How is it possible to erect such building—how is it possible to erect a sky-scraper at all? A partial answer may be given in one word—steel.
Generally speaking the method of building all these huge structures is much the same. Massive piers or pillars are erected, inside which are usually strong steel columns; crosswise from column to column great girders are placed forming a base for the floor, and then upon the first pillars are raised other steel columns slightly decreased in size, upon which girders are again fixed for the next floor; and so on this process is continued floor after floor. There seems no reason why buildings should not be reared like this for even a hundred stories, provided the foundations are laid deep enough and broad enough.
The walls are not really the support of the buildings. The essential elements are the columns and girders of steel forming the skeleton framework of the whole. The masonry may assist, but the piers and girders carry the principal weight. If, therefore, everything depends upon these piers, which are often of steel and masonry combined, the immense importance will be seen of basing them upon adequate foundations. And thus it comes about that to build high we must dig deep, which fact may be construed as an aphorism to fit more subjects than the building of sky-scrapers.
To attempt to build a sky-scraper without a suitable foundation would be tantamount to endeavoring to build a house on a marsh without draining the marsh,—it would count failure at the very beginning. The formation depends on the height, the calculated weight the frame work will carry, the amount of air pressure, the vibrations from the running of internal machines and several other details of less importance than those mentioned, but of deep consequence in the aggregate.
Instead of being carried on thick walls spread over a considerable area of ground, the sky-scrapers are carried wholly on steel columns. This concentrates many hundred tons of load and develops pressure which would crush the masonry and cause the structures to penetrate soft earth almost as a stone sinks in water.
In the first place the weight of the proposed building and contents is estimated, then the character of the soil determined to a depth of one hundred feet if necessary. In New York the soil is treacherous and difficult, there are underground rivers in places and large deposits of sand so that to get down to rock bottom or pan is often a very hard undertaking.
Generally speaking the excavations are made to about a depth of thirty feet. A layer of concrete a foot or two thick is spread over the bottom of the pit and on it are bedded rows of steel beams set close together. Across the middle of these beams deep steel girders are placed on which the columns are erected. The heavy weight is thus spread out by the beams, girders and concrete so as to cause a reduced uniform pressure on the soil. Cement is filled in between the beams and girders and packed around them to seal them thoroughly against moisture; then clean earth or sand is rammed in up to the column bases and covered with the concrete of the cellar floor.
In some cases the foundation loads are so numerous that nothing short of masonry piers on solid rock will safely sustain them. To accomplish this very strong airtight steel or wooden boxes with flat tops and no bottoms are set on the pier sites at ground water level and pumped full of compressed air while men enter them and excavating the soil, undermine them, so they sink, until they land on the rock and are filled solid with concrete to form the bases of the foundation piers.
On the average the formation should have a resisting power of two tons to the square foot, dead load. By dead load is meant the weight of the steelwork, floors and walls, as distinguished from the office furniture and occupants which come under the head of living load. Some engineers take into consideration the pressure of both dead and live loads gauging the strength of the foundation, but the dead load pressure of 2 tons to the square foot will do for the reckoning, for as a live load only exerts a pressure of 60 lbs. to the square foot it may be included in the former.
The columns carry the entire weights including dead and live loads and the wind pressure, into the footings, these again distributing the loads on the soil. The aim is to have an equal pressure per square foot of soil at the same time, for all footings, thus insuring an even settlement. The skeleton construction now almost wholly consists of wrought steel. At first cast-iron and wrought-iron were used but it was found they corroded too quickly.