BEAM (Sax. beam, a tree), in architecture, a piece of timber, long in proportion to its breadth and thickness, used either to support a superincumbent weight, or to bind together the parts of a frame as a tie, by resistance to extension, or to hold them apart, as a strut, by resistance to compression. The term is applied particularly to the largest piece of timber in a building, that which lies across the walls and supports the principal rafters. Important improvements have been introduced within the last few years, in various departments of practical construction, by the use of iron beams, especially in the building of fire-proof structures and bridges. Prior to their introduction the only method of securing safety from fire was by massive and cumbersome constructions of masonry. This system of groined arches involves great loss of room, the most solid foundations and heavy walls and piers to sustain their weight and thrust, and often an inconvenient arrangement and division of the interior of the edifice. It is not only not adapted to the purposes of business, but its expense is such as to preclude its use for ordinary warehouses, offices, and dwellings. The immense annual destruction of property by fires demonstrates the great importance of any improvements by which security can be obtained, without excessive cost and inconvenient restrictions on the plan of the building. By the introduction of cast-iron beams and light segmental arches, these results were, to some extent, obtained; but experience has shown that wrought-iron is much better adapted to resist transverse strains, and the testimony of eminent engineers and architects is unanimous in preferring it for this purpose, as both more to be relied on and more economical. The first instance on record of the construction of a building with cast-iron beams is that of a fire-proof cotton mill erected in Manchester by Boulton and Watt, in 1801. It was not, how ever, until after the elaborate experiments of Mr. Hodgkinson, in 1830, upon the strength and properties of cast-iron, that the best form of section was determined, or that iron beams were used for spans exceeding 14 feet. He found the resistance of cast-iron to compression to be about 6 times as great as its resistance to

extension, and that equal strength could be obtained with half the weight of material formerly used, by giving the proper proportions to the parts subjected to these respective strains. Much, however, was still to be desired, on the score of security and economy, and numerous accidents have justified the general want of confidence in beams of cast-iron, unless great precautions are observed in casting them and properly proportioning their parts; and even when these precautions are observed, and iron of good quality is selected, security can only be obtained by making the most ample allowances for unequal shrinkage in cooling, and for hidden imperfections not apparent on the surface, or to be detected only by the most careful examination. Other objections to cast-iron beams are, that they are liable to fail without warning, especially if subjected to concussion, and to be broken by the frequent application and removal of loads, much less than the permanent load they would sustain with safety. By a system of testing, in some cases, defective beams may be detected; but in others, the load applied in the test itself may so weaken the beam that it may afterward fail with a load much less than that employed in the test, especially if it is to be subjected to concussion or repeated deflections, even though small in amount.-Wroughtiron beams have been used only within the last few years. The successful construction of the tubular bridges, in 1849, over the Conway and Menai straits-the most novel and striking achievement of modern engineering-was one of their earliest applications, and on the most gigantic scale. The laws and the amount of the resistance of wrought-iron to the various strains to which it is subjected in its application to beams, were first determined by the most careful and elaborate experiments, and the superiority of wrought-iron for this purpose clearly demonstrated. By means of the data thus obtained, Mr. Stephenson was enabled successfully to carry out his conception of using for the bridges of the Chester and Holyhead railway, tubular beams of sufficient strength and rigidity to permit the passage of the heaviest railway trains at the highest speeds. These applications of wrought-iron beams on the grand

est scale have been followed by their more modest, but even more useful application to fire-proof buildings, whereby, at the same time, perfect security and a material reduction in the cost of fire-proof constructions have been attained. Wrought-iron is an elastic material of fibrous structure. Its ultimate strength of resistance to extension is greater than to compression, but when these strains do not exceed about one-half its ultimate strength, it offers equal resistance to either strain. Within these limits the amount of the extension or compression which it undergoes is about half that of cast-iron for equal loads; but the amount of its extension or compression, before rupture, is much greater than that of cast-iron. A wroughtiron beam will thus be more rigid than one of cast-iron, with any load that will in practice be permanently applied to it; but, unlike the latter, by its excessive deflection when overloaded, will give warning of danger before rupture can take place. This characteristic is of great importance in beams which may be subjected to impact, as the falling of a heavy weight, the resistance of the beam being in proportion not only to its strength, but also to the amount of deflection that it will undergo before rupture. The various processes of forging, rolling, &c., to which wrought-iron beams are subjected in their manufacture, will cause any serious defect to be detected. They can be used for much greater spans than beams of cast-iron, and it is often an important consideration to dispense with columns or division-walls, when large rooms are required. Their strength being about 3 times that of cast-iron beams of equal weight, while the comparative cost is in a much less ratio, they are not only more safe, but also more economical. For wrought-iron beams the most advantageous forms are the double flanched or I beam, and the box or tubular beam. Unlike those of cast-iron, the flanches or horizontal sides are usually of equal area. When lateral deflection cannot take place, there is little difference in respect to strength between these forms, the single vertical web and the horizontal flanches projecting from it, of the one, being respectively the equivalents of the 2 vertical and of the 2 horizontal sides of the other. For floor beams the I form is ordinarily employed. It is not only more economical, but has the great advantage of allowing the material of which the flooring between the beams is formed to rest upon its lower flanches, thus saving space, and surrounding and protecting the beams from the effects of fire. In the tubular beam not only do its upper and lower sides contribute to its lateral stiffness, but the vertical sides resist lateral flexure in proportion to the width of the tube, exactly as the horizontal sides resist vertical flexure in proportion to its depth, while in the I beam lateral stiffness is due principally to the flanches. A vertical load upon a beam is sustained by the resistance of its fibres to the forces of compression and extension. A

body subjected to compression, as a column, if its length be great in comparison with its lateral dimensions, will fail by bending, under a load much less than would be required to crush the material if the column were maintained in the direct line of strain. The tendency of a body subject to compression to yield by flexure being in proportion to the square of its length, while the vertical strength of a beam is in inverse proportion to its length simply, it may often happen that the limit of strength of a beam will be not its vertical but its lateral stiffness, and hence in some cases, as for girders without lateral supports, it may be advisable to use the tubular form, while for floor beams which are secured from lateral deflexion by the filling in between them, the I form is preferable. Wrought-iron beams of either form may be made by riveting together plates, angle bars, T bars, or other shapes; the rivets should always be fastened while hot in order that their contraction in cooling may draw the parts closely together. The manufacture of solid rolled beams has effected a further important reduction in the cost of fire-proof construction. This manufacture has been introduced in this country by the Trenton Iron company, at their works at Trenton, N.J. These beams have been adopted by the various departments of the government of the United States in the construction of the many custom houses, marine hospitals, and other public buildings erected since their introduction, to the entire exclusion of the system of groined arches and also of riveted beams, except in cases where solid rolled beams of sufficient size could not be obtained. This reduction in the cost of construction has also led to the erection of many fire-proof banking houses, warehouses, manufactories, &c., within the last 3 years, and the system is rapidly coming into general use. For filling in between the beams for fire-proof floors various systems have been adopted. In France, where fire-proof construction with iron beams is extensively used, the filling in is generally a concrete of refuse materials and plaster of Paris. Beams of the I form are placed 2 or 3 feet apart; their ends are built in the walls and secured by anchors; no beams are placed immediately at the walls parallel with the beams. The beam next each wall is connected to it, and each beam connected with the one next adjoining, by inter-ties of round or square iron of about half a square inch in sectional area, and placed 2 or 3 feet apart; the inter-ties pass through holes near the centre line of the beams, and are provided with a head at one end and riveted up at the other after they are put in; the ends that are built into the walls are bent to form anchors. Smaller rods parallel with the beams and 7 or 8 inches apart, are suspended from the inter-ties, the ends of the rods being bent up so as to hook over the inter-ties, while the rods themselves are on a level but little above that of the bottom of the beams; or the inter-ties may be supported upon the lower flanches of the beams and be bent up at the ends so as to