It is sometimes inconvenient or impossible to use a cast-iron girder of uniform strength. For instance, when it has to carry weight on both flanges both are required to be horizontal and both to be of convenient width throughout.
In these, whether uniformly loaded throughout their length or loaded at the outer end, the stresses are greatest at the fixed end and diminish from that point to the outer end of the cantilever, which may therefore be graduated in depth.
Figs. 252, 253 are the elevation and plan of a cast-iron cantilever, of which Fig. 2 5 4 is the cross section at A B.1
Fig. 252. Elevation.
Fig. 253. Plan.
Fig. 254. Section on A B, Fig. 252.
Further Particulars relating to Cast-iron Girders.
The webs of both girders and cantilevers have only shearing stress to bear (see Part IV.) which is comparatively slight. They may therefore safely be lightened by piercing the web with holes as in Fig. 252. Perforated webs are, however, liable to unequal cooling and air bubbles, and should therefore be avoided where heavy loads have to be borne.2
Cast-iron girders should never be fixed at the ends nor be continuous over more than one span, for this would subject parts of the small compression flange to tension, for which it would be quite unsuited.
1 The theoretical curve of the lower flange of a cast-iron cantilever, uniformly loaded throughout its length, is nearly a concave parabola, practically a circular arc may be used. 2 Wray.
The metal of cast-iron girders and cantilevers should, as explained above, be of equal or gradually varying thickness and the re-entering angles all rounded so as to avoid any internal stress (see Part III.) The thickness of metal in any part of cast-iron beams should as a rule not be less than 1/12 of the width of the part.1 The web should be equal in thickness to the flanges, tapering, where they differ, from the thinner to the thicker. Some engineers use, however, thinner metal when necessary.
So far as the requirements of good casting go, "for flanges 2 feet wide it is not wise to have a less thickness than 1 1/2 inches, and for flanges 18 inches wide not less than 1 inch, while for narrower flanges a somewhat less thickness may be used, although it is not a good plan to fine down the metal too much." 2
The depth of cast-iron girders should be ordinarily from 1/10 to 1/10 the span. Sometimes even 1/20 has been used, and cast iron should not be used for girders of over 20 feet span, nor at all for important girders.
Cast-iron girders are bedded at their ends on tarred felt. The ends are sometimes widened so as to increase the bearing area.
Small cast-iron girders are sometimes made of ┴ section; in which case the web should be of a good thickness, as the upper portion of it has, in the absence of a top flange, to withstand considerable compression.
Cast iron is easily moulded to girders of any ornamental pattern, and to any dimensions required to fit particular positions; but girders of this material are very heavy, are liable to contain dangerous flaws, are brittle and apt to break without warning, especially when cooled suddenly by water thrown upon them while they are very hot.
This is very likely to happen in a building on fire, and to cause serious accidents.
Cast-iron girders are heavier and less handy than wrought-iron girders of equal strength and are of much less reliable material. Cast iron is, however, useful when a girder or cantilever is required to be ornamental or of peculiar form - but as a material for girders generally it has been almost superseded by wrought iron, though it is still much used for cantilevers, brackets, etc.
1 Adams. 2 Wray.
On account of these objections to cast-iron girders they are not used so much as formerly, having been to a very great extent superseded by wrought-iron beams.