There are three elements which enter into the construction of the steel-skeleton building, — foundations, columns and floors. Of these three elements the column is the most import ant ; for while foundations may settle, derang ing the floor levels and causing the building to lean out of plumb; and while floors may bend or break without serious danger to any parts of the structure other than themselves,— col umns, if they fail, may entail the collapse of the entire structure. For this reason, in the best work, columns are made of the softer, less brittle grades of steel, while floor beams are permitted of °medium° steel, a harder and consequently more brittle grade.
Many sections of columns have been de vised, each having its own particular advan tages, but columns in which all the surfaces, except those between riveted plates, are ac cessible, are generally to be preferred. These columns usually consist of a single web or plate, with one or more flange plates connected to the web by riveted angles. Other sections have been devised, made of Z bars or of channels connected by lattice plates, and a very ingenious column is that known as the Gray column, made up of angles in pairs connected by ties. The choice of any one form depends upon the sta bility of its section, the ease of procuring the parts of which it is composed and the facility of connections. It is sometimes necessary to use the box column, but it is not to be recom mended by reason of the inaccessibility of the interior surfaces. Water and steam pipes are sometimes run inside the fireproofing of col umns next to the steel, but this is to be avoided if possible.
Columns are usually made in two- or three story lengths. The bearing parts are carefully ground normal to their axis, and the connec tions are made by riveted cover plates. The extraordinary weights which these columns are called upon to carry demand on ordinary soils a very extended footing. If this were attempted by the old method of brick or stone piers, the foundations would have to be carried to such depths that the system would not be economical. The customary method is that known as the grillage-beam system, in which the column starts from an iron or steel shoe which bears upon steel beams extending on opposite sides of the shoe, and bearing in turn upon one or more layers of beams bolted together and com pletely imbedded in concrete; under all is a layer of concrete whose area depends upon the compressive resistance of the soil. In rare cases
the foundation is of solid rock, and the area may then be reduced to a minimum.
A peculiar type of grillage foundations is re quired for columns on or near party lines, beyond which the foundations may not extend. A cantilever construction is then used, whereby the wall column foundation is united by beams to the nearest interior column foundation, so that the two act together and in a measure counterbalance each other. Where foundations occupy an interior corner of a property, and must be maintained inside two intersecting party lines, it is often necessary to combine four grillages in the same fashion. Where the soil is of a very compressible nature, as is frequently the case in many parts of Chi cago, the entire area of the building may have to be covered by a distributing foundation of concrete and beams, forming a pan upon which the building floats. This has been followed in some cases by settlements due to the leakage of the underlying soil, a result which might easily develop from the construction of other buildings in the immediate neighborhood. Where possible, it is preferable to penetrate through soft soils to a firm bed. In the lower part of Manhattan Island bed-rock has been reached by pneumatic caissons. These caissons are made of steel plates riveted together. The excavation is made under or in the caisson under air pressure suffi cient to hold back any water-bearing material which may underlie the foundations of adjoin ing buildings. After the caisson has been sunk to its proper depth it is filled with concrete or such other masonry as has been designed to form the foundation. Hydraulic caissons have also been used for the same purpose. Where excavations adjoin high buildings on sandy soil, and are car ried to a greater depth than the grillages, as in the case of the work on the subway in lower Broadway, New York, an artificial freezing proc ess is sometimes resorted to. A network of tiny pipes is inserted into the sand foundation, wind ing in and out among each other, so as to reach every part of the foundation soil. A cold salt solution is sent through the pipes, causing the sand foundation to freeze solid. This is a very expensive process and not to be employed unless other means fail. See FOUNDATION.