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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 RIQUIRED TO Crush CUBES OF Stone,
1. Granites (2-inch cubes):
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 experiinents 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
fore, be considered a useful practical rule, that, however soft a stone inay 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
certain height to show a proper compressive strength. The bulk of the examples are sith 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 condueted by the Commissioners; these experiments, executed with singular minuteness on some points, would have been useful, from their variety and specification of the localities, 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 was12 times the size of the base, the strength was
a little less
75 40 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, 15th 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 pioned-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.
15029. TABLE OF THE STRENGTH OF SHAFTS 12 INCHES LONG, 3 INCHES DIAMETER,
(Being experiments made by a committee of the Institute, as above-mentioned.)
1502h. l'airbairn, in a paper read at the Manchester Philosophical Society, and given in Pol. xiv. of the Proceedings; and also in his Useful Information, &c., 2nd Series, has detailed the following results of his researches :
comparatively wide opening. All blocks set upon it should have a clear bed along the
especially when supported in their length by a mullion or small pier, as is often introduced. We need hardly add that where impact or collision is likely to occur, no lintel of stone
15029. Marble mantles may sometimes be seen to have become bent by their own weight Beams of marble have been employed in Grecian temples as much as 18 feet in the clear in the propylæa at Athens; and marble beams 2 feet wide and 13 inches deep beams were about 13 feet in the clear in the north portico of the temple at Bassæ near walls. I. One of undoubted stability; II. A mean between the last; and the III. The one eighth part of the height: a mean stability is obtained when the thickness is one tenth
1504. The first case is that in which from many examples we find the thickness equal to however, to recollect that in most buildings one wall becomes connected with another, so part of the height; and the minimum of stability when one twelfth of its height. We are, that stability may be obtained by considering them otherwise than as independent walls.
1500. That some idea may be formed of the difference between a wall entirely isolated and 593. It is obvious that in the first case (fig. 591.). a wall acted upon by the horizontal and one connected with one or two others at right angles, we here give figs. 591, 592, force MN, will have no resistance but from the breadth of its base ; that in the second
or the compression of timber and metals, will be treated in a subsequent section (163le.
= 568.5 lbs.
9 inches, No. 3, set in cement, weighing 52 lbs., crushed with
= 454 3 lbs. per square inch, Hinches, No. 4, set in cement, weighing 554 lbs., crushed with
21 tons 14 cwt. Iqr. 17 lbs.
= 5210 lbs. The three last cubes continued to support the weight, although cracked in all directions ; they fell to pieces when the load was removed.
All began to show irregular cracks a considerable time before it gave way. The average weight supported by these bricks was 885 tons per square foot, equal to a column 583.69 feet high of such brickwork. (Fair. bairn, Application, &c., page 192.).
1502n. To crush a mass of solid brickwork 1 foot square, requires 300,000 lbs. avoir. dupois, or 134 tons 7 cwt.
15020. Besides compression, stone is subject to delrusion and a transverse strain, as when used in a lintel. Of these strengths in stone little is officially known, but we are perfectly aware of the danger of using any kind of stone for beams where there is much chance of
Its weakness in respect to this strain is manifest from all experimental evidence concerning it. Gauthey states the value of a constant S, for hard limestone = 78 lis ; for soft limestone = 69 lus. Hodgkinson, taking the power of resisting
crushing force as = 1000, notices
serious or of irregular pressures.
Black marble -
Mean Common bricks, S=64 lbs. (Barlow.) 1502p. The danger above noticed is so great, that it becomes essentially necessary in all rough rubble work to insert over an opening either an iron or timber lintel, or a brick or stone arch, to carry the superincumbent weight, and thus prevent any pressure upon the tone. This must be done more especially when heams or lintels of soft stone are used; the harder stones, as Portland, may in 'ashlar work support themselves without much danger. In rubble masonry, the stone arch may be shown without hesitation in the face of the work ; and also in domestic architecture, the brick arch may exhibit itself in the facework if thought desirable. Portland stone has been constantly used to extend over a middle of its length. Thus cills to windows should always be set with clear beds, or, as the Bework settles, they are certain to be broken. Lintels over even small openings worked in Bath or some of the softer stones, are very likely to crack across by very slight settlements,
should be used.
were bollowed out, leaving 48 inches thickness at the sides and 3 inchies at the bottım; these
1592r. The cohesive power of stone is seldom tested. The subject of crushing weights, et seq.); and the strength of some other materials will be given in the chapter Materials.
OF THE STABILITY OF Walls. 1.503. In the construction of edifices there are three degrees of stability assignable to
least thickness which they ought to possess.