Page:The American Cyclopædia (1879) Volume II.djvu/434

 414 BEAM by the most careful examination. Other ob- jections to cast-iron beams are, that they are liable to fail without warning, especially it' sub- jected 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 de- tected ; 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 elab- orate experiments, and the superiority of wrought iron for this purpose clearly demon- strated. 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 rigid- ity to permit the passage of the heaviest rail- way trains at the highest speed. These appli- cations 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 at- tained. Wrought iron is an elastic material of fibrous structure. Its ultimate strength of resistance to extension is greater than to com- pression ; 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 compres- sion 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 lat- ter, by its excessive deflection when overloaded, will give warning of danger before rupture can take place. This characteristic is of great im- portance 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. For wrought-iron beams the most advantageous forms are the double- flanged or I 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 be- tween these forms, the single vertical web of the one, and the horizonttil 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 I form is ordi- narily employed. It is not only more economical, but has the great advantage of allowing the ma- terial 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 con- tribute 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 stiflness 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 compari- son 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 ad- visable 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 riv- eted 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,