Foundation is also the substructure or bottom of a wall, consisting of one or more regular steps on each side of the wall, below the level of the under side of the floor of the lowest story of a house, in order to prevent it from sinking into the ground, by opposing a greater surface of resistance to it, and for preventing the wall from being overturned by a tempest or storm : each course of steps is called a footing.
The breadth of the substructure should be proportioned to the weight of the su perstructure, and to the softness of the ground on which it rests ; if the texture of the ground is supposed to be constant, and the materials of the same specific gravity, the breadth of the foundation will be as the area of the vertical section passing through the line on which the breadth is measured ; thus, for example suppose a wall 40 feet high, 2 feet thick ; to have a sufficient foundation at 3 feet in breadth, what should be the breadth of a foundation of a waIl 60 feet high, 21 feet thick : by proportion it will be 40x2 : 3 :: 60X21 : the ans. 5Veet. This calculation will give the breadth of the foundation of the required wall equal to the breadth of the insisting wall itself; when the height of the required wall is equal to the ratio, which is the first term 40 x 2 = 80, divided by the second term 3, that is 80= Thus a wall of feet would have the breadth of its foun dation equal to its thickness above the foundation, and less than 264 feet would have a thinner foundation than even the superstructure. But though the calcula tion in this case gives the foundation less breadth than the thickness of its super structure, it must be considered, that it only calculates the true breadth of sur face that should be opposed to the ground, in order to prevent the wall from penetration by its weight : though the rule gives all the breadth that is necessary, on account of the weight of the insisting wall, yet the breadth of the substructure should always be greater than that of the superstructure ; as it will stand more firmly on its base when affected by later al pressure, and be less liable to rock by the blowing of heavy winds. The least breadth that is commonly given to the substructure of stone walls is one foot thicker than the superstructure. In
damp foundations, the superstructure should always be separated from the sub structure by lead, tarred paper, or other means.
Stone arch. Stone arch is a number of stones so arranged,that, in consequence of theirpressure upon one another and upon their supports, they may be suspended over a hollow space ; every interior stone being such, that, if a plummet be depend ed by a line from any point in that stone, the line will fall within the hollow space.
Stone arches are generally hollow be low, and concave towards that hollow. The interior stones ought to be truncated wedges, and their faces, which fcrm the intrados, of less dimensions than the up per opposite surfaces which form the ex trados : so that when any stone endea vours to descend through the aperture which surrounds it, it will be prevented by the dimensions of the lower part of the aperture being less than those of the top of the stone which has to fall through it.
Wedge-like stones forming an arch are arch stones.
The joints between the arch stones are called sommerings.
The support or supports of an arch are called the reins of that arch.
When the support or supports of an arch are stone walls, the upper course or courses, on which the beds of the ex treme arch stones rest, are called the imposts.
Spring course, or chaptrels of the arch, are called the butments or abutments, or spring beds, or skew backs of the arch.
When an arch is either recessed in any piece of masonry, or forms the head of an aperture through that piece of mason ry, the arch stones, which are common to the intrados and to the face of the mason ry, are called voussoirs, and the middle voussoir is called the key-stone.
Stone arches are used for a variety of purposes, in supporting different parts of a building, over apertures, when the apertures are too wide for lintelling, and over a wooden or stone lintel, to assist in supporting the superincumbent build ing.
Arches are also used to prop the sides of a building, and in soft foundations in verted arches are used, between narrow piers, to prevent the pier from penetrat ing, by opposing a greater surface of re sistance to the ground.