In such a case the principal rafter is in compression throughout its length.
When, however, the weight is distributed throughout the length of the rafter by means of a number of small purlins, the principal rafter is subjected also to a transverse strain.
In either case the struts should not be so far apart as to necessitate the rafter being of too large a section for economy.
"For a tensile strain it is safest to have bolts instead of rivets, and sometimes, if much depends on their strength, bolts with a nut at each end, so as to avoid the risk of a flaw in the forming of the bolt head."
"In the main tension rods of a roof screwed ends at all the points of connection are advantageous, welds are also so avoided and there is an opportunity for adjustment."
Care should be taken in designing a roof to use such forms, sections, and scantlings of iron as can be readily found in the market. Sections of peculiar dimensions, though perhaps a little lighter than the nearest sections kept by manufacturers, will not only cause delay but cost more.
"In a roof which is rectangular in plan the distance apart of the Principals should be from 1/8 to 1/4 the span, and if these limits be overstepped there will be an unprofitable employment of material."1
It is sometimes economical to adopt the larger rather than the smaller interval, because, when the trusses are widely spaced, there is necessarily a large cross section given to the struts, but their length remains the same; they are, therefore, less liable to buckle under the thrust that comes upon them, and thus more resistance is obtained from an equal weight of metal.
A hipped roof is more expensive than one with gable ends, but the hipped end is a considerable support to the roof, and itself offers much less resistance to the wind than a gable.2
Trusses which do not contain vertical members are not so suitable for hipped roofs as those having such members.
The illustrations of iron roofs and parts of iron roofs in the foregoing pages are intended to show the student clearly the construction of different types and forms of such structures generally. Such illustrations would not, however, do for the working drawings required in practice, which must show the dimensions of the different parts of the actual roof required to be constructed.
1 Matheson, Works in Iron. 2 Maynard.
These dimensions vary of course according to the span of the roof and other minor considerations - the larger the span, the greater will be the average of the scantlings or dimensions of the different members of the truss. To figure dimensions on the illustration of a type form might lead the student into the serious mistake of considering that there were standard dimensions applicable to all roofs.
In order, however, that the student may have a good idea of the kind of drawings required in practice, the Plates IV. to X. may with advantage be studied by him. They are reduced copies of the actual contract drawings that were used for the roofs illustrated, which have all been erected within the last few years. In some cases, however, the drawings of unimportant parts such as skylights, etc., have been left out for want of room.
Some of the plates may seem to be unnecessarily crowded with dimensions, etc., but it was thought better to retain them, so that the figures might be actual complete copies of the drawings of the different parts of the roofs.
Plate IV. contains the working drawings of a roof for a shed erected alongside a dock.
The truss is of the trussed-rafter form with a single strut. The Principal Rafters of T iron; Purlins, wood; Tie Rods of fiat bar iron; Struts, fiat bars with distance pieces; Head and Shoes of fiat plates riveted together; and the Covering of corrugated iron, with a small skylight.
Plates V. VI. are from the contract drawings for the roof of a store. The Truss is of the king-rod form with additional rods (as in Fig. 362); the Rafters and Struts are of T iron; the Suspending Rods and Tie Rod of round iron; Purlins, timber; Covering, half of slates and half of glass. It will be noticed that this roof springs on one side from a rolled iron beam resting on columns, as in Figs. 400 and 414, on the other side from a wall, as shown in Figs. 401, 405, 406. There is provision made at a lower level for rails to carry a traveller for moving goods.
Plate VII. is a reduced copy of one of the type plans, signed by Sir Alexander Rendel and Sir Guildford Molesworth, for a roof on the Indian State Railways. The Truss is of the trussed-rafter form with two inclined struts; the Rafters of T iron; Purlins of angle irons fixed by angle-iron brackets; Struts of T iron; Heads, Shoes, and other joints of flat plates; Tension and Tie Rods of flat bar iron; Covering of corrugated iron, with a ridge cap of the same. The Rivets are of 3/4" diameter throughout.
Plates VIII. IX. are, by the kind permission of Sir John Coode, K.C.M.G., reduced from the working drawings of a roof for an engine house erected by him at the Cape. The Truss is of the trussed-rafter type with two struts, normal to the Rafters, which are of T iron; Struts, flat bars with distance pieces; Purlins, angle irons filled in with wood; Head of flat plates;