span in feet, and divide the product by the cubeof the thickness in inches. For fir, multiply the quotientby 0.96, which will give the depth in inches.

Case 2.—To find the scantling of a principal rafter, whenthere are two queen-posts ;

Rule. —Multiply the square of the length of the rafter infeet, by the span in feet, and divide the product by the cubeof the thickness in inches. For fir, multiply the quotientby 0.155, which will give the depth in inches.

Straining Beams. —A straining-beam is a horizontal piecebetween the heads of the queen-posts. In order that thisbeam may be the strongest possible, its depth should be toits thickness as 10 is to 7.

Rule. —Multiply the square root of the span in feet, by thelength of the straining-beam in feet, and extract the squareroot of the product. Multiply the root by 0.9, for fir, whichwill give the depth in inches.

To find the thickness, multiply the depth by the decimal 0.7.

Struts and Braces. —That part of a roof that is supportedby a strut or brace, is easily ascertained from the design ;but the effect of a load must depend on the position of abrace ; when it is square from the back of the rafter, thestrain upon it will be the least; and when it has the sameinclination on the roof, the same strain will be thrown on thelower part of the principal rafter, as is borne by the strut.But as the degree of obliqueness does not vary much, weshall not attempt to include its effect in the rule for thescantling.

Rule. —Multiply the square root of the length supportedin feet, by the length of the brace or strut in feet, and thesquare root of the product multiplied by 0.8 for fir, will givethe depth in inches, and the depth multiplied by 0.6, willgive the breadth in inches.

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