STRENGTH OF MATERIALS. 625 STRENGTH OF MATERIALS. MATERIAL Pouuds per sq. inch Timber, lougitudiual 600 Timber, transverse 3,000 Cast iron 20,000 Wroub'bt iron 60.000 Steel 70,000 Actual Values. The figures given in the pre- ceding tabuhir statements show rough average values, useful chielly to be memorized as a basis for appro.ximation. In careful designing tlie 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 : Hemlock White pine... Chestnut Red oak Yellow pine.. White oak.... Pounds per square inch Tensile Compressive strength strengtn 8,000 5,000 8,000 6,500 12,0(X) 6,000 9,000 6,000 16,000 9,000 12,000 8,000 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 cent, accord- ing to time of cutting, ])lace 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: Pounds per square inch TIMBER Alon(^ the grain Across the grain White pine Chestnut 600 600 600 2,500 1,500 4,000 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 jioiinds 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: Pounds per square inch Minimum Maximum Trap rock Granite Marble 20,000 12,000 8,000 7,000 6,000 24.000 21,000 20,000 20.000 15,000 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 poorly defined and the elongation is j)raetieally 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 all'orded 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 25,000 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 e.xtra 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 196 tons per square inch. The tensile strengths of some of the other more common metals are about as follows: MATEiiiAL, Pounds per sq. inch Brass, cast 23,600 Bra.ss wire 49,000 Copper, cast 24,000 Copper wire 60,000 Gold, cast 20,000 Gold wire 27,600 Lead, cast 2,050 Lead wire 1,660 Platinum wire 66.000 Aluminum 26,000 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 600 to 800 poimds 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, Eiifiiiiccrs' Pocket Book (New York. 1900). Working Stkes.ses. The loads carried by a structure should never strain the material to an amount at all close to its ultimate strength.
ere they to do so the elastic limit of the ma-
terial would be exceeded and distortion would ensue even though ultimate rupture did not oc- cur. Therefore, in designing a structure a unit stress is adopted which is certain not to cause distortion or rupture, and the size of each mem- ber is determined by dividing this unit stress into the total load on the member. This as- sumed unit stress is called the working stress, and it varies for any material according to the character of the load it is to support. Thus the safe working stress for a varying load is less than for a .steady load, and that for a sudden load or shock is less than for a varying load : a load producing tension and compression alter- nately requires the use of a smaller working stress than does a steady load or a varying load in tension only. In all cases the working stress
