Abbildungen der Seite
PDF
EPUB

from the angles of the plan as there are ribs intended, until they mutually intersect each other. The curvatures of the ribs will be elongated as they recede from the primitive arch, till they reach the centre on the place where the groins cross, and where of course the elongated curve is a maximum The ribs thus form, when they are of the same curvature, portions of an inverted conoid.

14992. In the next example (fig. 590h.), the primitive arches are unequal in height, the arch A being higher than B The plan remains the same as in that immediately preceding: but from the inequality of height, a d, c b, must be joined by curved lines, determined on one side by the point a. where e a intersects the longer arch. A curved summit rib, as well longitudinally as transversely, may occur with equal or unequal heights of primitive arches (as in fig. 590i.); but the stellar form on the plan still remains, though differently modified, with the same, or a less or greater, number of ribs on the plan (fig. 590k.). By truncating, as it were, the summit ribs, level or otherwise, with the tops of the primitive arches, and introducing on the plan a polygon or a circle touching quadrants inscribed in the square, we obtain, by means of the rising conoidal quadrants, figures which perform the office of a key. stone. In this, as we have above observed, the construction of the work is totally different from rib vaulting, inasmuch as each course, in rising, supports the next, after the manner of a dome, and is not dependent on ribs for carrying the filling-in pieces. Hence the distinction between fanwork and radiating rib work so judiciously made by Mr. Willis.

Fig. 5901.

1499aa. The sixth example (fig. 5901) has primitive arches of different heights, forming an irregular star on plan, that is to say, the points are of different angles. The figure will scarcely need explanation after what has been already said in relation to the subject.

1499bb. A polygonal space may be vaulted in three different ways. First, by a central column serving for the reception of the ribs of the vault, the column or pillar performing in such case the office of a wall, as in the chapter-houses of Worcester, Salisbury, Wells, and Lincoln. This mode evidently admits of the largest space being covered, on account of the subdivision of the whole area by means of the central pillar. The second mode is by a pendent for the reception of the arches, as in the Lady Chapel at Caudebec, (given in the section MASONRY). This mode is necessarily restricted in practice to small spans, on account of the limits attached to the power of materials; albeit in theory its range is as extensive as the former. The last method is by at

[blocks in formation]
[ocr errors]

once vaulting the space from wall to wall, as in fig. 590m., like the vaulting to the kitchen of the monastery of Durham Cathedral, or. fig. 590n., similar to the chapter-house at York, of which, the upper part being of wood, Ware quaintly observes, The people of Yorkshire fondly admire and justly boast of their cathedral and chapter-house. The principle of vaulting at the chapter-house may be admired and imagined in stone; not so the vault of the nave; it is manifestly one of those sham productions which cheat where there is no merit in deceiving." The principle, as Ware justly observes, is perfectly misonic, and might be easily carried out with stone ribs and panel stones, it being nothing more than an extension of that exhibited in the third example of simple groining (fig. 590f.) above given; and the same remark applies to the Durham kitchen.

1499cc We propose to offer explanations of the nature of the vaulting at King's College Chapel at Cambridge, and the silly story related by Walpole of Sir Christopher Wren, saying, "that if any man would show him wnere to place the first stone he would

engage to build another" (vault like it). The vault of the chapel in question is divided into oblong severies, whose shorter sides are placed longitudinally (fig. 5900.) It must be evident that the curves of the inverted quadrants must intersect each other previous to the whole quadrant of the circle being completed. Hence these intersections form a curved summit line lowest against the windows or smaller sides of the oblong. This summit line of the vaulting of the building in the direction of its length forms a series of curves, though from the angle under which it is seen it is scarcely perceptible. Mr. Ware says, " It is observable, in the construction of this vault, that the principle of using freestone for the ribs, and tufa for the panels, has not been followed; but the whole vault has been got out of the same description of stone, and with an uniform face, and the panels worked afterwards, and reduced to a tenuity hardly credible except from measurement. The artists of this building might be trusted in the decoration of a vault with what is now called tracery; they knew how to render it the chief support, and what was the superfluous stone to be taken away : every part has a place, not only proper, but necessary; and in the ribs which adorn the vault we may in vain look for false positions. This is the ocular music which affords universal pleasure."

Fig. 5900.

1499dd. We now return to the consideration of two more modes of simple vaulting. In England, the summit ribs of the vault are almost always found running longitudinally and transversely in the various examples. In Germany the summit ribs are more frequently omitted than introduced. Thus in the example fig. 5907, the scheme is merely a square diagonally placed within the severy, subdivided into four parts and connected with the basepoints of the groins by ribs not parallel to the alternate sides of the inserted square. This, bowever, sometimes occurs in English buildings, as in the monument of Archbishop Stratford, at Canterbury Cathedral; though in that the central portion is not domical. It is to be remarked that the intersecting arches are not of equal height, otherwise the arrangement could not occur.

1499ee. In the example fig. 590p, the arrangement completely assumes what Mr. Willis calls the stellar form. Here in the soffit a star of six points is the figure on which the projection depends, the points radiating from the angles of an hexagon, and thus forming a cluster of lozenges whose middle longitudinal sides produce another longitudinal lozenge to connect the centres of the pattern. The longitudinal arches are, as in the preceding figure lower than the transverse arches. Mr. Willis says, "the principal distinction between these and our own fanvaulting is the substitution of lozenge-headed compartments in the fans, for the English horizontal transom rib. We have also lozenge-headed compartments in our early vaulting, but they are never so symmetrically arranged in stars throughout."

Fig. 590p.

1499ff. From the simple lines or principles above given, it is easy to perceive through what numberless ramifications of form they may be carried. Another form is that called hexpartite vaulting, where the ribs spring from the angles, and two others from a shaft placed in the middle of each long side, thus making six divisions. This is a step beyond the quadripartite groining shown in fig. 590f. Examples of hexpartite vaulting are scarce in England, but it may be seen in the chapel of St. Blaise in Westminster Abbey, the choir of Canterbury Cathedral, and in many parts of Lincoln Minster.

1499gg. It would be difficult to find a system of vaulting more unlike any English example than that in Anjou generally, of which the Hospital at Angers is a fair specimen. It is always excessively domical in its sections, both longitudinal and transverse; and has eight ribs, the cells being filled in with stones exactly parallel with the centre or ridge of each cell the ribs are edge-roll mouldings.

:

1499hh. Besides the books named above. Prof. Willis On Vaulting, and by T. Eagles, 1874, both read at the Royal Institute of British Architects, the Dictionnaire by Violletle-Duc, the Lectures by Sir G. G. Scott, R.A., and the paper by W. H. Wood, in Builder for 1883, xliv., 55, should be referred to. A very complete outline of the subject has been printed by Prof. Babcock, of the Cornell University, Ithaca, New York, for his courses of lectures.

SECT. IX.

WALLS AND PIERS.

1500. The thickness which is to be assigned to walls and points of support, that theit stability may be insured, depends on the weight they have to sustain, and on their forma tion 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).

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

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 material 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.

[merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

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):

[merged small][merged small][ocr errors][merged small][ocr errors][ocr errors][ocr errors][merged small][ocr errors][merged small][ocr errors][merged small][merged small]

Indep ndent Quarry, Whit-bed-
Waycroft Quarry, Base-bed

[ocr errors][merged small][ocr errors][merged small][merged small][ocr errors][merged small][ocr errors]
[ocr errors]
[ocr errors][merged small][ocr errors][ocr errors][merged small][merged small][ocr errors][merged small][ocr errors][merged small]

He 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 ex tent, by seasoning."

15026. 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.

[blocks in formation]

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 castern window of Carlisle Cathedral, built with a fiiable 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, there

« ZurückWeiter »