RESISTANCE TO COMPRESSION.] 



APPLIED MECHANICS. 



801 



Fig. 61 



But if we endeavour to trace the practical effect of 

 crushing any material, we shall find it difficult to establish 

 the truth of this simple law, without making great allow- 

 ances for irregularities in the consistency of the material, 

 and in the application of the compressive force. Some 

 materials of a soft consistence, such as clay or lead, on 

 being subjected to vertical compression, merely spread 



out sideways. Thus, a 

 piece of lead compres- 

 sed by a heavy weight 

 bulges in width, and 

 thickness, to the form 

 marked by the dotted 

 lines (Fig. 62). Other 

 materials of a fibrous 

 texture, such as wood, 

 become splintered 

 compression, owing to 

 some of the fibres being 

 driven or wedged into 

 the interstices between 



others, and thus bursting them asunder. Hard anc 

 brittle materials, such as cast-iron, glass, stones, or bricks, 

 are riven and splintered in most irregular and unex- 

 pected ways, gome portions being actually pulverised in 

 the process, or cracking away and occasionally flying ofl 

 with considerable force frem the rest. 



These results, however, occur when the compressive 

 strain is carried beyond what the material is capable oi 

 sustaining without disintegration ; for all practical pur- 

 poses, where we make the strength more than adequate 

 to meet the load, we may very safely follow the simple 

 law, and reckon the strength to be very nearly as the 

 area of section at the weakest part. This, however, only 

 holds good when we pay no regard to the height of the 

 mass pressed upon. As we increase the height we must 

 increase the strength, for several reasons, which we shall 

 endeavour to illustrate. Suppose we take a cylindrical 

 piece of cast-iron o/ie inch in diameter and six inches 

 lii^'li, and placing it upright, load it on the end until it 

 will bear no more. As we place more and more weight 

 upon it, the particles become pressed closer and closer 

 together, and the mass yields or subsides a little in the 

 direction of its length. After being compressed to a cer- 

 tain extent, the particles cannot get into closer contact ; 

 but some of them must insinuate or wedge themselves 

 into the interstices between others, and thus the mass 

 will tend to bulge sideways ; or, being of a hard unpliable 

 consistency, the sides will become fractured, and burst 

 away. The mass thus weakened will finally all crumble 

 under the pressure. Now let us take a cylindrical piece 

 of the same material, and of the same diameter ; but, 

 instead of being six inches high, let it be merely a thin 

 disc one-eighth or one-sixteenth of an inch thick. Here, 

 although the area of section pressed upon be the same as 

 before, yet the height being so much less, the number of 

 particles that can be forced to one side or the other is 

 much less, the number of interstices to receive pressed 

 particles is less, the amount of bulging is diminished, 

 and the whole resistance is enormously greater. As an 

 example of the effect of increased Height. Crushing Force, 

 height in diminishing strength in. 



to resist vertical pressure, we $ 



may quote some experiments 

 made on pieces of cast-iron, all 

 having square bases, a quarter of 

 an inch each way, and various 

 heights ; whence it appears that 

 as the height was increased, the strength to resist com- 

 pression was diminished, so much, that the force to crush 

 tho piece f inch, or 1 inch high, was little more than tivo- 

 thirds of that required to crush the piece & inch high. 



Farther, when the height is considerable, as in the 

 case of columns, a slight want of uniformity in tho ma- 

 terial, or a slight want of equality in the distribution of 

 the load, will cause one side to be affected more than tho 

 other, and thus produce flexure in the case of pliable 

 materials, such as wood or wrought iron, or oblique 

 Mporation in the case of stone. An extreme load press- 



Ibs. 



9006 

 8845 

 8362 

 6430 

 6321 



ing on a column just tottering on the versje of ruin, wil 

 certainly find out the weakest place, aud there begin tlw 

 demolition ; and having once begun it, will very rapidly 

 complete it. Thus, a knot in a wooden post, or a vein in 

 a marble pillar, may become important elements in deter- 

 mining the mode of their fracture, and the amount -n: 

 compression required to effect it. It appears, from some 

 experiments made with wooden columns, that when the 

 height is more than eight times the diameter, there is 

 considerable risk of the column bending under an ex- 

 cessive load. With timber of uniform strength, and a 

 load well balanced upon the column, when its heighl 

 does not exceed this proportion, the wood will generally 

 splinter under a crushing force. In columns of stone or 

 brickwork, destined to sustain a heavy load, the ratio ol 

 height to diameter may be increased considerably above 

 that of eight times, without the danger of their breaking 

 by a sloping fracture. In such cases, however, the eye 

 seems instinctively to judge of proportion ; for a very tail 

 slender column conveys a notion of instability, and 

 is consequently wanting in that grace which depends, 

 mainly, on the fitness of the object for the work it has 

 to perform. 



Cast-iron is found to be capable of supporting a very 

 considerable vertical load without flexure or crushing ; 

 aud columns of this material are, therefore, with safety 

 made of tall and slender proportions. Even here, how- 

 ever, there is a limit beyond which the eye becomes dis- 

 satisfied ; and accordingly, when the column is required 

 to be of considerable height, it is generally made of 

 greater diameter, the inside being hollowed out to save 

 weight and material, without sensibly affecting the 

 strength. Where, from inequalities of the load, there is 

 any risk of unequal pressure on the summit of a cast- 

 iron column, and of consequent tendency to bend, the 

 material used in constructing the column is much more 

 advantageously applied in the shell of a hollow column, 

 or in the ribs of one, the section of which is a cross, than 

 in one solid. We only advert to this now, as we shall 

 have an opportunity of recurring to it when we discuss 

 the question of transverse strain. 



Some very valuable experiments have been made upon 

 the crushing of building materials, such as stones and 

 bricks. The compressive forces in most of them have 

 been applied to cubical pieces of 1 inch or 2 inches. In 

 some, where the pieces had a square base 1 inch broad, 

 and a height of 2 inches, the force required to crush, has 

 been found less than two-thirds of that required for the 

 pieces only 1 inch in height. The increase of height, 

 therefore, considerably diminishes the strength to resist 

 compressive force. In connection with this subject, it 

 may be interesting to inquire how high a brick building 

 might be carried without becoming crushed by its own 

 weight. We may suppose a massive brick pier or column 

 divided into numerous separate columns, each 1^ inch 

 square in section, and each supported along its whole 

 height by those around it, against lateral pressure or 

 flexure. The lowest part of each of these columns would 

 thus have to support the vertical weight of all the column 

 above it ; and if that weight exceeded the load required 

 to crush a brick cube of 1J inch, the lowest part of the 

 column would of course give way. Now, it has been 

 found, by experiment, that the load necessary to crush a 

 li inch cube of brick, varies from 1,200 Ibs. up to 3,000 

 b"s., according to the different qualities of the brick. It 

 would perhaps be safest to take the lower number, 1,200 

 bs. The weight of 1 foot of brickwork having a 1 inch 

 square base, is about 2 Ibs. ; therefore it would require a 

 leight of 600 feet to crush the base. An architect, wish- 

 jig to secure the permanence of his building, would cer- 

 .ainly not venture to raise it one-third of this height 

 without having recourse to the ordinary expedient of 

 spreading the base or foundation by extensive footings, 

 so as to diffuse the crushing pressure over a much larger 

 surface of material. 



Table V. contains the average results of experiments 

 made on the resistance of various materials to crushing 

 force. We would, however, caution the mechanic against 

 placing too much reliance on these results, as we think 



5 K 



