AS the machinery for rolling iron bars has improved, and the facilities for obtaining these of any required section have become greater, so iron has gradually, to a great extent, taken the place of wood for roofs, especially those of large span.
Some examples of these roofs in the transition state, composed of wood and iron combined, are given in pages 173 to 177.
In process of time iron was substituted, first for one member of the roof, then for another, until the whole truss was composed of iron in different forms.
The result of this gradual change was that the early iron roofs were nearly of the same form of construction as the ordinary timber trusses.
It was soon noticed, however, that the material could be better applied, and different forms were adopted for iron roofs, some of which will now be described.
The various forms of iron roofs have been classed as follows: - 1
1. Roofs with straight rafters.
2. Roofs with arched rafters.
3. Mixed roofs, which form a transition between the other two. Of these, the second and third classes are used chiefly for very large spans, far exceeding those of 40 feet (to which this course is limited). It will, therefore, be unnecessary further to notice them, except in the case of two very simple examples of arched roofs, which may here be described before the whole of Classes 2 and 3 are dismissed as not coming within the scope of these notes.
1 Umvin's Wrought-iron Bridges and Roofs.
This simple form of arched roof consists merely of sheets of corrugated iron riveted together into the form of an arch. The edges of the resulting large arched sheet are secured at the springing to wall plates, angle irons, or to the inner sides of iron gutters. Tie rods, king bolts, and struts are used for moderate spans and curved iron Principals for larger roofs.
Figs. 339, 340 show two forms of this kind of roof.
For spans of 12 feet the rods as shown in Fig. 3391 are necessary (even for 10 feet spans they are desirable), and roofs of this form have been used for spans up to 3 0 feet. It is better, however, to restrict them to 20 feet.
For spans of over 20 feet struts must be added as shown in Fig. 340. The ends of the tie rod are secured to plates on the heads of the columns, or walls supporting the roof, or to cast-iron gutters made specially thick, and for the large spans, strengthened by flanges, and stiffened by arch-shaped cast-iron stays across them at intervals of about 10 feet, and the covering is fastened near the eaves by hook bolts to angle irons secured to the head of the columns. The covering and columns should be well held down, as the wind has a great effect upon roofs of this kind. Such roofs may be used up to spans of 3 0 or 35 feet, but beyond this curved Principals must be used with purlins to carry the roof covering. The corrugated iron may generally be of 18 to 20 Birmingham wire gauge, and the tie rods 8 feet apart.
1 From Messrs. Rownson, Drew, and Co.'s catalogue.
It may be convenient to bring the different forms of trusses ordinarily used for iron roofs with straight rafters into one view before they are described in detail.
Figs. 341 to 359, PL III., show forms of trusses, most of which are in common use, and the spans for which each is adapted.
When the principal rafters are long they require support at intermediate points, which with roofs of ordinary construction should not be more than 8 or 9 feet apart. This support may be given in two distinctly different ways.
In these the principal rafters are supported by one, two, or more struts at right angles or nearly at right angles to them, which together with tension rods form the principal rafters into a pair of trusses, joined at the ridge of the roof and prevented from spreading by the tie rod.
Sometimes, though rarely, the struts supporting the principal rafters are vertical as in Fig. 344.
In these the trusses are of the same skeleton form as in timber roofs, the principal rafters being supported by inclined struts which with the tension rods and tie rod form the whole into a truss.
Occasionally, however, though but rarely, the struts are made vertical and the tension rods inclined as in Figs. 353, 355. The vertical struts are more convenient when the roof has hipped ends.