TRUSS, (from the French trousse,) a frame of timbers so disposed, that if suspended at two given points, and charged with one or more weights in certain others, no timber would press transversely upon another, except by timbers exerting equal and opposite forces.
When one or more exterior timbers of a frame, suspended from two given points, are propped by the disposition of interior timbers at certain points in each of the exterior pieces, so as to resist the pressures of several weights, each acting upon one of the said points, without any tendency to bend or break any timber employed in the construction ; the frame is called a truss, and each of the exterior timbers so propped in their length, are said to be trussed.
It is a principle in every such frame to have as few quad rangles as possible ; all the interstices, or openings, should be triangles; and the intersections of the timbers should be as direct as possible, because oblique pressures exert pro digious strains, which require strong timbers of large scant lings to withstand them, and these would press upon the abutments so much as to make the truss sag by the compres sion of the intermediate joggles. Wherever two oblique thrusts press to the same point, no transverse timber should be interposed ; for the shrinking of the transverse piece will also make the truss liable to sag. A truss of any extension may be made with a series of triangles composed of very short timbers ; but then it will be necessary that every two adjoining triangles have the same common side, otherwise transverse strains will be produced. A truss may also be made of very short timbers, by making them balance each other by their position only.
Trusses arc used for several purposes in building, as in partitions for supporting the floor above, to prevent it from communicating its pressure to the floor below, which may also be hung to the truss; particularly when there are neither bearing partition, nor trussed girders, which are shallow trusses, put between the ceiling and the floor, in order to stiffen the platform for walking upon. A roof of any considerable extent cannot be executed without one or more trusses; nor yet the centre of a bridge, or large vault.
Trusses employed in roofs and eenterings, are placed from eight to ten feet distance in the clear, and in equidistant ver tical planes. They may also be employed in the inclined sides of a roof, having their plane parallel to that of the covering, to counteract the pressure of the rafters downwards, and keep the lateral force thereby occasioned from acting upon the walls.
In all regular trusses, inclined timbers stand in pairs for mutual resistance, or counteraction. The names of the tim bers which most frequently occur in trussed work are as follow : all inclined timbers are called braces ; braces which either meet, or have then direction to a point below their extremities, are called struts ; hence every strut is a brace, but every brace is not a strut; those braces which form the exterior part of the truss, are called principal rafters : braces under the principal rafters, and parallel to them, are called principal braces, discharging braces, auxiliary rafters, or cushion rafters. Beams have various names, according as
they have a higher or lower situation in the truss, or accord ing as they perform the office of a tie or straining-piece ; beam acting as a tie, is therefore called a of which description it is always the lowest ; a beam extending above the tie-beam, between a pair of principal rafters, is called a or simply a collar, or which name indicates its use; when a beam terminates the upper part of a truss, it is called a because it is made to slope in a small degree both ways from its middle towt.rds each extreme on the upper edge ; beams placed above the tic-beam, between a pair of posts, are called Posts when employed in trusses, stand always in pairs, except there be one in the middle : every such post is called by the general name of : when the head of a truss-post stands at the apex of a pair of principal rafters, it is called the crown-post, or A- : a pair of truss possts, each of which is placed equidistant from the middle or ends of the truss, are called : when there is no crown-post in a tress, but one or more pairs of side-posts, the pair next the middle are called queen-posts. The an nexed Plate shows the various parts of a truss in detail, together with various methods of connecting them together.
Figure 1.—No. 1. The top of the king-post, with part of the principals, and a strap connecting the three members.
No. The method of joining the king-post with the tie beam, and of screwing it up.
No. 3. The edge of the king-post, and a section of the tie beam.
Figure 2.—Another method of joining the principals, king-post, and tie-beams.
No. 1. Parts of the principal rafters and king-post secured together by a branched strap.
No. 2. The method of strapping the king-post and tie beam.
No. 3. Parts of the king-post and tie-beam, showing the method of wedging them.
Figure 3.—The method of forming a joggle, when the thick part at the bottom of the king-post is not sufficient for receiving the shoulder of the struts at right angles to their directions.
Figure -1.—A similar method, with a little variation.
Figure 5.—No. 1. Another method of joining the prin cipals and king-posts by means of an iron dovetail, which is received into a mortise in the head of each principal.
No. 2. View from the top of the principals, showing the head of the wedge.
Figure G.—Method of securing the tie-beam and principals when the is made of an iron rod.
No. 1. The principals, with a part of the iron king-rod, and the hanging up of the tie-beam.
No. 2. The struts fixed to the iron king-rod, and the hanging up of the tie-beam.