the square root of the length supported in feet. by the length of the brace or strut in feet, and the square root of the product multiplied by 03 for fir, will give the depth in inches, and the depth multiplied by 0.6, will give the breadth in inches.
Purlins.—The stress upon purlins is proportionable to the distance they are apart, and the weight being uniformly ditlbsed, the stifiliess is reciprocally as the cube of the length.
Rule.—Multiply the cube of the length of the purlin in feet, by the distance the purlins are apart in feet; and the fourth root of the product for fir will give the depth in inches; or multiplied by 1.01, will give the depth for oak ; and the depth multiplied by the decimal 0.6, will give the breadth.
Common Rafters.—Common rafters are uniformly loaded, and the breadth need not be more than from 2 inches to 2 inches. The depth for Welsh slate may be found by the following rule: Rule.—Divide the length of bearing in feet, by the cube root of the breadth in inches; and the quotient multiplied by 0.72 for fir, or 0.7• for oak, will give the depth in inches.
The largest roof ever executed was that of the Riding House, built at .Moscow, in 1790, by Paul 1. Emperor of Russia. The span was 235 feet, and the slope of the root' about 19 degrees. The principal support of this immense truss consisted in an arch or curved rib of timber, in three, thicknesses, indented or notched together, and strapped and bolted with iron. The principal rafters and the tie-beams, were supported by several vertical pieces notched to the curved rib ; and the whole stiffened by diagonal braces. The disposition of the parts of this roof is extremely ingenious ; but it was too slight for the immense extent of the span, and it appears that it settled so much, that it was proposed to add another curved rib to the original design. This example affords an instanee of the impropriety of adding material so near to the neutral line of the framing ; a like want of atten tion to principle is sometimes found in a first design ; a roof designed by Bettancourt for a riding-school, as given by Krailit, is an example. The external dimensions of the build ing were 1,920 feet by 210 feet ; it was lighted from the top by a lantern ; and there was a gallery round the inside of the building for spectators. The method of notching the Umber
in the curved rib, is objectionable, on account of the danger of the splitting of the timber under a considerable strain. This system of trussing is termed the bow suspension truss, and has been much used of late fur bridges for railway works.
Of a somewhat similar construction are the roofs executed by Philibert de Lome, in the sixteenth century, which are constructed with a series of arched timber ribs in lieu of trusses, these ribs being formed of planks in short lengths, placed edgewise, and bolted together in thicknesses, the planks in one thickness breaking joint with those in the adjoining thickness.
A line example of this class of roof was that constructed over the Halle an 1116, at Paris, l•0 feet in diameter, which has been destroyed by fire.
A smaller example, but nearer home, is to be found over the central compartment of the Pantheon, Ox rord.street ; the span measures 39 feet. The roof is circular, and is sup ported by nine semicircular ribs. Each rib is in three thick IIQSSCS ; the middle thickness is of teak, the side thicknesses or flitches are of fir; the pieces of timber forming these thicknesses are scribed on the under side to the curve of the roof, (and ceiling,) but the top edge is left straight ; therefore at their abutting ends they are considerably broader than at their middle. Each abutment of every piece, throughout every rib, has a shoe of cast-iron interposed. Iron belts con nect the three thicknesses together at each end, with a washer connecting each pair of bolts, as explained on the section. The ribs rest alternately on the pillars, (hereafter described) and on the centre of the longitudinal plate extending the - whole length of the roof. in the former case the ribs are trained into strong upright pieces, which become the king post of a semi-truss. These semi-trusses, besides carrying the timber of the roof and ceiling over the galleries, arc cal culated to receive the lateral thrust of the semicircular roof, a large cast iron shoe laid upon a stone template, receives the end of the tic-beam of these seini-trusses next the outer wall ; the object of this shoe being to prevent the possible decay of this end of the beam from affecting the stability of the roof.