FIREPROOF BUILDING.
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 build ing 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 con tents of the building, but haying 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 con struction.
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 settle ment, and to so distribute the loads that the settlement shall be uniform.
Height of Buildings. As the owner will naturally desire to obtain as many floors as possible, it will be necessary to consult, first of all, the building laws of the city in which the building is erected, to determine the height to which our building may be carried. This determined, the number of floors must be decided. As the lower floors are less likely to be subdivided than the others, they must be given a height which will be in proportion to the probable size of the rooms, so that the height of the lower stories must be added together, and the sum, with the addition of the thickness of the floors, must be taken from the whole height of the building before we can establish a unit of height for the rest of the stories.
The Building Site. While these matters are under consid eration by the owner, an examination of the proposed site will be profitable, to gain necessary information in regard to party walls and the condition of adjoining property. The nature of the soil, the location of sewers and other underground works, will be factors in the proposed construction, and all data relating to these matters must be recorded and carefully preserved.