ACTUAL VALUES. The figures given in the pre ceding tabular statements show rough average values, useful ehiellv to be memorized as a basis for approximation. In careful designing the in gineer requires actual values as determined by authoritative tests upon the particular material he is using or proposes to use. These values vary within quite narrow limits for each kind or quality of any material and within broader limits for all kinds and qualities of the same material. The ultimate average strengths of six of the most common structural timbers are as follows: These values have been obtained by testing small pieces. Timbers of large size such as are actually used in engineering structures will fall from 20 per cent. to 50 per cent. below these values in strength. These values are also sub ject to a variation of about 25 per eent, accord ing to time of cutting, place of growth, and methods of seasoning. The shearing strength of timber is more variable than either the tensile or the compressive strength. The values average about as follows: The elastic limit of timber is not well defined; it varies from one-third to one-half the ultimate strength in tension. The elongation at the point of rupture in tension is from 1 per cent. to 2 per cent. Tensile and shearing tests of brick are seldom made, but the compressive strength varies from 500 pounds per square inch for soft brick to 10.000 pounds for pressed brick, and to 15.000 pounds for the best qualities of hard burned pav ing brick. The crushing strengths of the prin cipal building stones of the United States are about as follows: Cast iron is so variable in quality that similar specimens will vary from 10 per cent, to 20 per cent. in strength. Fair average values are 20,000 pounds per square inch in tension and 90.000 pounds per square inch in compression. The elastic limit is puorly defined and the elongation is practically nil, indicating clearly the fact, known to every one familiar with this material, that it is a hard and brittle substance. A direct
contrast to cast iron is afforded by wrought iron with a tensile and a compressive strength nearly equal and ranging between 50.000 and 00.000 pounds per square inch, and with an elongation of 20 per cent. to 30 per cent. and an elastic limit well defined at about 2,5,000 pounds.
Steel is manufactured in a variety of grades distinguished by the relative amounts of con tained carbon, as mild steel, medium steel, hard steel, and extra hard steel. Ordinary structural steel for bridge and building construction has an ultimate tensile strength of from 00.000 pounds to 70.000 pounds per square inch and an elastic limit of from 30,000 pounds to 40.000 pounds per square inch. Nickel-steel, or steel containing a small percentage of nickel, has been made with a tensile strength of 277,000 pounds per square inch and an elastic limit of 100.000 pounds per square inch. The compressive strength of steel is always greater than its tensile strength. The maximum compressive strength recorded for hardened steel is 392,000 pounds or 190 tons per square inch.
The tensile strengths of some of the other more common metals are about as follows: Common mortar composed of one part lime and five parts sand has a tensile strength of from 15 to 30 pounds and a compressive strength of from 150 to 300 pounds per square inch at the age of six months. Natural hydraulic cement will test from 100 to 200 pounds per square inch in tension when one month old, and Portland cement will test from GOO to 800 pounds per square inch at the same age. Mortars and pastes of cement and lime increase in strength with age. For a concise statement of strengths of a great variety of other materials, see Traut wine, En.ryi»errs' Pocket Book (New York, 1900).