21. Steel. The hard steels have the highest compressive strength; there is a recorded value of nearly 400,000 pounds per square inch, but 150,000 is probably a fair average.

The elastic limit in compression is practically the same as in tension, which is about 60 per cent of the ultimate tensile strength, or, for structural steel, about 25,000 to 42,000 pounds per square inch.

22. Cast Iron. This is a very strong material in compression, in which way, principally, it is used structurally. Its ultimate strength depends much on the proportion of "combined carbon" and silicon present, and varies from 50,000 to 200,000 pounds per square inch, 90,000 being a fair average. As in tension, there is no well-defined elastic limit in compression (see curve for cast iron, Fig. 5).

23. Brick. The ultimate strengths are as various as the kinds and makes of brick. For soft brick, the ultimate strength is as low as 500 pounds per square inch, and for pressed brick it varies from 4,000 to 20,000 pounds per square inch, 8,000 to 10,000 being a fair average. The ultimate strength of good paving brick is still higher, its average value being from 12,000 to 15,000 pounds per square inch.

24. Stone. Sandstone, limestone and granite are the principal building stones. Their ultimate strengths in pounds per square inch are about as follows:

*Compression at right angles to the "bed" of the stone.

Examples For Practice

1. A limestone 12 x12 inches on its bed is used as a pier cap, and bears a load of 120,000 pounds. What is its factor of safety ? Ans. 12.

2. How large a post (short) is needed to sustain a steady load of 100,000 pounds if the ultimate compressive strength of the wood is 10,000 pounds per square inch ? Ans. 8 X10 inches.

25. Materials in Shear. The principal materials used under shearing stress are timber, wrought iron, steel and cast iron. Partly on account of the difficulty of determining shearing strengths, these are not well known.

26. Timber. The ultimate shearing strengths of the more important woods along the grain are about as follows:

"Wood rarely fails by shearing across the grain. Its ultimate shearing strength, in that direction is probably four or five times the values above given.

Fig. 6 a.

Fig. 6 b.

27. Metals. The ultimate shearing; strength of wrought iron, steel, and cast iron is about 80 per cent of their respective ultimate tensile strengths.

Examples For Practice

1. How large a pressure P (Fig. 6 a) exerted on the shaded area can the timber stand before it will shear off on the surface abcd, if ab = 6 inches and bc = 10 inches, and the ultimate shear-ing strength of the timber is 400 pounds per square inch ?

Ans. 24,000 pounds,

2. When a bolt is under tension, there is a tendency to tear the bolt and to "strip" or shear off the head. The shorn area would be the surface of the cylindrical hole left in the head. Compute the tensile and shearing unit-stresses when P (Fig. 6 b) equals 30,000 pounds, d = 2 inches, and t = 3 inches.