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WALLS AND Piers.
1500. The unickness which is to be assigned to walls and points of support, that their stability may be insured, depends on the weight they have to sustain, and on their formation with proper materials; still more on the proportion which their bases bear to their heights. The crushing of stone and brick, by mere superimposed weight, is of extremely rare occurrence in practice, even with soft stone and with bad bricks. The result of some few experiments that have been made as to the resistance of some of our bricks and stones to a crushing force, by George Rennie, in 1818, are here subjoined. Some later experiments made by the Commissioners mentioned in Book II. chap. ii., and appended to their Report on Stone, &c., in 1839; with a few others; as well as some important trials made in 1864 by a committee of the Institute of British Architects, given in Transactions, 1863–64, are likewise added.
TABLE of CRUsHING Force of MATERIALs, BY GEoRGE RENNIE (Phil. Trans. 1818).
| Materials. # Q's
Portland stone, 2 inches long, 1 inch square - - - || - - 805
1501. The above experiments lose much of their practical value from our knowledge that the interior particles in granulated substances are protected from yielding by the lateral resistance of the exterior ones; but to what extent it is impossible to estimate, because so much depends on the internal structure of the body. We are, however, thus far informed, that, taking into account the weight with which a point of support is loaded, its thickness ought to be regulated in an inverse ratio to the crushing weight of the nuaterial employed. In Gothic structures we often see, for instance, in chapter houses
with a central column, a prodigious weight superimposed. It is needless to say that, in such instances, the strongest material was necessary, and we always find it so employed. So in the columns, or rather pillars, of the naves in such edifices, the greatest care was usually taken to select the hardest stone. Generally speaking, the thickness of walls and piers should be proportioned rather to their height than to the weight they are to bear; hence often the employment of a better material, though more costly, is in truth the most economical.
1502. TABLE of THE WEIGHT REQUIRED To CRush CUBEs of StoNE. Materials. # ' | #
I. Granites (2-inch cubes): lbs. lbs.
II. Limestones (2-inch cubes):
Bramley Fall - - - - - || 2506 - 6.0.53
1502a. In the above list B stands for Bramah, and C for the Commissioners' Report, &c. It is of very great importance to notice that the size of the cubes experimented upon by the latter, was only two inches; those by Rennie were only one and a half inch cubes. A set of experiments on Portland stone, of the weight sustained up to the point of fracture, i.e. the crushing weight, by accurately cut cubes of two inch faces placed
between perfectly smooth lead surfaces, were carried out with the well-known American mechanical testing machine, by Mr. Abel (Builder, 1863, p. 860):— War Department Quarry, Vern Hill - - - - 14,795.8 lbs.
inmos hay Quarry, Whit-bed - - - - - 14,591 ‘8 ..
** * Whit-bed - - - - - 13,979.5 ,
** * * Base-bed - - - - - 13,775 O , New Maggott Quarry, Whit-bed - - - - 12,857-1 ., Old Maggott Quarry, LI Whit-bed - - - - 12,244.8 ..., * ** ** I T Base-bed - - - - 12.857: 1 , - * * ** L I Base-bed - - - - 8, 163.2 , Indepndent Quarry, Whit-bed- - - - - 1 1,632 6 ..., Waycroft Quarry, Base-bed - - - - - 1 1,836-7 ,
IIe also observes that “no definite conclusion can be drawn from the comparative properties of the specimens of stone from one and the same locality, quarried at different periods of time, regarding the influence exerted by exposure, after quarrying, upon the quality of the stone. On the whole, the evidence may be considered as a little in favour of the opinion that an improvement in the strength of the stone is effected, to some extent, by seasoning.”
1502b. A very instructive set of experiments on the strength of Portland stone (brown bed), a material now so greatly employed in building, was made by a committee of the Institute, above-mentioned.
1502c. C. H. Smith has cbserved (Transactions of the Institute of British Architects, 1860, page 174.), that “the stone which possesses the least cohesive strength, or that which will crush with less pressure than any other, is nevertheless strong enough, when well fixed in a building, for almost all practical purposes. No architectural members have to sustain greater pressure, in proportion to their size, than mullions of large Gothic windows. The tracery in the great north window of Westminster Hall is now executed in Bath stone, which is remarkable for having the least cohesive strength of all the specimens described as experimented upon in the Report on Stone, &c. Some of the mullions of that window are less than nine inches wide and more than forty feet high sustaining not only their own weight, but also that of the whole of the tracery beneath the arch. The eastern window of Carlisle Cathedral, built with a fiable red sandstone, is fifty feet high, the mullions are smaller, and the tracery much heavier than in that at Westminster, yet in neither of these examples are there any symptoms of crushing. The cohesive strength of stones is never more severely tested than during their conversion by workmen from the rough state to being fixed in their final situation in a building. During these operations, iron levers, jacks, lewises, and various other implements are applied, frequently with but little regard for the mechanical violence which a stone will safely bear ; and it may, therefore, be considered a useful practical rule, that, however soft a stone maay be, if it resist the liability of damage until out of the masons' hands, there can be little doubt of its possessing sufficient cohesive strength for any kind of architectural work. If the foundation be insufficient, or any part of the edifice give way, so as to cause an unfair or unequal pressure, a soft stone will, of course, yield sooner than a hard one.” 1502d “Unfortunately,” writes Warr, Dynamics, 1851, “those experimental results which we possess were obtained without attention to the fact that the specimens should be of a certain height to show a proper compressive strength. The bulk of the examples are: with cubes, a fault excusable with those experimenters who made their work public before those peculiarities were well known, but the same cannot be said of the investigations conducted by the Commissioners; these experiments, executed with singular minuteness on some points, would have been useful, from their variety and specification of the localitics, but they were made on (2-inch) cubes, at a period when the laws of fracture were as public as at present, and are therefore of limited value.” 1502e. Hodgkinson (Phil Trans., 1840, p. 385), found that in small columns of one inch to one and three-quarters inch square, and from one to forty inches long, a great falling off occurred when the height was greater than twelve times the side of the base. Thus, when the length was—
12 times the size of the base, the strength was - - 138
He also found that with pillars shorter than thirty times the thickness, fracture occurred by one of the ends failing, and as the longer columns deflected more than the shorter, they presented less of the base to resist the pressure, and therefore more readily gave way. Thus the practical view from these experiments points out an increase of area at the ends as being most economical, and that in proportion to the middle as 13,766 to 9,595 nearly. From the experiments it would appear that the Grecian columns, which seldom had their length more than about ten times the diameter, were nearly of the form capable of bearing the greatest weight when their shafts were uniform; and that columns, tapering from the bottom to the top, were only capable of bearing weights due to the smallest part of their section, though the larger end might serve to prevent lateral thrust. This last remark applies, too, to the Egyptian columns, the strength of the column being only that of the smallest part of the section. (British Association for the Advancement of Science, 15 h. Report, 1845, p. 27.) 1502f. It might be asked, how does this apply to those small shafts or colonettes so freely used with piers in pointed architecture, and which are generally in height upwards of thirty times their diameter. We would refer the student to the paragraph 1502c., respecting the mullions in windows, and to the circumstance that the small shafts are not pinned-in to the work, but are left free, so that they only apparently carry the weight imposed on their capitals. Where no attention has been paid to this necessary precaution, in modern work, the shaft has fractured when of soft, or shaky, stone.
1502g. TABLE of THE STRENGTH of SHAFTs 12 INcHEs LoNG, 3 INCHES DIAMETER,
(Being experiments made by a committee of the Institute, as above-mentioned.)
Materials. Cracked. | Crushed. o're Remarks. Tons. Tons. Tons. Portland stone: All yielded vertically. ' Worked - - - 7.3 10-25 1'48 Bedded in leather. Rough tooled - - I - - 8'57 1-00 | Bedded in plaster. Devonshire marbles: Ipplepen, mottled red - 9.2 10-7 1:37 [with vein. Poltesco, grey green - 4-3 4-3 0-60 | Went across and not Ditto - - - - - 6:0 0.84 | Went at once. f • - 33-5 4.73 | Went into fragments. Signal Staff red and black 2O.O 22.5 3.18 Ditto. 12.75 16:25 2.29 Cadgewith, green and black | 16.92 17.62 2:49
1502h. Fairbairn, in a paper read at the Manchester Philosophical Society, and given in vol. xiv. of the Proceedings; and also in his Useful Information, &c., 2nd Series, has detailed the following 1esults of his researches:
Grauwacke from Penmaenmaur - - - - 16,893
1502i. He further shows that the resistance of strong sandstone to crushing in a direction parallel to the layers, is only six-sevenths of the resistance to crushing in a direction perpendicular to the layers. The hardest stones alone give way to crushing at once, without previous warning. All others begin to crack or split under a load less than that which finally crushes them, in a proportion which ranges from a fraction little less than unity in the harder stones, down to about one half in the softest. The mode in which stone gives way to a crushing load is in general by shearing. The factor of safety in structures of stone should not be less than eight, in order to provide for variations in t e strength of the material, as well as for other contingencies. In some structures which have stood it is less; but there can be no doubt that these err on the side of boldness, as urged by Rankine, Civil Engineering, page 361.
1502k. TABLE of THE WEIGHTS REQUIRED To CRush BRicks. Experiments by T. Cubitt. Yielded to. Crushed by. Remarks.
Good place bricks - - ll tons l64 tons Bedded on plaster.
l)itto - - - - 16 , 29% , Ditto.
Two common stocks - - 1 O , 16 ** No plaster. 16} ,
Good stock - - - 30 ,, 34 **
Superior washed stock - - 36 , 44} ,
Ordinary place brick - - 3 *, 9 ** |
l)itto - - - - 3 * 6 - , ,
Common ditto - - - - 5 to 23 ,
A brick made by Beale's machine being placed on bearers seven inches apart, was broken in the middle by the weight of 2,625 lbs. A common hand-made brick was broken by 645 lbs. The hollow or frog formed in the underside of a brick necessarily lessens its resisting power. Young (Nat. Phil.) states that the cohesive strength of a square inch of brick is 300 lbs., but the quality is not stated. Other experiments give the following strength of bricks:—
Cwt. Tons. Cwt. Brick of Huntingdonshire clay, perforated, per square inch - 31 58 18 ** Suffolk clay, solid: (broke across) * , - 8.75 16 l 2 ** Made by Prosser's machine, bore ** * - 90 () ** Arsley, and not crushed ** ** * 83 0 ** Ordinary stock, - - 140 0 * Common fire clay, $per square foot - - 157 O ** Good ditto, - - 396 O
** Used at Edinburgh Gas Works, of fire clay and iron stone,
15021. Brickwork.—Brick piers 9 inches square, 2 feet 3 inches high, made of good sound Cowley stocks, set in cement, and proved two days afterwards:Cracked at Broke at Brick flat, compressed quarter of an inch - - - 25 tons 30 tons Brick on edge, did not compress - - - - 30 , S5 ..., 1502m. Mr. L. Clarke's experiments for the works at the Britannia and Conway tubular bridges, on brickwork in cubes, showed that9 inches, cemented, No. 1 or best quality, set between deal boards, weighing 54 lbs., crushed with 19 tons 18 cwt. 2 qrs. 22 lbs. - - - - = 551-3 lbs. per square inch 9 inches, No. 1, set in cement, weighing 53 lbs, crushed with 22 tons 3 cwt. 0 qis, 17 lbs. - - = 6 12.7 lbs. **