MATERIALS, STRENGTH O 



MATERIALS, STRENGTH OP. 



(.1 



under the condition* discussed under ROOF. The precaution* to be 

 ohm-red in all these oases are 1, that an efficient circulation of air 

 ahould be kept up withiu the roof ; 2, that when metals are used, the 

 contraction and expansion ahould not be interfered with ; and 3, that 

 when two metals are employed, they should be placed in uch manner 

 an not to allow a destructive galvanic action to take place. 



Metals are also uaed in building operations in other mode* than limply 

 a* covering*, and iron in iU various forms has of lte become an article 

 of the greatest importance to the constructive or to the decorative 

 builder. Cast-iron U largely used for girders, columns, railings, pipes, 

 sashes, Ac. ; wrought-iron is becoming daily of more and more import- 

 ance for girders, for roof framing, for pillars, railings, tanks, doors, and 

 countless purposes of common life. Lead, copper, zinc, tin, and their 

 alloys are used for many details of building, aud for the distribution of 

 those essential elements of modern comfort, gas and water : precautions, 

 however, are requisite in their application to secure them from certain 

 chemical actions which may be produced by the extraneous matter* 

 contained in the fluids so distributed. [WATER SUPPLY.] It would 

 appear that if the new metal aluminium were obtainable at a cheap 

 rate, it would constitute one of the most valuable additions to the .list 

 of building materials, on account of its lightness, its tenacity, and its 

 powers of resisting the action of the atmosphere. 



Glass in its various forms is a material so generally known that it 

 will be sufficient merely to mention it ; and the use of oil-painta for 

 purposes of decoration or of preservation will be discussed under 

 PAINT. The use of the solutions of silica, and of the other various 

 substances applied of late years for the protection of woods from wet 

 and dry rot, will be noticed under the article PRESERVATION Of STONE 

 AND WOOD, already referred to. Wall linings, or PAPER HANUIM.S, 

 will be noticed under the latter head. 



MATERIALS, STRENGTH OF. The strength of any material 

 object, as a rod, bar, beam, chain, or rope, is that power by which the 

 substance resists an effort to destroy the cohesion of its parts. It 

 evidently depends on the disposition of the particles relatively to each 

 other, on the intensity of the force by which the particles cohere 

 together, and on the manner in which the straining power is applied. 

 The inquiry into the laws by which the materials employed in the con- 

 struction of edifices or machines resist the strains to which they are 

 subject is of considerable importance, because upon a just adaptation 

 of the strength at any one point to the strain there experienced (and 

 an excess or deficiency of the former is sometimes equally injurious) 

 depend* the stability of the whole structure. 



Whatever be the constitution of a rod or of a beam, the relation 

 between its strength and the strain to which it may be exposed can be 

 made the subject of mathematical investigation only by imagining the 

 material to consist of an infinite number of particles arranged in lines 

 (like fibres or threads) parallel to each other in the direction of its 

 length. The particles in each line must be supposed to cohere together 

 by Dowers exerted in that direction, and the several lines to cohere 

 laterally with forces which may or may not be equal to those exerted 

 longitudinally. In homogeneous bodies, as glass and some of the 

 metals, the particles may be supposed to be symmetrically disposed 

 throughout the masses, and to attract each other in every direction 

 with equal powers : but the case is different in other bodies, particu- 

 larly in timber; for in such bodies the lateral cohesion of the fibres is 

 much less powerful than the longitudinal cohesion of the particles in 

 each fibre. In ropes the fibres have no lateral cohesion, and the 

 strength depends on the twisting of the fibres together; in consequence 

 of this, as the latter can scarcely be separated from each other in the 

 direction of the length of the rope, the cohesion of nearly all the 

 particle* in any transverse section must be destroyed before a disruption 

 can take place. 



A rod of any material, consisting of parallel fibres as above supposed, 

 It-ing placed in a vertical position, and strained by a weight applied ut 

 the lower extremity, the particles in every fibre will be separated from 

 each other by the action of the weight, and consequently its length 

 will be increased. The cohesive power by which the particles are 

 kept together will, in most oases, be diminished by the separation ; and 

 if the weight be sufficiently great, or if it be allowed to act during a 

 sufficient length of time, the cohesive power will be eventually over- 

 come ; that is, the rod will in some parts of its length be torn asunder. 

 But before this occurs, since all bodies possess a certain degree of elas- 

 ticity, on removing the weight the attraction of cohesion will cause the 

 separated particles to return towards their original positions ; or the 

 rod will become nearly of the same length as at first. That it does not 

 exactly become so, in general, arises from the imperfect elasticity of 

 the material, on which account the particles come to a state of rest at 

 augmented distances from each other. The elongation of the rod 

 when strained by a weight, and the amount of the weight necessary to 

 produce fracture, will depend on the nature of the material ; and, from 

 a want of uniformity in the constitution of materials even of the same 

 kind, though the rods be of like dimensions, great irregularities are 

 found to exist in then- power of resisting direct strains. Numerous 

 experiments performed on each of the different kinds of material can 

 lone afford a mean value on which reliance may be placed when it is 

 required to determine the capability of a bar or beam to resist the 

 train arising from the action of any given force. 



If the materials were perfectly clastic, so that the length of the rod 



became the same after the' removal of the suspended weight as before 

 that weight w*s applied, the force of cohesion would evidently be pro- 

 portional to the intensity of the straining power ; and this is generally 

 adopted as an hypothesis in investigations concerning the equilibrium 

 between strengths and strains ; it being understood that the latter haro 

 only that moderate degree of intensity, compared with the former, 

 which is consistent with the permanent stability of the edifice or of 

 the machine. The law just mentioned appears to have been discovered 

 by Dr. Hooke ; and as the separation of the particles in any fibre is 

 proportional to the straining power, it follows that, within certain 

 limits, the cohesive power between two particles of an elastic body ia 

 proportional to the distances to which one of them is removed by the 

 straining force from the place where it was before at rest. The same 

 law is considered to hold good when the particles of an elastic body are 

 made to approach each other by the action of a compressing force like 

 that of a weight on the top of a vertical pillar. 



The power by which the particles in any body resist the action of a 

 force tending to separate those particles in the direction of the length 

 of the body may be considered as constituting the direct or absolute 

 strength ; and it is evident that, if the body were of a homogeneous 

 texture, that strength would be proportional to the number of particles 

 in a tranverse section ; that is, to the area of such section, while the 

 strain is proportional to the weight applied. Therefore, if p designate 

 the cohesive power in a unit of such area, as a square inch, a square 

 foot, &c. ; also, if A represent the area and w the weight applied, 

 including that of the body itself, we should have r A = w when the 

 strength and strain are in equilibrio. This formula for the absolute 

 strength may be considered as nearly correct with respect to most of 

 the bodies in nature ; and hence (r being determined by experiment) 

 the strength by which a rod of any material resists this kind of strain 

 may be found when the dimensions of the rod are given. 



For complete details concerning the experimental values of r, the 

 reader must be referred to the extensive tables which have been pub- 

 lished by Barlow (' Essay on the Strength of Timber '), lU-nni.- C 1'liil. 

 Trans.,' 1818), Tredgold ('Principles of Carpentry'), Hodgkinson 

 (' Experimental Researches into the Strength of Cast Iron,' &'< 

 Morin (' Lecous de Mecanique Pratique,' vol. iv.), our limits permitting 

 us to introduce only the few determinations which follow. 



The area of a transverse section of each rod is one square inch, and 

 the values of F are expressed by the breaking weights in pounds 

 avoirdupois. 



Notwithstanding the irregularities in the column containing the 

 values of F for ropes, it may be concluded that large ropes of a given 

 diameter have less strength on each square inch of tin -ir ti.i 

 section than those of less diameter ; and this is owing, no doubt, to 

 their threads being less twisted together. It may be observe! 

 that those woods whose fibres are nearly straight bear much y 

 weights suspended from them than those whose fibres have considerable 

 curvatiin-. 



According to the experiments of Mr. Barlow, it appears that a bar 

 of malleable iron is extended one ten-thousandth part of its length by 

 a direct strain equal to one ton for each square inch in the area uf the 

 transverse section : when stretched with ten tonx per inch its elasticity 

 was injured, or the bar did not return to its original state. 



If the fibres in any material body were exactly rectilinear, so that, a 

 rod being placed on one end in a vertical position, no one of the particles 

 were opposite to the intervals between any two in a transverse section 

 below it, it might be conceived that no force compressing the rod in 

 the direction of its length would produce any other effect than that of 

 diminishing its length. But as we find that all bodies when so com- 

 pressed may be bent, and finally broken, such a disposition of tliu 

 particles is destitute of I'M!,.,!., lily. In fact, when n j 

 pressed by a great weight above it, either iK fil.nw, already cmv. < I, 

 have their curvature increased, so that the whole pillar bends ; or the 



