Preliminary Work. As soon as the instructions of the owner, or the recommendations of the architect, have been definitely adopted, the construction of the building will be laid out. 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 bring the necessary vertical supports as much out of the way as possible. Another consideration will be the thickness of the floors. As the number of floors is sure to be considerable, it will readily be seen that an excessive thickness will result in considerable loss of height; and. if the thickness of each floor can be kept down to a reasonable it may be the means of obtaining a story more for rentable purposes, which is a matter worthy of consideration.
These factors, which do not enter so strongly- into the problems which we have previously considered, must have careful consideration in constructing mercantile buildings of the class which we have now before us; and upon their skillful employment will depend the suc cess of the enterprise.
In the bringing together of these elements to form a whole composition, certain external elements must be considered. Among these are rase of access, the maximum of light, rentable area, ease of rearra gemont to suit tenants, and the minimum of cost. Ease of access will require that the elevators and stairs shall 1 placed in direct and obvious connection with the entrances and as nearly as possible at the level of the sidewalk, readily seen on entering the building or on leaving the offices to which they give access. The position of the elevators will be of more importance than the stairs, as the latter will be little used in high buildings. To obtain the best light possible it will be well to provide that the areas (which will be 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 not be narrower than six feet and should contain win dows as large and as near the ceiling as possible, and the rooms should be as nearly rectangular as may be, thus avoiding dark corners.
The structural conditions will enter more or less into the arrange ment of the offices, as already suggested, and to this must be added ease of adjustment to the needs of the average tenant. The unit of size and arrangement of the offices should be such that a tenant who wants a single office may have one of average size, or a tenant who wishes, may have the whole floor, except the miuimom of space necessary for toilet rooms, elevators and stairs.
The Structure. When the character of the building and the general divisions of the floor space have been established, the location and design of the the kind of wall and floor con struction, and the size and spacing of floor beams and girders must he determined. 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. In general, the most econom ical spacing of columns and girders will vary from 14 feet to 16 feet.
The spacing of the floor beams will depend upon the system of floor construction used.
Columns. The columns used in skeleton construction are generally made up of a combination of the standard shapes of steel bars. One of the commonest forms of steel column is the Z-bar ,colmn, shown in Fig. 196. This column is made by bolting gether four standard Z-bars with reinforcing plates, the bars and plates increasing in size and thickness as the loads to be supported increase. The column shown in Fig. 196 is of the type known as "closed columns" and can be used only where there is no necessity for repainting the steel, as only the outside surfaces are accessible. The open form of column, Fig. 197, is more generally used. Other forms of steel columns are formed from standard beams, Fig. 198, or from channels and angles, Fig. 199. As these columns will be required to support a heavy load in most cases, bases must be used which will distribute the weight over a surface large enough to sus tain the load. This may be done by the use of a cast-iron base, Fig. 200, or a lase built up of steel, Fig. 201.
The selection of a particular form of column section will depend upon varying conditions, being generally determined by the amount of load and the way in which the floor beams come to the column. The shape and size desired for the finished column, the thickness and detail of enclosing piers, and the availability of certain shapes, are also factors.
Erection. To save time and labor, it is customary to run the columns in lengths of two stories each. This takes more metal, but the gain in time and labor will generally offset it. The splicing of columns must be made with a perfect bearing of the metal, so that the load is transmitted from column to column, the splice-plate giving the full strength against lateral movement. (Fig. 202.) In this connection attention should be called to the importance of starting and maintaining the columns perfectly plumb and always in line. To do this, it is necessary that the bases should be set absolutely level, as the slightest variation at this point will increase as the work is carried to a height. For this reason bases should never be set by means of an ordinary mason's level, but an engineer's level should be employed to insure accuracy. If this is done, and the end sections of the columns are milled off properly, there is no reason why perfectly plumb work should not be maintained, per mitting the spliced joints of the columns to bear perfectly upon each other without the use of thin "shims," which should not be allowed.