teel and concrete have largely replaced them. With these newer materials we can build to greater heights, span wider gaps, and carry heavier loadings than ever before. In many countries today, mountainous masses of steel and concrete are reaching to the skies on a scale undreamed of in earlier times. Engineers have been able to do this because the rise of the science of structures and the advent of steel and concrete have enabled them to conceive and build their buildings and their structures as frameworks.
Before this, the walls of buildings were "bearing" walls: they carried the dead weight and the live load of the structure. The modern large building, however, is essentially a framework or skeleton of steel or concrete that supports the roof and floors. The outside walls are enclosing or "curtain" walls, and often their weight at each floor is also taken by the framework. So the "sidewalk superintendent,'' gaping curiously at the busy scene on a building site, may see the walls rising simultaneously from the ground and midway between roof and ground. Since the walls do not earn' the structure, they are sometimes made of glass or plastic.
When the lift-slab method was first devised, some people described it as a way of constructing a building from the top downward. The description, however fanciful, draws attention to the novelty of the system. The method does, nevertheless, give an excellent illustration of the framework principle employed in most modern buildings.
One begins by constructing the various floors and the roof on top of one another, and around the columns, at ground level. These are then lifted by powerful hydraulic jacks to their correct positions in the structure, and permanently connected to the columns. In the case of a nine-storey building, the columns may be erected in three stages, and the tops of each stage temporarily braced together by the ascending roof. As the roof and higher floors move upward, they leave behind lower floors which, secured to the columns, serve to brace them permanently together. In this type of structure, therefore, the floors and the roof serve the added purpose of forming part .of the framework. When roof and floors are all finally in place and connected to the columns, the protective walls can be built up from ground level.
In large buildings, the civil engineer's main task is the design of frameworks and foundations.
Left, model by Mies van der Rohe for a glass skyscraper, 1920. Right, Tokyo hotel designed by Frank Lloyd Wright; it rents on a concrete raft (diagram).
Where a building is supported by its walls, the weight is spread out over a large area. This gives relatively low pressures on the ground and makes foundation work easy. But in a modern building framework, the load is carried by a small number of columns, which leads to highly concentrated ground pressure. Hence the foundations must be strong, particularly if the ground is weak.
If the ground is reasonably firm, it is often enough to place the columns on spread footings. These are blocks of concrete of such area as to spread the load to within the carrying strength of the ground. On soft ground, a building is sometimes put on a concrete raft, which is a footing covering the whole ground plan area. A building on its raft is virtually floating on the ground beneath. A method often used is to drive a number of piles under each column foundation. The piles are made long enough to rest on the hard ground below the soft surface. The building is then literally on stilts. Another method is to carry the columns on fairly large diameter cylinders sunk deeply into the ground.
Below, the Empire State Building lowers above :he skyscrapers of New York.
II. Answer the following questions.
1) What factors have led to the revolution in the "history of modern structures"?
2) Why is the «framework principle» used widely in modern constructions?
3) How does the "lift- slab" method work?
4) Why is it necessary for civil engineers to pay special attention to the design of frameworks and foundations?
5) What types of foundations a