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FIREPROOF BUILDING

IMPORTANT QUESTIONS FOR PRELIMINARY STUDY

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

Land Values. 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 demand which the owner will make, 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 the thinnest walls that safety justifies and the building laws allow, and the same consideration will force us to build as high as possible. From these tendencies has arisen the modern skyscraper, a construction consisting of a steel skeleton, covered with masonry simply as a protection for the steel and for the contents of the building, and having no weight-bearing value in 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.

This type of construction is fully treated in Part III, so that mention need be made here of only some important details which affect mercantile buildings in relation to the general layout of the plan.

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 structure is to be erected, to determine the height to which the 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; the height of the lower stories must, therefore, 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 remaining stories.

Building Site. While these matters are under consideration by the owner, an examination of the proposed site will be profitable, to gain necessary information with regard to party walls and the condition of adjoining property. The nature of the soil and 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.

General Construction Problems. Vertical Supports. As soon as the instructions of the owners, or the recommendations of the architect, have been definitely adopted, the construction of the building will be laid out. Certain factors, which do not enter so strongly into the problems we have previously studied, must have careful attention in constructing mercantile buildings of this class, and upon our skilful management of them will depend the success of the enterprise.

As the problem will resolve itself into the disposition of a greater or less number of isolated points of support, instead of the ordinary continuity of walls, it will be necessary to study the construction at the same time with the laying-out of the offices in order to place the necessary vertical supports as much out of the way as possible. Thickness of Floors. Another matter of importance will be the thickness of the floors. Since the number of floors is certain to be considerable, it will be readily seen that an excessive thickness would result in an appreciable loss of height; if the thickness of each floor can be kept down to a reasonable rate, it may be the means of obtaining one more story for renting purposes, which is a matter worthy of consideration.

External Factors. In bringing together the elements to form a whole composition, certain external conditions must be taken into account. Among these are ease of access, maximum of light, rentable area, ease of rearrangement to suit tenants, and minimum of cost. Ease of access requires that the elevators and stairs shall be placed in direct and obvious connection with the entrances and as nearly as possible at the level of the sidewalk, that they may be readily seen on entering the building or on leaving the offices to which they lead. The position of the elevators is of more importance than the stairs, as the latter will be little used in high buildings. To obtain the best light possible it is well to provide that the

areas (which are a necessity in a building in the interior of a block or in a wide building on a corner) shall have a general direction north and south. These areas should contain windows as large and as near the ceiling as possible, and the rooms should be as nearly rectangular as may be, to avoid dark corners.

The structural conditions determine more or less the arrangement of the offices, as already suggested, but the adjustment of the office space to the needs of the tenants is a matter that must also be given consideration. The unit of size and the arrangement of the offices should be such that the tenant who wants a single office may have one of average size, and the tenant who wishes may have a whole floor, except the minimum of space necessary for toilets, elevators, and stairs.

FLOOR CONSTRUCTION

When the character of the building and the general divisions of the floor space have been established, the location and design of

[graphic]

Fig. 231. Section Showing Typical Brick Floor Arch Construction

the columns, the kind of wall and floor construction, and the size and spacing of floor beams and girders must be decided. The location of the columns and girders will be determined by the size and shape of the floors, the position of present or future partitions, the floor loads, and other considerations which arise with different locations and uses.

Original Fireproof Floor. The size and spacing of the floor beams will depend upon the style of floor construction which may be adopted. The original fireproof floor construction consists in spacing beams 5 or 6 feet apart, and turning a brick arch from flange to flange of the beams, Fig. 231. The space above the arch is filled with concrete up to about an inch above the beams, completely enclosing the steel; and screeds, running parallel to the beams and bedded in the concrete, give a sufficient nailing for the floor

boards. The ceiling underneath may be finished by simply plastering on the underside of the brick arch, or a level ceiling may be hung to the rough floor. As an arch of such a shape will exert considerable thrust, this must be taken up by the use of tie rods inch or inch in diameter, spaced along below the center line of beams at intervals of about 6 feet. This style of flooring weighs about 70 pounds to the foot, and has been practically superseded by the lighter constructions employing terra cotta arches or concrete.

Terra Cotta Floor Arches. Hollow terra cotta blocks, molded in the form of a flat arch, are used to a large extent for fireproof floors, and are to be obtained in a variety of patterns and devices. These various patterns may be divided into two principal classes

[graphic]

Fig. 232. Section Showing Side Method Terra Cotta Arch Construction

according to the method of laying-the side method and the end method.

Side Method. The side-method arch, where the blocks are laid with the webs parallel to the beams, Fig. 232, is the original form of terra cotta floor arch, while the end-method, where the blocks are laid end to end at right angles to the beams, is an improvement designed to present the full end-section of the material to resist the great thrust of the arches. These side-method arches are usually made of dense terra cotta and may be obtained of various depths from 6 to 15 inches; they should be set with close joints and thoroughly cemented together. Specially molded blocks, called skewbacks, are made to fit the lower flanges of the beams and project about 2 inches below the beam, which is covered by a thin strip of tile. The space above the blocks is filled with a cinder concrete, in which beveled wooden strips are imbedded as a nailing for the wooden floors.

Side-method arches are made to break joints endways, so as to give a bond; and they are usually strong enough for all ordinary floor loads. The joints in the blocks are generally made parallel to

the sides of the key block, as this gives a uniform pattern and so is less expensive than a radial jointing; the latter makes a stronger

IF

Fig. 233. Section Showing End Method Terra Cotta Arch Construction

arch, but on account of the expense of making and adjusting the different patterns, it is little used.

End Method. In this method, the blocks are usually made of porous terra cotta and are set end to end, giving greater resistance to the thrust by forming a series of continuous webs from beam to beam, Fig. 233. The blocks are generally set in continuous lines, not breaking joints. The jointing of the tiles must be done with great care, as the open ends do not give so good a surface for cementing as do the tiles laid by the side method, and, the bearing on the beams being given by the thin webs, it is necessary that the joints be perfect.

Combination Method. Because of the difficulty in jointing, a combination method is often used, shown in Fig. 234. This gains the extra strength of the end blocks, and the better bearing of the

[graphic]

Fig. 234. Section Showing Combination Terra Cotta Arch

flat skewbacks, the skewbacks being made with many webs and of small sections.

Setting. Floor tiles of either pattern must be set upon plank centers which are hung from the beams, and should be crowned inch in an arch of 6 feet. All joints must be close, and made with cement. The centering should be left in place until the cement is thoroughly set, which will require from twelve to thirty-six hours,

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