Trusses are strong frames of carpentry, so contrived as to act like a solid body, and support certain weights at given immovable points, the truss being suspended from two such immovable points; they are generally made of a triangular form, and placed at equal distances on the wall plates, in vertical parallel lines, at right angles to the walls. Some excellent examples of trussing, especially on the suspension principle, are given under the article Beam. See also Roof. Various methods are adopted for the construction of floors according to the different bearing of the timbers. In small rooms the floors consist in general of single joists, but in large rooms two rows of joists are employed, one supporting the other above, and fixed at one end into a beam called the girder, which is usually placed transversely in the middle of the space; if the size of the room requires three compartments of joisting, two girders will be needful. The upper row of joists is called bridging joists, and the lower, binding joists. Stairs are used as the means of communication to the different storeys of a house, and the space in which they are enclosed is called the staircase.
The utmost attention on the part of the builder is necessary in fixing the staircase, and in determining all its just proportions. The steps, whether of marble, free-stone, or wood, etc, may be supported at both ends, or at one only; when the latter, it is usually the broadest, though in small wooden stairs the steps are sometimes made to project from a newel, to which the narrow ends are fixed. The height of large steps must never be less than 6, or more than 71/2 inches. The breadth should not be less than 10, and never exceed 18 inches. The length of them cannot conveniently be less than 3 feet, or more than 12 feet. Geometrical stairs have their outer ends fixed in the wall, and the under edge of every step supported by the edge of the one below it. The upper part of the joint may be level from the face of the riser to about 1 inch within the joint; the plane of the tread of each step is thus continued about an inch within the surface of each riser; the lower part of the joint has a narrow surface perpendicular to the rake of the stair at the end next the newel.
Geometrical stairs constructed of stone depend upon the following principle: - that every body must be supported by three points at least, placed out of a straight line; and consequently, if two edges of a body, in different directions, be secured, they will become relatively immovable; such is the case in a geometrical stair; one end of the step is always tailed into the wall, and one edge rests either on the ground, or on the edge of the inferior step or platform. The methods of forming staircases are so various that it would be impossible to give even an intelligible definition of them that would be compatible with our limits; we shall, therefore, at once refer the reader for information on this and all the other departments of this subject to The New Practical Builder, which unquestionably is the most complete modern work upon this important art, whether as respects the plainest or the most magnificent structures.
Building en Pise. An extremely simple, durable, and economical method of building walls, in very general use in the neighbourhood of Lyons, and which has been introduced into this country. It consists simply in ramming earth firmly between moulds formed of boards set upon the ground-plan of the walls; and although it may seem that buildings erected upon this plan must resemble the mud cabins which disfigure many parts of this country, they nevertheless differ widely both in appearance and durability, since they may be made as neat as brick buildings, and equally durable, nouses of 150 years standing, built in this manner, being by no means rare in France. The following instructions for this mode of building are taken from a paper in the Transactions of the Society of Arts, Vol. XXVII. by Mr. Salmon, who has practised it to some extent on the estate of the Duke of Bedford, at Woburn. The only tools necessary for building en pise", besides the ordinary ones used in building, are the moulds and rammers.
The mould consists of two side pieces, each composed of two planks 12 feet long, 10 inches wide, 1 inch thick, strengthened by several pieces of board nailed across them on the outside, at equal distances apart.
Holes are made through these pieces of board at top and bottom, to receive iron bolts, which hold the two boards parallel to each other, 14 or 16 inches asunder, which is the thickness of the walls intended to be formed between them. The bolts have a large head at one end, and a key passes through the other to keep the planks together. Two boards, equal in length to the thickness of the wall, are placed between the planks at the ends to form the ends of the mould. The rammer consists of a short staff with an iron head, which should not be more than half an inch wide on the edge, in order that it may more forcibly compress the earth in every part, which a flat rammer could not do so well. In forming the angles of a building, four mould boards are requisite: each of the boards for the internal angle have two eye-bolts through, and the boards being set together at the proper angle, a bolt is passed through the four eye-bolt, forming a kind of hinge; the boards are retained in their position by an iron stay, hooked into a staple in each of the boards.
The boards for the outer angle are connected by two short pieces of iron projecting from one of the boards which pass through corresponding holes in the sides of the other board near the end, and are keyed up on the outer side of the mould. The process is conducted as follows: - the foundations of the intended building must be laid of brick work, stone or rubble, and be carried at least 9 inches above the surface to protect the pise" from the rain; the mould frames are then set up on the foundation walls and bolted together, the thickness of the foundation regulating the distance at bottom, which distance is maintained at the upper part of the mould by pieces of wood cut to the proper length, which are laid upon the upper connecting bolts; head boards are also placed at each end to retain the earth. Loose earth is then thrown into the mould to the depth of about 3 inches, which is afterwards drawn back, and cleared from the face of the mould, and the space filled up with a facing composition, forming, on an average, about an inch in thickness; the whole is then firmly rammed (on which, and properly preparing the facing stuff, the perfection of the work will greatly depend,) until it is quite hard, when it will be compressed to about 11/2 inch in thickness.