Fireproof Building

While

these systems of floor construction are typical of the main features of all, they are not necessarily the best or only kinds, for many other systems, embodying variations of these features, are in constant use, some making use of twisted wire strands in place of expanded metal to support the concrete plate, others, steel bars of special pattern.

Selection.

With so many styles of floor available, each pos sessing merits of its own, the architect will often be at a loss to decide upon a special system. Cost, which is an ever-present factor, will decide in some cases, and in others, local considerations may require particular methods. In the main, consideration should be given to lightness, strength, and fire-resisting qualities, as well as speed of erection and the skill of available workmen. Of the respective merits of the two rival systems of terra-cotta arches and concrete, much has been written, especially in regard to fire-resisting qualities, but the question of durability cannot be said to have been settled at the present time. For very high buildings one of the systems requir ing the use of steel floor beams and girders will usually be adopted, and the beams spaced with reference to the load and the requirements of the floor construction; these beams may rest entirely on the top of the girders, but are usually framed flush with the top, allowing the girder to project if it is deeper than-the beam. The beams are connected with the girder by means of angles riveted to the webs of the beam and girder. (Fig. 222.) Connections of the floor beams with the columns, which will occur where the spacing of the floor beams brings a beam opposite to a column, are made as described for light girders. The outside beams of the floor are sometimes allowed to run behind the exterior columns, forming no part of the exterior construction, but more often they are framed between the columns, and, in connection with other supports, help to carry the enclosing walls of the building. (Fig. 223.) Roof and Ceilings. As the roofs of high buildings are in general made with very little pitch, the same construc tion may he used here as for the floors, but a special treatment will be required to retain a level ceiling in the upper story. This is usually done by hanging steel T-bars at a level below the roof, and , upon these a ceiling may be laid either of terra-cotta blocks to be plastered, as in Fig. 224, or of metal lathing, Fig. 223.

Exterior Walls. The construction of the exterior skeleton of a fireproof building will require special treatment according to the covering material to be used. While the adopted type of floors may be carried throughout one or several buildings, by a simple repetition of methods, the con struction of the outside frame will vary with the different materials used in differ ent buildings, or often in different por tions of the same building.

In general, the weight of the exte rior walls, especially above the first two stories, must be carried by the steel frame. The two or three lower stories, on the street side being largely glass area, the piers are often carried by the foundations below. These piers are sometimes of stone enclosing the steel columns, but brick or terra-cotta are used to great extent. For the upper stories, especially- of a fireproof building, brick or terra-cotta are to be recommended, not only on account of the fire-resisting qualities, but also because of the ease with which they may be built around the skeleton frame.

In general, the exterior treatment of a building of skeleton con struction will resolve itself into a series of piers, more or less marked in character, which enclose the vertical supports, the space between being to a great extent occupied by windows, with a horizontal piece of masonry separating the windows of each story and covering the floors.

The masonry enclosing the columns is generally supported by brackets or angles riveted to the columns, and the horizontal bands of masonry between the windows, called spandrels, are supported by beams of requisite size and shape, which run be tween the columns and are riveted to them.

The character of these supports will vary with the design of the building; but in general the inner beam will of neces sity be of such form that it may also be used to support the floor construc tion. (Fig. 22G.) Wind Pressure. A distinctive prob lem of high building construction is the provision for lateral resistance to wind pressure. Unless the building is more than four times its width in height, the effect of the wind pressure is not a serious consideration. The danger of overturning the building bodily is very remote, the chief danger being a tendency to shear the connections or twist and distort the frame. Sometimes the ordinary framing of the floors and columns will be sufficient, and in other cases special provision must be made. This is done by diagonal ties where the construction will allow, as shown in Fig. 227. When the spaces between the columns must be used for passage or for windows, knee braces and deep girders are used, as in Fig. 228, and portal bracing is sometimes adopted, but to no great extent. (Fig. 229.) Inspection. Steel and iron members are inspected in the mill, the shop, and on the job. Mill inspection is to determine the quality of the steel, while shop inspection relates to the preparation of the mem bers.