It sometimes occurs in dwellings that it is necessary to use very long trimmers or headers around stair wells, and in most cases it is desirable that they shall be of the same depth as the floor joists. In such cases it is difficult to get sufficient stiffness without using iron or steel. Where steel beams are too expensive to use, a compromise may be made by using a flitch plate girder, which is a beam built up of two pieces of timber bolted to each side of a steel beam or plate, as shown in Fig. 51.
For such beams as may be required in dwellings a piece of boiler iron plate, of the same length and depth as the timber, and from 3/8 to ½ inch thick, will generally answer.
The total thickness of the timber should be about twelve times that of the plate.
The pieces should be bolted together by ¾-inch bolts, spaced 18 inches to 2 feet apart, and 2 inches from the edge. It will generally be better to keep the iron ½ inch narrower than the timbers, to allow for shrinkage in the latter.
The safe distributed had may be calculated by the formula
W = 2*D^2/ L (f b +750 t); D being the depth of the iron in inches,b the total breadth of the timber, t the thickness of the iron, L the clear span in feet and f the coefficient of strength for the timber (too for Georgia pine, 70 for spruce, 60 for white pine). Such a beam will be much stiffer than a wooden beam of the same strength, and considerably cheaper than a steel beam, especially when the furring of the beam for plastering is taken into account.
Example. - What is the safe distributed load for a flitch plate girder, built up of two 3x12 spruce beams and a ½x11½-inch plate, the clear span being 16 feet ?
Answer. - W = 2 * (11½)^2 / 16 (70 * 6+750 * ½) = 13,142 pounds.