STEEL AND IRON CONSTRUC

TION. ‘The uses of steel and iron in construction are chiefly exemplified in beams, columns, and braced structures. All longitudinals subject to a bending load may be cla ified as beams. Beams supported at both ends are termed girders. Beams supported (and fixed) at one end only are known as cantilevers. Columns are upright posts sustaining a vertical load, usually imposed upon them by the ends of beams or roof principals which they support. Braced structures include all contrivances for spanning an opening, or sustaining (while composing) an erection, which consist of a jointed framework of steel or iron members. Steel has now practically superseded wrought iron for beams and braced structures, being slightly stronger, le susceptible to corrosion, and not more costly. The use of cast iron is restricted to columns, base-plates, and other parts where compre ion only has to be borne. The object to be kept in view in designing structures of steel or iron is to make provision in every part for the greatest stre that can be brought upon it by any condition or combination of external forces or loads, at the same time avoiding waste of material and extravagance of proportion. For this purpose it is nece ary to arrive at the magnitude and direction of the external forces, the stre es which these forces produce in the structure, and the degree in which the component parts of the structure are fitted to resist the stre es. In beams the upper member is in compre ion, the lower in tension, while the web or portion separating the two flanges is in shear. Columns are, as regards their main bulk, in compre ion alone. Braced structures of all descriptions are so designed that their component members are in_ either compre ion or tension, shearing stre being, with the exception of such parts as may be treated as beams, confined to the joints. The first step in designing any structure is to calculate the loads upon it, and the points at which these loads will act. In the case of bridges the former consist of the dead load, due to the actual weight of the girders and flooring, and the live or rolling load, due to the impact of the train. In the ease of buildings, the calculations are simplified by the absence of any live load. The weight of roof, walls, and flooring is constant and easily arrived at; that of snow (on a roof) is estimated according to the severest conditions liable to occur; while the additional load liable to be imposed by the presence of persons on the upper floor of a building is taken to be that due to a crowd of people standing closely throughout. All these loads act vertically downwards. In the case of roo is and high-braced structures generally, a zontal pre ure due to the wind has to be taken into account. The and their position having been mined, the requisite strength of member is arrived-at by calculation, aided by graphic methods, based on actual measurements of diagram on paper. Steel construction of buildings as residential flats and offices is a of modern cities, being exemplified ticularly in New York and Other forms of steel structure bridges, roofs of railway stations, puildings demanding an extensive roof (e.g.), the Machinery Hall of Paris Exhibition (1889), the roof which spans a clear width of 375 For the same exhibition the Tower was constructed, and serves a typical example of a high-braced structure. Lighthouses are frequently constructed of steel.