GROINS. In our article CARPENTRY, we have already treated the subject of groins at sonic length. We propos ed, under the present head, to have investigated the sub ject of Domes, from which we have made a reference ; but we have found .it necessary to include this subject un der that of ROOFS, to which the reader is referred.
In the article CARPENTRY, we have mentioned the great improvement in the construction of brick groins, rising from rectangular piers, as made by Mr George Tapper. The following account of this improvement has been drawn up for our work by Mr John Naricn.
If a square or rectangular area be covered with two vaults, penetrating each other at right angles, and forming two ridges which cross the area diagonally, and intersect each other at the common summit of the vaults, the arch thus formed is called a groined vault : the penetrating vaults may be either semicircular, or semi-elliptical, or one of them may be semicircular and the other semi-el liptical. The intersections of the circular or elliptical vaults, for mingthe ridges or groin angle's, will be ellipses, because every oblique section of a cylinder or elliptical prism produces an elliptic curve, and the case w ill be the same when one of the vaults is circular, and the other elliptical ; for if horizontal lines were drawn from points in the diameter or chord of the circular vault perpendicu larly to that diameter, and cutting in different points a line drawn diagonally' across the area, the ordinates drawn up to the ridge or groin angle from these points, will be re spectively equal to the ordinates at the corresponding points under the circular vault, and consequently their ends will be in the periphery of an ellipse.
The four vaults or arches forming the groined vault, .spring froth the angles of four square abutment piers, and if the intended vault is to be built of stone, the courses in each arch respectively are laid upon the centering, in lines parallel to the axis of the arch they compose, in such a manner that the voussoirs of each arch meet the voussoirs of its adjacent arch at the groin angle, where the faces of the angular voussoir in each course are wrought in such a manner as to form the curve of the groin, which springs from the angle of the pier on which the arches stL,nd. The upper surfaces of these anguka voussoirs are also w rought, so that on both faces they may coincide with tie other stones of the same course, by which means these sou faces meet in a r idge which is always perpendicular to the curve of the groin angle. When the intended vault is to be built
of brick, the internal faces of the brick voussoirs in each course are cut away at the groin angle, to receive the wedge-like end of a brick in the adjacent arch in the same course, in order to bind the arches together more firmly : (See Plate cclxxxiv. Fig. 1.) But as bricks have not, like wrought stones, the form of a frustrum of a wedge by which they may sustain themselves when arranged in the. shape of an arch, their stability must depend upon the strength of the cement placed between them, which, unit ing them into one mass, renders a structure of this kind a sort of vault hewn out of a rock.
From a consideration of the above mentioned mode of constructing groined vaults, it will be evident that the pressures, both vertical -and lateral, of all the arches of which they are composed, are resisted and sustained by the mutual intersections of the courses of masonry at the groin angles; these intersections may be considered as squares upon the corners of the piers from which they spring, the side of the square being equal to the thickness of the course of voussoirs, so that the diagonal ribs of the vault form as it were two arches, which are kept in a state of equilibration, by the weight of the spandrils imme diately over them, and serve as bases upon which the side arches with their spandrils, and all the superincumbent loading, ultimately rest ; hence it will be evident that, if the weight of the arches, with their loading over them, were in a constant ratio to the weight necessary for keep ing the ribs in a state of equilibration, the whole vault will be in equilibrio in all its parts. This, however, cannot be attained in practice, because the distances between the ribs at any part are never in proportion to the height of the spandrils over the ribs at those parts, and therefore the groined vaulting will always in some degree be defective in its equilibration ; besides the disadvantage arising ftom the whole of the weight falling entirely upon the ribs, which receive no support from the voussoirs of the conti guous side arches, whose joints are all oblique to the di rections of the ribs.