Lintels of cast iron were at one time extensively used for supporting brick walls over store fronts and door openings, and even at the present time are used to some extent. On account of the brittle character of this metal, however, and its low tensile strength, it should not be used for beams subjected to a moving load, such as floors upon which heavy articles are moved.
Cast iron beams of long span are also not as economical as those made of rolled steel. About the only places, therefore, in which cast iron lintels may be suitably and economically used, are over store fronts where the span does not exceed 8 feet, and over door openings in unfinished brick partitions where a flat head is necessary. The relative economy between cast iron and steel lintels will depend largely upon the distance from the rolling mills and upon freight rates. Foundries for casting iron are much more widely distributed than rolling mills, so that castings of almost any shape can usually be obtained in any city of twenty thousand inhabitants, while mills for rolling steel beams are comparatively few in number and located mostly in the extreme eastern portion of the country.
The common shape for cast lintels over door openings is that shown in Fig. 207. The width of the flange is usually made the full thickness of the wall, and the extreme height of the lintel at the centre not less than two-thirds nor greater than the width of the flange. The strength of the lintel may be somewhat increased by stiffening the web at the centre by brackets, as shown by dotted lines at A.
Where the width of the flange must be over 16 inches two webs should be used, as shown by the section drawing, Fig. 208. For handling and moulding it is best not to make the flange more than 24 inches wide; if a greater width than this is required, several lintels should be placed side by side. The thickness of the metal should not be less than ¾ inch, and the web should be about 1/8 inch thicker than the flange.
When proportioned as above the strength of the lintel to support a dead load may be safely made equal to 9700 X area of bottom flange X extreme depth span in inches.
Fig. 210. - Store Front Lintel.
Thus a lintel of 6 feet clear span with 12-inch by ¾-inch flange and extreme depth of 12 inches should safely support
9700 X 9 X 12 / 72 = 14,550 pounds.
Lintels over store fronts should be made with ribs at the ends, as shown in Fig. 209, with holes for bolting the lintels to each other and to columns. Store front lintels are also occasionally made as shown in Fig. 210, to give a finish above the openings.
Fig. 211 shows details for cast iron lintel and sill, sometimes used for windows in external walls. The thickness of the sill need not exceed 3/8 of an inch. 324. Cast Iron Arch Girders are also sometimes used to support brick and stone walls where the opening is from 10 to 30 feet in width. Fig. 212 shows a girder of this kind that was used to support a central tower over the crossing of the nave and transept on St. John's Church, Stockton, California, Mr. A. Page Brown, architect. The clear span is 29 1/3 feet, and the height of the wall above the girder 18 feet. One object in using such a girder in this place was to get the height in the centre without also raising the supports, which could not be obtained with a steel plate girder. The church has a vaulted ceiling which comes just below the arch of the girder, the tie-rod being exposed.
The rise of the casting in this case is rather more than common, the usual rise being from 1/10 to 1/8 of the span. The end of the girder is generally cast in the shape of a hollow box, with shoulders to receive the ends of the rods. The tie-rod is often made with square ends, and about 1/8 inch shorter than the casting, and is heated until the expansion permits of its being slipped into its place in the casting. As it cools the contraction binds it tightly into its place. If tightened by means of a screw and nut, the nut and bearings should be dressed to a smooth surface and the rod turned up with a long-handled wrench. It is very essential that the rod shall be fitted in place so tightly that no tensile strain can come on the casting, and, on the other hand, it should not be expanded so as to bring an initial strain on the arch.
This form of girder is comparatively little used now, but there may be conditions, as in the church mentioned above, where it can be used to advantage.