1332
THEORY AND PRACTICE OF ENGINEERING.
Book II.
Purlins of 6 feet bearing may be 6 inches by 4 ; of 8 feet, 7 inches by 8 ; of 10 feet,8 inches by 6 ; and of 12 feet, 9 inches by 7 : where the common rafters have a bearing of8 feet, they may he made 4 inches by 2J ; 10 feet, 5 inches by 2J ; 12 feet, 10 inches by 2J.
Roof of St. James's Church , Piccadilly, is admirably contrived, and is one of Sir ChristopherWren ’s best designs for a covering to a church or large building where tie-beams cannot beintroduced. The breadth of the church is 63 feet, which is divided into five equal parts,three of which are given to the nave, the others to the side aisles. The principal raftersare framed into a plate which rests on the outer wall, and on the capitals of the columns anupright post, immediately over the capitals, supports the principal at about one-third ofits length, and a semicircular arch simply cradled is placed between these upright posts, andabuts against them: above this arch the principal rafters are held together by stout collars ;the timber framing or the principals is prevented sliding by the St. Andrew’s cross intro-duced over the side aisles. Sir Christopher Wren describes this roof as the cheapest hecould invent; it is both convenient and beautiful in its application and arrangement, and itis astonishing that it is not more generally adopted: over the side aisle galleries between everypair of principals are semicircular arches at right angles with the main arch, which restupon the returned entablatures, so that no portion of this roof is hidden or unemployed.'Die length of this church is 84 feet, the breadth 63, and height at the centre of the vaulting42: with its galleries it will contain upwards of 2000 persons.
Roofs trussed xcith Iron Rods. — Over the passengers’ shed at the Croydon railway station,London Bridge, is an ingeniously constructed roof, 54 feet span, formed by a series of strutsand iron rods, placed 4 feet apart: the tie-beams camber about 6 inches, and are 12 by 6inches; the principals are 9 by 6 ; the struts 6 by 4, and the par.ins, which have a 12 feet
Fig. 2133.
CROYDON RAILWAY ROOP.
bearing, are 8 by 3 ; l \ inch boarding with a zinc covering lays upon the purlins withoutthe aid of common rafters: the iron rods or bolts are 1| inch in diameter; that whichforms the centre or king passes through the ridge piece, against which the two principalsabut; there is great stiffness and economy in this system of construction, and the whole hasa light effect, and it is evident that the more at right angles to the inclination of the prin-cipals the wooden struts are placed, the greater will be the strength.
The Roof of the Hall of Christ's Hospital , London , constructed by Mr. Shaw out ofBaltic timber, possesses considerable strength: the walls, 3 feet 6 inches thick, are15 feet apart in the clear ; the rise of the roof in the centre, from the under side of the tie-beam to the top of the principals, is 9 feet 4 inches; it is queen-post trussed, and the tie-beams are held up at five different points, or at every 8 feet 6 inches; the principals aredistant from each other 17 feet; the length of the hall is 187 feet, and the breadth 51:every precaution has here been taken to unite the feet of the principals with the ends of thetie-beam, and their weight at the ends is partly borne by iron standards, which rest onshoes worked into the wall below. The principals taper, and are 12 inches by 9 inches atthe feet, and 9 inches by 9 inches at the top; the tie-beams are 14 inches by 14 inches;the straining piece between the heads of the queens, 12 inches by 9 inches ; the struts6 inches by 6 inches: between each pair of principals is a pair of main rafters, supportedby five longitudinal trusses, and which are also made to carry the ceiling-joists. 'These lon-gitudinal trusses bear upon the principal tie-beams, which are 17 feet apart from centreto centre; the middle longitudinal truss comes under the ridge, and is very stronglybraced; the lower beam is 12 inches by 7 inches, the Ling-post 6 inches by 6 inches, andhead 12 inches by 6 inches; the struts 6 inches by 6 inches: into the head of the kings arelodged the main rafters, which are 7 inches by 5 inches; on these are laid the commonrafters longitudinally to receive the boarding, which is laid in the direction of the slope ofthe roof; so that the lead which covers it is not so subject to derangement as when the