Beam (Sax. beam, a tree), in architecture, a piece of timber or iron, 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 resist-, ance 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 a 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 introduction of cast-iron beams and light segmental arches to some extent obviated these inconveniences; 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 trustworthy and more economical than cast iron. 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, however, 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 six 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 be obtained only 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.

The successful construction in 1849 of the tubular bridges over the Conway and Menai straits was one of the earliest applications of wrought-iron beams, 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 speed. These applications of wrought-iron beams on the grandest 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 tire-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 wrought-iron 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, etc, 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. - For wrought-iron beams the most advantageous forms are the double-flanged or X beam, and the box or tubular beam. Unlike those of cast iron, the flanges 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 of the one, and the horizontal flanges projecting from it, being respectively the equivalents of the two vertical and of the two horizontal sides of the other. For floor beams the X 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 flanges, 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 X beam lateral stiffness is due principally to the flanges. 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 deflection 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 was first introduced in this country by the Trenton iron company, at their works in 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 the latter are used because solid-rolled beams of sufficient size cannot be obtained. This reduction in the cost of construction has also led to the erection of many fire-proof banking houses, warehouses, manufactories, etc, and the system is rapidly coming into general use. For tilling in between the beams for lire-proof floors various systems have been adopted. In France, where fireproof construction with iron beams is extensively used, the filling in is generally a concrete of refuse materials and plaster of Paris. Beams of the X form are placed 2 1/2 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 1/2 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 flanges of the beams and be bent up at the ends so as to hook over the upper flanges, and the smaller rods parallel with the beams be laid upon the inter-ties. A flat centring is placed against the bottoms of the beams, and broken bricks or other refuse materials suitable for concrete are put upon the centring; and plaster of Paris being poured in, the whole mass soon becomes sufficiently set to allow the centring to be removed, and the concrete to be sustained by the iron framework between the beams.

Id some cases the plaster concrete fills up the whole space between the beams, and flooring tiles are laid directly upon it; in others the depth of the concrete is less than that of the beams, and wooden strips are laid across the beams perpendicular to their length, to which ordinary flooring boards are nailed. A finishing coat of plaster put directly on the concrete forms the ceiling below. Hollow potteries placed upon the iron latticework, with the interstices filled with plaster, are frequently used instead of concrete. A very light and superior floor is thus made, and the rigidity of the whole svstem considerably increased. - The use of plaster for the tilling in between the beams has not been adopted in England or America, because of the greater cost and inferior quality of the plaster that can be obtained. The system known as that of Fox and Barrett has been used extensively in England. Light strips of wood with narrow spaces between them are supported on the bottom flanges of the beams, and reach from beam to beam. On these strips is spread a layer of coarse mortar, which is pressed dawn between them. Concrete, made with cement, is filled in between the beams, and a tile or wooden floor is laid immediately upon it.

A rough and a finishing coat of plaster are put directly on the cement to form the ceiling below. Floors have also been made by the use of arched plates of wrought iron or of corrugated sheet iron supported upon the lower flanges of the beams, with a filling of concrete above the arched plates or corrugated iron on which the floor is laid. The system of light segmental brick arches springing from the lower flanges of the beams and levelled up with concrete is that most generally employed in this country and in England. It is more strictly fire-proof than any other, and much more economical than the use of arched plates or corrugated sheet iron, and, except in France, where plaster is cheap, than the French system. The weight of the floors themselves forms a much greater part of the total load to be carried by the beams than in the lighter French system; but on the other hand, the arches and concrete add materially to the strength and rigidity of the beams, not only by preventing lateral deflection, but by adding to some extent the resistance to compression of so much of the arches or concrete as is above the neutral line to that of the upper parts of the beams, whereby they become in fact an integral part of the beams themselves.

Long beams should be supported in the middle of their length by wooden scantlings until the cement of the arches or concrete is set, in order to get the full advantage of this additional resistance. The arches should have a rise of not less than one inch to the foot of span, and are generally the width of a brick in thickness, unless the span exceeds 6 or 8 feet, when they should be 8 inches at the soffit and 4 1/2 inches at the crown. If a wooden flooring is to be used, wooden strips parallel with the beams are laid in the concrete filling above the arches, to which the flooring can be nailed. To form the ceiling below the beams, wooden strips may be secured to the lower flanges of the beams, to which ordinary furring, lathing, and plastering can be nailed; or the plaster may be put directly upon the arches, so as to show the system of construction, and thus with suitable mouldings a good architectural effect can be obtained. Any inequality in the thrust of the arches on the beams is counteracted by the tie rods perpendicular to the length of the beams connecting them together. The load to be sustained by the floors of dwellings, offices, and buildings, other than manufactories and buildings for the storage of heavy goods, is ordinarily assumed at 150 lbs. per square foot.

The weight of the beams, arches, concrete, etc, forming the floor, will ordinarily be about 75 lbs. per square foot, leaving 75 lbs. per square foot for the variable load. This is as great a load as can be brought upon a floor by a crowd of people. For wrought-iron floor beams the actual or safe working load should not produce a greater strain than 12,000 lbs. per square inch of section at the part of the beam which is subjected to the greatest strain by the action of the load. In the following part of this, article the term "safe load" will mean the load corresponding to that strain. The safe load will be less than one third of the ultimate or breaking strength of the beam, thus allowing a sufficient margin of strength to insure safety. The deflection of floor beams should not exceed 1/30 of an inch for each foot of span. If the depth of the beam is not less than 1/24 of the span, the deflection will be within that limit for the safe load. For spans for which a greater depth than 15 inches is not required, solid.rolled beams are ordinarily used, and for greater spans riveted beams. - The following table gives the dimensions, weights per yard, and coefficients to determine the safe loads for rolled wrought.iron beams of the sizes most used in this country:

The safe load, uniformly distributed over the span, when the beam is supported at both ends, and lateral deflection is presented by the filling between the beams, will be found, in pounds, by dividing the coefficient given in the table by the span estimated in feet. If the span be less than that given in the column headed "Limitation of coefficient," the load should nevertheless not exceed the safe load for that span, in order that the shearing strain upon the stem shall not exceed the safe limit. The | deflection at the middle of the span, for the safe distributed load as given by the above rule, will be found by dividing the square of the spun, estimated in feet, by 70 times the depth of the beam, estimated in inches; and for any less load, it will be proportionally less. If the beam is free to deflect laterally, the coefficient given in the table must be modified, to allow for the increased strain brought upon the beam, as follows: multiply the coefficient by the number given in the column headed " Correction for lateral resistance," and divide the product by the sum of that number and the square of the span estimated in feet.

The strength of various forms and dimensions of riveted beams may be determined by the or. dinary formulas for the strength of materials.