438
BUILDING
inverse proportion to the resistance to the flow of water through the material in which the caisson is sunk, and that the air-pressure which it is necessary to maintain in the air-chamber, to expel the water and dry the bottom, is always much less than, and frequently not more than 33 per cent, of, the pressure due to the column of water equal in height to the depth of immersion of the cutting edge. In transferring the loads from the column bases to the bottom of the footings the greatest care must be taken in all systems of construction that the stresses o umns. ^}ir0Ugq0U|- n0 p0int exceed the safe limits of stress for the various materials used. Steel is generally used for columns in preference to cast iron, because it affords greater facility for securing satisfactory connexions, because its defects of quality or workmanship are more surely detected by careful test and inspection, and because, on account of its superior elasticity and ductility, its fibre is less liable to fracture from slight deformations. It is used in preference to wrought iron on account of its lesser cost. Columns are generally built of riveted work of zeebars, channels, angles, plates, or lattice, of such form as will make the simplest and most easily constructed framing in the particular position in which the column is placed. The columns are sometimes run through two or more storeys and arranged to break joints at the different floors. In buildings to be used as offices, hotels, apartments, &c., it is usual in establishing the loads for the purpose of computation to assume that the columns carrying the roof and the upper storey will be called upon to sustain the full dead load due to material and the maximum computed variable load, but it is customary to reduce the variable loads at the rate of about 5 per cent, storey by storey towards the base, until a minimum of about 20 per cent, of the entire variable load is reached, for it is evidently impossible that the building can be loaded by a densely packed moving crowd in all of its storeys simultaneously. In the case of factories and buildings used for storage purposes the maximum variable load which can be imposed for any serious length of time on each floor must be used without reduction in computing the loads of the lower column, and proper allowances must be made for vibrating loads. In the case of very tall exposed buildings of small depth, the vertical load on the columns due to wind-pressure on the opposite side of the building must be computed and allowed for, and in case the lower columns are without lateral support their bending moment must be sufficient to resist the lateral pressure due to wind and eccentricity of loading. In computing the column sections a proper allowance must be made for any eccentricity of loading. It is usual to limit the height of sections of columns without lateral support to 30 diameters, and to limit the maximum fibre stress to 12,000 lb per square inch. The sectional areas are computed by the use of the ordinary formulae for columns and struts. For girders of small spans “ I ” beams or channels are generally used, but for greater spans girders are built of |riveted in side the form boxes withwith top Girders. anc bottomwork plates, plates,of and angles proper stiffening bars on the side plates, or “ I’s,” or lattice, or other forms of truss work. In girders and beams the ma.Yimnm fibre stress is usually limited to 16,000 lb. In very short girders the shear must be computed, and in long girders the deflexion, particularly the flexure from the variable load, since a flexure of more than -5^ of the length is liable to crack the plastering of the ceilings carried by the girders. The same necessity for computing shear and flexure applies to the floor beams. The floors between the girders are constructed of “ I ” beams, spaced generally about 5 feet between centres; their ends are usually framed to fit the form of the girders, and rest
either upon their lower flanges, or upon seats formed of angles riveted to their webs, being secured to them by a pair of angles at each end of the beam riveted to its web and to the web of the girder. Sometimes the beams rest upon the girders, and are riveted through the flanges to it; in this case the abutting ends of beams are spliced by scarf plates placed on each side of the webs and secured by rivets. A similar construction is followed for flat roofs, the grades being generally formed in the girder and beam construction, and a flat ceiling secured by hanging from them, with steel straps, a light tier of ceiling beams. The floor beams are tied laterally by rods in continuous lines placed at or above their neutral axis. It is usual in both girders and beams to provide not only for the safe support of the greatest possible distributed load, but for the greatest weight, such as that of a safe or other heavy piece of furniture which may be moved over the floor at its weakest points, the centres of the girders and beams. It must always be borne in mind that the formulae for the ultimate strength of the “ I ” beams only hold good when the upper chord or flange is supported laterally. Steel cages may be divided into two classes. In the first and more frequent type the exterior columns and girders are embedded in and hidden by the masonry of the walls, in some cases being stee[ cage covered on their exterior surface by as little as 4 and generally by only 8 inches of masonry. In the second system the columns are placed inside of the wall, and beams or channels in couples are run in each side of and past the wall columns, or a floor beam is run through the column and allowed to project beyond it, or the floor beams are allowed to run over a wall girder between the columns and project beyond it, the projections in each case serving as cantilevers to carry the girders which are incorporated in the masonry of the wall for its direct support. The wall girders in both forms of construction are formed of “Z’s,” “ T’s,” or channels, sometimes carrying plates of proper forms which are riveted to them. The wall girders are always incorporated in the masonry. When the exterior columns and girders are embedded in the wall, the cement masonry and painted surfaces of the steel are expected to guard it from corrosion. Time only can prove whether this protection will be permanently efficient; for masonry, particularly when of brick, unless absolutely free from defects or cracks, and unless kept perfectly painted, will not only fail as a waterproof covering, but will maintain a damp surface in contact with the frame for a considerable time after each beating storm. It is claimed that a coating of cement mortar will prevent the oxidation of steel, but it is certain that it is very efficient in preventing the oxidation of paint. For the cantilever system it is claimed that all eccentricity of column loads is avoided, that the columns standing free within the wall are as readily protected from corrosion as any interior columns, that they can be at any time uncovered, examined, and if necessary replaced, and that while the failure of a column may result in the collapse of the building, the failure of a wall girder would be a local injury of relatively small importance, since in almost all cases there is a sufficient margin of strength in the wall to transfer the load to the wall girder next below. The steel used throughout the entire structure should be subjected to the most thorough chemical and mechanical tests and inspection, first at the mill and sub- ^ terjais sequently at the fabricating shops and the MS^ra building, to insure that it shall not contain more than *08 per cent, of phosphorus or ‘06 per cent, of sulphur, that it shall have an ultimate strength of between 60,000 and 70,000 5) per square inch, with an elastic limit of not less than 35,000 K) per square inch, and an elongation
