Chap. XXII.
TIMBER ROOFS.
1883
Fig. 2135.
Fig. 2136.
boarding is laid the reverse way. The two other trusses on each side are similarly framed,the heights being varied to suit the top of the roof: that of the pairs on each side of thecentre is 5 feet from the under side beam to the under side the main rafter; the outsidepair are only 2 feet 9 inches in height from the same points.
Pantheon., Oxford Street, constructed from the designs of Mr. Sidney Smirke , exhibits somenovelty in the roof, which is admirably adapted for its purpose ; it is formed of teak and fir:the centre opening, or the width between the pillars, is 37 feet 3 inches; the pillars are inthickness 2 feet 1 inch, and the side rooms in the clear 23 feet 5 inches, so that the clearspace between the walls is 88 feet 3 inches : a semicircular rib, formed of three thicknessesbolted together, and having iron abutments, spans the centre • the middle rib is of teak, 5
inches thick ; the other or side flitches are fir, each inches thick, and their depth in thesmallest part 16 inches : the under side of these is scribed to the curve of the roof; the topis left polygonally, so that the abutting ends of each piece is broader than in the middle;cast-iron shoes receive the feet of each rib, and the three thicknesses are bolted together atthe ends.
These nine semicircular ribs, when they rest on the pillars, are framed into upright tim-bers, which form king-posts to half trusses; these half trusses assist in opposing the lateralthrust of the semicircular ribs, as well as carry the roofs and ceilings of the side galleries.
The tie-beams are 14 inches by 6£ inches; the principals taper, being at bottom 13 inchesby 6£ inches, and at top 11 inches by 6J inches; the large strut is 11 inches by 6^ inches,and the smaller 9 inches by 6£ inches, and 7 inches by inches: a cast-iron shoe,resting on a stone template, receives their ends where they rest upon the wall, whichit is calculated would be of service, should the ends of the timbers at any time decay. Atthe other end, where the king-post unites with the tie-beam, is a cast-iron shoe, withsockets and flanges, made to unite by means of a longitudinal iron plate, at the headof the main pillars, which are also of iron : the lateral compression of the timbers isprovided for by the introduction of iron shoes.
4 q 3