The student will have noticed (in Chapter XI (Iron And Steel. Pig Iron).) how the use of iron tends to supersede that of wood in roofs originally designed and executed in wood; and as excellence in the production of wrought-iron of various forms increased, so it gradually displaced wood in the construction of roofs, first taking the parts of various members of the different trusses, for which it was in many ways the more suitable material, until now it has almost completely taken its place for the roofs of such buildings as stations, warehouses, factories, and similar constructions of a plain, substantial character, and where no ceilings are required; whereas, in trusses for the roofs of domestic buildings, the difficulty of dealing with them in a simple, practical manner precludes the exclusive use of iron - though it must be pointed out that iron is still used for the trusses only in all buildings, seeing that the purlins must necessarily be of wood to receive the rafters, which must also be of wood, for the purpose of securing the roof-boarding or laths which carry the slates or other external coverings. These wooden members - or, rather, the longitudinal parts of roofs, such as purlins or ribs, and ridges, to which the covering framing is secured - are supported in their respective positions by angle-irons and other similar means, which are, in their turn, secured by bolts or rivets to the principal rafters, etc., of the iron trusses.
Before proceeding to deal with the iron trusses themselves it will be well to fully explain this part of the subject.
The ridge is generally treated as in fig. 431, being let into a special casting fitted and secured to the apex of the iron truss; or it is held up between two short pieces of angle-iron, also secured to the truss, as fig. 432.
The purlins, or ribs, which support the common rafters - or to which, when placed closer together, the boarding itself is affixed - are secured by coach screws to angle-irons, as fig. 433, bolted to the back of the principal rafter, just as cleats assist purlins on wooden trusses.
Iron trusses have a lighter appearance than wooden trusses, though they can be made much stronger, according to requirements, both wrought and cast-iron being employed in their construction in the first instance. The cast-iron is used for the members in compression, as rafters and struts; while wrought-iron is, of course, used for ties and suspension rods, whether kings, queens, or princesses, as they are called in wooden roofs. As cast-iron was very fragile - liable to snap under sudden shocks, etc., and otherwise troublesome - it was in course of time displaced by wrought-iron, which has the advantage of being almost equally as strong to resist compression as tension - a great point in some members which are liable to both compression and tension, almost touching each other, as it were (as will be explained in Chapter XXIV (Stresses. Definitions). on "Strains"), so that now iron roof trusses, with the exception (and that not always) of the shoe, connecting the truss to the bearings on the supports, are completely built up of wrought-iron.
When iron trusses first came into vogue they were made of similar form to wooden trusses, as in figs. 434 and 435 - which, it will be seen, are identical with "king " and "queen"- post wooden trusses - but subsequently it was found out that these old lines of principle could be deviated from with advantage, causing a saving of labour and material without any loss of strength whatever. Among the various novelties in form then and afterwards introduced were trusses similar to figs. 436 to 441; or they were oftentimes made of circular form - i.e., with a circular roof formed by a circular principal rafter, the framing to support it being on lines following one or other of the triangular-shaped trusses hereinbefore illustrated.
In the study of iron roofs it helps the student considerably to have a knowledge, however slight, of the different strains, whether of tension or compression, which the members are generally subjected to; and with that end in view the writer would suggest that the following simple comparison be employed to distinguish one from the other - viz., if a rope can be substituted for the piece of iron, then it may be taken for granted that that member is in tension; whereas, if such substitution cannot be made without detriment to the structure, it is in compression. This being ascertained, rods must be used for tension, and T's for compression, generally.
With this knowledge the student should be perfectly able to apply he principle of the following illustrated explanations of the ordinary joints to whatever ordinary roof is required. It would be an endless undertaking to point out the peculiarities of all joints of the many forms of iron roof-trusses, so that an exposition of the principles must suffice once for all.
Side Elevation Fig 442.