a trus9, which is supported at either end by an upright tim-ber-post above the columns, and at two intermediate pointsby braces from these posts to the tie-beam, which is sus-pended from the principals at three points by a king-post andtwo queen-posts ; struts are carried from the foot of the king-post to the head of the queen-posts, and from the feet of thelatter to the principals. Above the aisles, hammer-beamsare supported at one end on upright posts in the wall; and, at'the other, on the post above the columns, and two interme-diate bearings are afforded by struts from the same posts.The main principals have a continuous bearing on the raftersof the truss over the roof, another on a post which rests uponthe hammer-beams, and at the extremities are strapped to thehammer-beams. From the last-named post there is a brace tothe top of post over column. The scantlings of the timberare rather full, and are as follow :—
Main principal, 13 inches by 10 inches at bottom, and 11inches by 10 inches at top.
Straining-brace or principal of nave-truss, 14 inches by 10inches at bottom, and 11 inches by 10 inches at top.
King-post, 9 inches by 9 inches.
Strut on king-post, 7 inches by 7J inches.
Queen-post, 8 inches by 9-Jr inches.
Strut on queen-post, 7 inches by 7 inches. '
Tie-beam, 14 inches by 9£ inches.
Post over column, 14 inches by 9J inches.
Brace below tie-beam, 7 inches by 7 inches.
Hammer-beam, 14 inches by 9^ inches.
Brace.from uprights to hammer-beam, 8 inches by 8 inches.
Post on hammer-beam, 8 inches by 9 inches.
Purlin rafters, 4 inches by 6 inches.
The roof the basilica of St. Paolo fuori le murk, executedin the fifteenth century, has double trusses, each consistingof two similar frames nearly 15 inches apart, and placed atintervals of 10-^ feet from each other. The tie-beam, whichis in two lengths scarfed together and secured by three ironstraps, is suspended at three points by a king and two queen-posts, the latter being separated by a straining-beam, andhaving auxiliary rafters abutting against their heads. Theking-post is of curious construction, and consists of threepieces for the double truss; that piece, against which the prin-cipals abut, is very short, only extending as low as the strain-ing-beam. Between the trusses another piece is placed, andsustained by a strong key of wood passing through it and thetwo upper short pieces; and this piece, in its turn, sustainsthe tie-beams by means of another strong key. The scant-lings are:—
Tie-beams, 221 inches by 15 inches.
Principal rafters, 21 f inches by 15 inches.
Auxiliary rafters, 13£ inches by 13 J inches.
Straining-beam, 15 inches by 12f inches.
Purlins, 8-Jj- inches by 8-^ inches.
Common rafters, 5£ inches by 4£ inches.
The purlins are 5 feet 7 inches apart, and the rafters 8£inches.
The span of the roof is 78 feet.
The roof over the Passengers’ Shed of the Croydon Rail-way, at London Bridge, is of simple construction, consistingonly of tie-beam, principals, and iron suspending-rods, withtimber-struts between each two, the tie being suspended ateleven points.
The tie-beams, principals, and struts being framed toge-ther, the suspending-rods are introduced, and screwed up bynuts upon their lower ends, until the whole is firmly united.The tie-beams camber 6 inches in the centre. There are14 of these trusses, 12 feet 6 inches apart, connected by theridge-piece and purlins. These extend to within 25 feet of
similar constru e »r igUC J]
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either end, where a half-truss offramed into the centre of each extreme truss atto the same, to form the hip, with ridge-pieces as us fl ,j|icbwhole is covered in with 1^-inch rough boarding, U P ^ tb® 1zinc is laid and joined. The under side of tl* e . c> 9 n“covers the joint is grooved to admit the edges of the^ ^ c oli-the angle next it rounded, to allow the zinc to exp an j vvU' 1tract. The rolls are painted, and are sometimes covzinc or lead. , j c h tb' 9
The entire length of the Passengers’ Shed (to w , g r0 o»>roof belongs) is 212 feet, and 53 feet wide. I^er® j; n esfor about 24 carriages, which is divided into t “J e Da rtu*' e . ?rails ; the outer lines being for the arrival an< ^/j a e a c ' 19 ftrains, and the centre line for spare carriages- ^ o
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of the shed is a platform raised nearly level ffb® j,
floor'
this
for
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the carriages, 11 feet wide, and connected at the s toi* e ‘
covered with asphalte, and coped next the rails n io'’ e9
In the centre of the shed is a carriage-truck, ca i-ri®^ e ft
transversely to the rails, for the purpose of shitunb.^j '**from one line to another, when any extra are re fi lefSjJtgtrain. Between the rails of each line (170 fee^ to e . jthe ground is sunk between walls a sufficient dep ^ e njZ' ,,eworkmen to get at the under side of the carriages fit j 0 uwhen required. This is the general plan of cons
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The roof of Christ’s Hospital is 51 feet wide, ^ebu* 1 ^long. It consists of principal trusses running acr ° tw o i®* 1 ^ing, with smaller longitudinal trusses between e T e . '
trusses, to carry the rafters. In the main truss t w j,ieU ®suspended from five points by king and queen-P 0S .g’ etvV eeii lt>)however of iron, and bolted through the tie. . ra i n ing'^ ei |[,e
two innermost queen-posts, or suspenders, is a s j te( j to tvV o
which abuts against their heads, and is also g( j 9 tprincipals; its length is 16 feet, and it is supp^_ ^intermediate points by struts from the f eet ^ g jde 0 lje .posts; there is also another strut for the ot q’),e eS ipost to the head of the further or outermost on g j j0 eS,mities of the tie-beams and rafters rest in i^ gtone c otpartially upon cast-iron standards supported o10 feet below the ceiling-line. . main r ® a -
The five longitudinal trusses, to support eaC bbetween the principal trusses, is extended ir° j n the 0 t j,epost in one cross truss to the corresponding o tbe>- ef0 ’-mpk>the distance between the main trusses, a are s „length of the tie-beams, being 17 feet. ■*_ ffla in r , gjtedking-post trusses without struts, and carry t t ;mberi %e9 etop of the king-post, which, in this case, is 0 jj e tW eeI1 r tb* 9to the tie-beam. Filling-in beams are P aC gcan tling 9 0trusses to receive the ceiling-joists. Theroof are as follow:
Principal truss:—
Tie-beam, 14 inches by 14 inches.
Principal rafters outside the innermo^^ by12 inches by 9 inches at lower end ; i° 1 .po^’
at upper end. . rm0 st q ueeI1
Principals within and above the > nne9 inches by 9 inches. .
Straining-beam, 12 inches by 9 inches. tj-uss® 9 ’'Struts, 6 inches by 6 inches, (longitudinTie-beams, 12 inches by 7 inches.
King-post, 6 inches by 6 inches.
Struts, 6 inches by 6 inches.
Main rafters between trussesAt lower ends, 12 inches by 7 inches. ^ 7 iUAt upper end, above queen-posts, 9 i aC , 5 in c ,: 0 ji, l ’
Common rafters, (Ion gitudinal) 7 inches ^ ons£r ucnThe roof over Exeter Hall is of simp
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