Page:EB1911 - Volume 23.djvu/732

fixed on the principal rafters with intervals of about 8 ft., and

framing, usually of rough northern pine or spruce, is generally In such large spans the straining beam often becomes of such hidden from view by the ceilings. The spans usually are not a length as to require support and this is effected by congreat, and extra support is obtained at various points from partitions and cross walls. Where the span is large, that is, above zo ft. without intermediate support, it is necessary to employ roofs with “ principals” and “purlins, ” sometimes called “double tinuing the principal

° '2, mrafters up to the ridge

and introducing a

y

short king-post to

sustain the beam in

rafter roofs.” Principals are strong trusses of timber 4” the middle of its rigidly framed together and placed at intervals of about length. ro ft. to support the weight of the roof covering., ,/ - Open timber roofs Purlins-stout timbers running longitudinally-are of various types but " principally Qpgg

construction were used in the middle ages

where stone vault-M,

ing was not em-

ployed. Many of

these old roofs still

exist in good pre-7/

serration and exhibit

the great skill of the Y medieval carpenters

who designed and erected them. Such forms are still used, chiefly for ecclesi-SM MPM » astical buildings and

FIG. 3.-Queen-post Roof Truss; half elevation, 38' o” span. on these the common rafters are fastened. Principals, or “ roof trusses” as they are more often called, are framed together in various ways, and the members may be entirely of wood or reinforced by ties of iron rods or bars; the latter are called “ composite trusses.”

The “ king-post truss ” may be used for spans up to 30 ft. and is constructed as shown in figs. I and 2. It has a central post sustaining the “ tie-beam ” in the centre with struts projecting from its base to support the principal rafters at their centres at a point where the weight. of the purlins renders strutting necessary. The members are connected by wrought iron straps and bolts; the strap connects the king-post and tie-beam and is often fitted with a gib-and-cotter arrangement (really a pair of iron folding wedges) which allows the Whole truss to be tightened up should any settlement or shrinkage occur. “ Queen-post trusses ” have, in place of the king-post dividing the tie-beam into two, two queen-posts supporting it at two points (fig. 3). The joints the roofs over large

halls. In the best

periods of Gothic

architecture the pitch

sometimes as much

of these roofs was made very steep, as 60° with the horizon. In the hammer-beam type of roof the tie-beam at the foot of the rafters is omitted, a. collar being thrown across connecting the principal rafters at a point about half-way in their length, the lower portion of the principal consisting of a number of struts and braces rigidly connected in such a manner as to throw as little thrust as possible upon the walls serving as abutments. There are two kinds of-hammer beams, the arched and the bracketed; the chief examples are Westminster Hall and Middle Temple Hall (Plate I. figs. 24 and 25). The “hammer beam” projects from the top of the Wall and is bracketed from a corbel projecting from the Wall some distance below. This form of roof has a style and dignity of its own, and gives greater height in the upper part of the building as well as being more ornamental and lighter in effect than tie-beam trusses, which have a rather heavy effect.

between the members are made in a, m, ,, {{{Hm@, ,, (, similar manner to those of the king-;', , m;“.'w -»»W§ ii""" mls; post principal with wrought-iron straps. I Milf, The purlins are two in number on each 3 44 'E slope, one supported at the top of each, W an “ queen, ” the other half-way between that . 7| I u -point and the wall-plate and resting upon vw 5; 1%. '| . W, -". the principal rafter at a point where i ' ; / strutted from the base of the queen- 53,  ;, ,.-», . s. VU post. A stout straining beam connects -t, L f rjmjl the heads of the queens. In fig. 4, a /f', 1 lqll "WM, ~l and b are details at the foot of the g /g.f;' I l' nw queen-post, and c at the head. Trusses § /, ., of this type are suitable for spans up to 45 ft. In roofs of a larger span than fl I, " 'this and up to 60 ft. the tie-beam requires "“h / W" 0 to be upheld at more than two points, - and additional posts called “ princesses ” are introduced for this purpose. This also entails extra struts and purlins. FIG. 4.-a. Detail of queen-post truss at b. b. Vertical section through queen-post. c. Detail of queen-post truss at head; purlin and wrought-iron straps are omitted for the sake of clearness.