STRAWBERRY HILL 



STRENGTH OF MATERIALS 767 



berry -time ' it is amazing to see the fruit pouring 

 into Covent Garden, from ship and from train, ano 

 by the English grower's van from all the nearer 

 counties. The imported fruit is coarse, insipid, 

 and generally in bad condition, but it serves to 

 keep the prices low. 



For further instructions, see works cited in our article 

 upon Gardening ; also Professor Decaisne's Jardin 

 Fruitier du Miaee ; the Illustrated Dictionary of Gar- 

 dening, by George Nicholson ; and the Strawberry and 

 haw to grow it, by E. W. Harrison. For 'strawberry 

 leaves,' see CORONET. 



Strawberry Hill. See TWICKENHAM. 



Streatham, a suburban parish in Surrey, 6J 

 miles SSW. of St Paul's. The Thrales's house, 

 visited by Dr Johnson, is gone ; but the church, 

 though rebuilt in 1831, retains some interesting 

 monuments. Pop. (1881) 25,553; (1891) 48,742. 

 See F. Arnold's History of Streatham ( 1886). 

 r Streator, a mining-town of Illinois, on the 

 Vermilion River, and on five railways, 94 miles 

 SW. of Chicago. Pop. (1880) 5157 ; (1890) 11,414. 



Street, GEORGE EDMUND, architect, born at 

 Woodford in Essex, 20th June 1824, was edu- 

 cated at Camberwell and Crediton, and studied for 

 five years with Gilbert Scott. Starting in practice 

 for himself in 1849, he designed many cnurches 

 throughout the country, and restored more the 

 chief restoration being Christ Church Cathedral in 

 Dublin. Cuddesden College and Uppingham 

 School are by him ; but his most famous work 

 is the new Law Courts in London, the subject 

 of so much controversy (see Vol. VI. p. 703). 

 Street became an A.R.A. in 1866, an R.A. in 1871, 

 and P.R.I.B.A. in 1881. He died in London, 18th 

 December 1881, and was buried in Westminster 

 Abbey. He published The Architecture of North 

 Italy in the Middle Ages (1855) and Gothic Archi- 

 tecture in Spain ( 1865 ). See Memoir by his son ( 1 888). 



Streltzi, or STRYELTSY. See RUSSIA, p. 46. 



Strength of Materials is the heading under 

 which it i.s usual to discuss the elastic or resisting 

 properties of the materials used in engineering or 

 building operations (see ELASTICITY, also STRAIN 

 AND STRESS ). When a structure is being designed 

 the engineer must know first of all the amount and 

 character of the stresses (loads, wind-pressures, 

 &c.) that will act upon the structure. He must 

 then decide as to the size and shape of the pieces 

 that are to compose the structure, so that they 

 may easily stand these stresses. For this purpose 

 he must know beforehand what 'strength of 

 material ' is possessed by the steel, iron, or wood 

 that is to be used. 



When any substance is strained beyond a certain 

 limit it will break, and the greatest stress which 

 the substance can bear without being torn asunder 

 is called its ultimate strength. The value of this 

 for any given piece of material will depend upon 

 the kind of strain to which it is being subjected. 

 But whatever this strain be, whether extension, 

 compression, flexure, or twisting, there are two, 

 or at most three, distinct kinds of ultimate strength 

 which practically fall to be considered. The one 

 is the ultimate tension or pressure applied in one 

 direction, usually longitudinally ; and the other is 

 the ultimate shearing stress, such as comes into 

 play in simple torsion. In certain cases, such as 

 in steel, wrought-iron, and ductile metals gener- 

 ally, the strength under tension and that under 

 longitudinal pressure in other words, the tenacity 

 Ana the resistance to crashing are practically the 

 same. In other cases, however, of which cast-iron 

 is the most interesting instance, the resistance to 

 crashing is much greater than the tenacity. The 

 ultimate strength under shearing is generally less 

 than that under tension or compression. For 



example, the ultimate tensile strength of steel 

 varies from 30 to 45 tons' weight per square inch 

 of section, while the ultimate shearing strength 

 varies from 22 to 35. Castiron, again, which has 

 a tensile strength of 74 tons' weight per square 

 inch, lias a strength under crushing or 45 and a 

 shearing strength of 12. 



It is out of the question to make a structure in 

 which the pieces are strained up to their ultimate 

 limits. For, even though the limit is not exceeded 

 and the material not torn asunder, the excessive 

 straining to near the limit will produce a per 

 manent deterioration in strength. In other words, 

 the ' working strength ' is much smaller than the 

 ultimate strength, being obtained from it by divid- 

 ing by a number known as the 'factor of safety.' 

 In the case of steel this factor is about 6 ; so that 

 in no structure should a hard steel rod be subjected 

 to a greater tension than 7J tons' weight per square 

 inch. Experience is the sole guide as to the value 

 of this factor, which must be taken large enough 

 to provide a margin of strength for all possible 

 contingencies. Now, in the first place, the ulti- 

 mate strength of a material that is to be used in 

 a bridge or roof is somewhat uncertain. It is 

 obtained by testing a sample. But no two samples 

 of the same material have ever quite the same 

 strength. Again, although theoretically a long 

 column should have the same tensile strength as 

 a short one of the same material and section, 

 practically it is not so. There is greater chance 

 of there being weak places in the longer column, 

 and at the weakest place the material will begin 

 to yield. Thus a greater factor of safety must-be 

 used in estimating the working strength of the 

 longer rod. Then, in the second place, the char- 

 acter of the stress to which the material is to be 

 subjected must be considered. If it is to be a 

 fluctuating and not a steady stress the factor of 

 safety must be increased, and similarly a wider 

 margin of strength must be provided if the material 

 is to be subjected to sudden shocks or impacts. 

 For example, a bridge which is strong enough to 

 allow a train to rest on it or to crawl over it, may 

 be unable to support the train dashing at full 

 speed. In fact, under a stress which fluctuates 

 between wide limits the ultimate strength is 

 diminished ; hence if the ultimate strength has 

 been measured by testing a sample under a steady 

 stress, and if the substance is to be subjected to a 

 sudden shock, the factor of safety is doubled. 



A very important part of the subject is the con- 

 sideration or the form best suited to resist certain 

 strains. A glance at any fine modern structure, 

 such as the Forth Bridge, will show how the form 

 is varied, according as the member is in compression 

 or in extension. Here the question of flexibility 

 enters in. For although the strengths under ex- 

 tension and compression may be the same, yet 

 if a rod is taken too thin and subjected to a longi- 

 tudinal pressure, it will bend long before the true 

 :ompression limits are reached. This bending or 

 buckling must be prevented, and the only way of 

 doing so is to increase the section. Thus hollow 

 iubes resist buckling better than rods of the same 

 ength and mass. Herein also lies the great virtue 

 of the I-shaped rod, which if laid horizontally and 

 supported by its ends bends under its own weight 

 very slightly as compared with the bending of a 

 solid cylindrical rod of the same length and mass. 



See Todhunter and Pearson, A History of the Elasticity 

 and Strength of Materials (1886-94); H. T. Bovey 

 Theory of Structures and Strength of Materials ( New 

 York and London, 1893 ) ; Barlow's Strength of Materials 

 6th ed. 1867 ) ; Fairbairn's Mechanical Properties of 

 Steel (Brit. Assoc. Reports, 1867) ; Burr's Elasticity and 

 Resistance of the Materials of Engineering (New York 

 1883; newed. 1889); and W. G. Kirkcaldy's Strength 

 and Properties of Materials (New York, 1891). 



