Beam, in Building, a piece of timber resting upon walls, and used to support the floors or fronts of houses, to suspend weights from, etc. The strength of beams to each other is inversely as the length, and directly as the breadth and the square of their depth; their depth, therefore, is generally made greater than their breadth, as, by this means, greater strength may be obtained with less material; thus diminishing both the weight and the cost. Beams are variously named, according to the situations they occupy in a building. A tie-beam is a horizontal beam, extending from the opposite walls of a building, and having notches near the extremities against which the lower ends of the principal rafters of the roof abut, by which means the thrust of the rafters is exerted against the tie-beam, instead of acting against the walls. A girder is a stout beam, extending from one side of a building to the opposite one, and used to carry the joists: when the distance between the walls is great, it at the same time serves as a tie to the walls. A bressummer is a beam used to support a portion of a building above a considerable opening similar to a lintel over a door-way. When a beam projects from a wall, the outer end being unsupported, it is termed a cantilever.
In cases where the distance between the points of support is great, and the load to be sustained by a beam is considerable, instead of a beam formed of one piece, a frame of timber, called a truss, is employed, which, in its simplest form, consists of two inclined beams, abutting against notches in a horizontal beam near its extremities; and the upper ends of the beams meeting in a point upon which the load rests, by which means the pressure is transferred from the centre of the beam, to the part of it which is over the points of support, and the stress upon the beam is changed from a transverse strain to a tensile strain, operating in the direction of the fibres. Roofs and centres of bridges are specimens of trusses, and are framed upon the same principle, although more complex in the form. The methods of forming trusses are very varied, depending partly upon the uses for which they are intended; the following description of a very neat, simple, and effectual method of constructing rafters for a nearly flat roof, invented by Mr. Smart, of the Ordnance Wharf, Westminster Bridge, is an extract taken from the inventor's communication to the Society of Arts. "l take a square spar of the usual size for a rafter, and, by means of a circular saw, make an incision in it as shown at b b, Fig. 1 page 156. l then make the cut c at right angles to the former, and equidistant from the two ends; lastly, l make the two cuts d d, taking out a thin wedge from each place. The two pieces c d are then to be gently raised up, till they form an angle of 10° or 12°, with the piece b b, and are secured in their place by the insertion of a key-wedge e of seasoned oak, as represented at Fig. 2. It is obvious that a weight pressing on the key-wedge of this rafter (the ends being properly supported), will be sustained till either the fibres of the wood forming the string are drawn asunder, or till the lateral cohesion of the wood forming the butt-ends of the rafters be destroyed: at the same time there is no lateral pressure on the wall." The above rafter is commonly known by the name of the "Bow and String Rafter."
Another rafter upon the same principle, by the inventor of the foregoing, is shown in the margin. The rafter, which is 56 feet between the bearings, is made out of a scantling 10 inches by 4. An incision is made by a circular saw, from the middle nearly to each end; a transverse cut is then made at a, through the middle of the upper part of it, and at each end b b of the incision, a transverse piece, of a wedge shape, is cut out, reaching nearly to the longitudinal incision, but not so near as to separate the parts. Short pieces of wood are then inserted between the upper and under part of the rafter, and the whole are secured by iron straps. It appears that Mr. Smart was led to the foregoing method of forming rafters, by the following experiment, which he made for the purpose of ascertaining whether the strength of a beam is increased in the proportion of three to two, as 6tated by Belidor, by fastening its ends so as to prevent their approach when loaded in the middle. He placed a lath, an eighth of an inch thick, in a strong frame, as shown by the annexed cut, which broke with a load of 11 lbs.placed on its middle.
He next took a lath of the same wood, and fixed it firmly at the ends, by means of the projecting pieces b b and the wedges c c, and ascertained that it would sustain, by this arrangement, a load of 270 lbs. whence it appears that the strength of the lath was increased nearly 25 times, by merely securing it well at the ends.
The following figure represents a trussed girder of wrought iron, similar in construction to the foregoing. This girder is made by welding an arched bar of wrought iron to a longer straight bar, and then turning the ends of this latter either up or down, as may be most convenient for the particular use to which the girder is to be applied. t t are the places where the bars are welded together that compose the girder; u u the ends of the straight bar turned either up or down. The arch is prevented from buckling, when the weight presses upon it, by means of blocks of well-seasoned wood, inserted at intervals between the two bars, and secured in their places by the iron straps v v. Beams of wrought iron made in this way will, in Mr. Smart's opinion, support a weight so much greater than cast iron ones of equal dimensions, that they may be made of any given strength at half the cost of equivalent beams of cast iron.