There are two classes of steel construction, the cage and the skeleton. In the cage construction the frame is strengthened for wind stresses and the walls act as curtains. In the skeleton, the frame carries only the vertical loads and depends upon the walls for its wind bracing. It has been found that the wind pressure is about 30 lbs. for every square foot of exposed surface.
The steel columns reach from the foundation to the top, riveted together by plates and may be extended to an indefinite height. In fact there is no engineering limit to the height.
The outside walls of the sky-scraper vary in thickness with the height of the building and also vary in accordance with the particular kind of construction, whether cage or skeleton. If of the cage variety, the walls, as has been said, act as curtains and consequently they are thinner than in the skeleton type of construction. In the latter case the walls have to resist the wind pressure unsupported by the steel frame and therefore they must be of a sufficient width. Brick and terra-cotta blocks are used for construction generally.
Terra-cotta blocks are also much used in the flooring, and for this purpose have several advantages over other materials; they are absolutely fire-proof, they weigh less per cubic foot than any other kind of fire-proof flooring and they are almost sound-proof. They do equally well for flat and arched floors.
It is of the utmost importance that the sky-scraper be absolutely fire-proof from bottom to top. These great buzzing hives of industry house at one time several thousand human beings and a panic would entail a fearful calamity, and, moreover, their height places the upper stories beyond reach of a water-tower and the pumping engines of the street.
The sky-scrapers of to-day are as fireproof as human ingenuity and skill can make them, and this is saying much; in fact, it means that they cannot burn. Of course fires can break out in rooms and apartments in the manufacturing of chemicals or testing experiments, etc., but these are easily confined to narrow limits and readily extinguished with the apparatus at hand. Steel columns will not burn, but if exposed to heat of sufficient degree they will warp and bend and probably collapse, therefore they should be protected by heat resisting agents. Nothing can be better than terra-cotta and concrete for this purpose. When terra-cotta blocks are used they should be at least 2 inches thick with an air space running through them. Columns are also fire-proofed by wrapping expanded metal or other metal lathing around them and plastering.
Then a furring system is put on and another layer of metal, lathing and plastering. This if well done is probably safer than the layer of hollow tile.
The floor beams should be entirely covered with terra-cotta blocks or concrete, so that no part of them is left exposed. As most office trimmings are of wood care should be taken that all electric wires are well insulated. Faulty installation of dynamos, motors and other apparatus is frequently the cause of office fires.
The lighting of a sky-scraper is a most elaborate arrangement. Some of them use as many lights as would well supply a good sized town. The Singer Building in New York has 15,000 incandescent lamps and it is safe to say the Metropolitan Life Insurance Building has more than twice this number as the floor area of the latter is 2-1/2 times as great. The engines and dynamos are in the basement and so fixed that their vibrations do not affect the building. As space is always limited in the basements of sky-scrapers direct connected engines and dynamos are generally installed instead of belt connected and the boilers operated under a high steam pressure. Besides delivering steam to the engines the boilers also supply it to a variety of auxiliary pumps, as boiler-feed, fire-pump, blow-off, tank-pump and pump for forcing water through the building.
The heating arrangement of such a vast area as is covered by the floor space of a sky-scraper has been a very difficult problem but it has been solved so that the occupant of the twentieth story can receive an equal degree of heat with the one on the ground floor. Both hot water and steam are utilized. Hot water heating, however, is preferable to steam, as it gives a much steadier heat. The radiators arc proportioned to give an average temperature of 65 degrees F. in each room during the winter months. There are automatic regulating devices attached to the radiators, so if the temperature rises above or falls below a certain point the steam or hot water is automatically turned on or off. Some buildings are heated by the exhaust steam from the engines but most have boilers solely for the purpose.