The most modern feature of city building construction is the erection of the high business blocks which form the main part of the commercial districts of all cities. Here a variety of problems present themselves, more or less complicated according to the nature of site and surroundings, of size and usage.

In the first place, the value of the land which the proposed building will occupy must be considered. Real estate in the heart of our large cities attains almost fabulous value, and the first consideration which the owner will require, will be the maximum of rentable area within the walls of his building. This of itself will force us to adopt a system of construction which will permit us to erect the building with the thinnest possible walls that safety and the building laws will allow, and the same consideration of rent will force us to build as high as possible. From this tendency has arisen the modern "sky scraper," a construction consisting of a steel skeleton covered with masonry, simply as a protection for the steel, and for the contents of the building, but having no weight-bearing value of itself. The floors and walls are supported wholly by the steel frame, which is carried usually on isolated supports far below the sidewalk. This is the common form of high building and is known as skeleton construction.

Sometimes the exterior walls are made strong enough to be self supporting, the steel frame carrying only the floor loads, but in this case there is danger of unequal settlement between the frame and the enclosing walls, whereas if the whole of the load is carried by the steel frame, the footings can be proportioned so as to give equal settlements. In doing this, it is customary to use only the dead load, - i.e., the weight of the building material - in establishing the proportions of the footings, for if the live load of people and merchandise were to be included, the interior footings would have a much higher percentage of live load than the exterior footings, and as the live load is not constant, it would be impossible for the building to settle uniformly. This has been proved by the present condition of existing buildings which have been erected upon a compressible soil, the tendency being for the interior footings, which are subjected only intermittently to their full live load, to settle less than the exterior footings where the dead load of the walls is constant, unless due allowance is made. Some authorities allow 25 per cent of the live load in addition to the dead load upon the footings, but difference in soil will require especial allowances. The essential point in any foundation is not to overload the soil, so as to cause excessive settlement, and to so distribute the loads that the settlement shall be uniform.