Different materials break or fail under compression, in two very different ways: 1. Ductile materials (structural iron, etc.), and wood compressed across the grain, do not fail by breaking into two distinct parts as in tension, but the former bulge out and flatten under great loads, while wood splits and mashes down. There is no particular point or instant of failure under increasing loads, and such materials have no definite ultimate strength in compression.
2. Brittle materials (brick, stone, hard steel, cast iron, etc.), and wood compressed along the grain, do not mash gradually, but fail suddenly and have a definite ultimate strength in compression. Although the surfaces of fracture are always much inclined to the direction in which the load is applied (about 45 degrees), the ulti mate strength is computed by dividing the total breaking load by the cross-sectional area of the specimen.
The principal materials used under compression in structural work are timber, wrought iron, steel, cast iron, brick and stone.
19. Timber. As before noted, timber has no definite ulti mate compressive strength across the grain. The U. S. Forestry Division has adopted certain amounts of compressive deformation as marking stages of failure. Three per cent compression is regarded as "a working limit allowable," and fifteen per cent as "an extreme limit, or as failure." The following (except the first) are values for compressive strength from the Forestry Division Reports, all in pounds per square inch: vlaitZgatutg:itt.h Hemlock 6,000 White pine 5,400 700 Long-leaf yellow pine 8,000 1,260 Short-leaf yellow pine 6,500 1,050 Douglas spruce 5,700 800 White oak 8,500 2,200 Red oak. 7,200 2,300 20. Wrought Iron. The elastic limit of wrought iron, as be fore noted, depends very much upon the size of the bars or plate, it being greater for small bars and thin plates. Its value for com pression is practically the same as for tension, 25,000 to 40,000 pounds per square inch.
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 compres sion, in which way, principally, it is used structurally. Its mate strength depends much on the proportion of "combined car bon" 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 pav ing 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: Sandstone,* 5,000 to 16,000, average 8,000.
Limestone,* 8,000 " 16,000, " 10,000.
Granite, 14,000 " 24,000, " 16,000.
*Compression at right angles to the "bed" of the stone.
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. 10 X 10 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: shearing strength in that direction is probably four or five times the values above given.
27. Metals. The , ultimate shearing strength of wrought iron, steel, and cast iron is about 80 per cent of their respective ultimate tensile strengths.
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 abed, if ab =6 inches and be = 10 inches, and the ultimate shear ing strength of the timber is 400 pounds per square inch ? Ans. 24,000 pounds.