THE STEEL FRAME.
The problems to be met with in laying out the steel frame and designing the different elements are never twice the same but, vary with each special case. Different classes of buildings give rise to different problems. Some of the problems that naturally arise can best be explained by going in detail through the process of framing the office building of which plans are given in Figs. 40 to 45.
In a building of this character, and in all buildings where the interior arrangement is a feature, the designer of the steel frame must base his work on the architect's layout. For this purpose it is most convenient, in making the preliminary study and provisional framing plans, to use tracing paper, which can be placed over the architect's plans, and thus show the position of all partitions, ducts, etc.
Position of Columns. The first step is the location of columns. These should always come in partitions, unless there is a large hall or like arrangement in which the columns form a feature. The position of the columns fixes, of course, the spans of beams and girders. A stiffer frame will result if the beams run transverse to the longest dimension of the building. The girder spans should also be shorter than the beam spans, as otherwise excessive depth of girders will be required. In general, therefore, the shortest spacing of columns should be in the direction of the longest dimension of the building. The length of this space will be limited also by the allowable depth of floor system. For an office building like the one in question, it is not desirable to use beams or girders over 12 inches deep, if possible to avoid it. With the above points in mind, we shall see what application can be made in this case.
Columns cannot be located by a study of one floor plan alone, for the arrangement of rooms may vary from floor to floor so as to result in columns interfering with doorways or not coining in par titions in certain floors, though being well adapted to the condi tions of some one floor. The natural method, therefore, is to take the typical floor plan, and then adapt the locations indicated therein to the conditions on the other floors. Figs. 40, 41 and 42 show respectively the basement, first floor, and typical floor plans of an office building ; and Figs. 43, 43A, and 44 show respectively the framing plans of the first and second floors and typical floor.
As will be seen, the lot is approximately of the same dimen sions on each side. There is only one right angle, however, and one side has two very obtuse angles. The interior arrangement of the typical floor shows a line of offices on three sides, with corri dors parallel on these sides, and an interior court. The effect of this court is to divide the building into sections whose longest dimensions are parallel to the exposed walls.
As before noted, it is an advantage for the sake of stiffness to have the girders run parallel with the long sides. It is further an advantage, and generally necessary, to have the girders of shorter span than the beams, and to have them come in partitions, as otherwise they would drop below the ceiling or necessitate a deep floor system. The first step, therefore, is to see whether the col umns can be so placed as to meet all of these requirements. In the present instance it will be seen that in general this can be done by placing the columns at the intersections of office and corridor partitions or walls. This is, moreover, a desirable location for the columns, because with the thin partitions used in offices, a column cannot be fireproofed without exceeding the thickness of partitions, and it is not desirable to have a large column casing in the middle of a partition.
The next point to fix is the exact position of the column cen ter with relation to the partitions and the direction of the column web. The corridor side should finish flush with the corridor par tition, leaving the necessary casing to come in the offices. There fore the center must come a little inside of the center of the corri dor partition, and coincident with the center of the cross partition. As the greatest dimension of the column is generally in the direc tion of the web, it will be necessary to set this in less if the web runs parallel with the corridor partitions and with the girders. This is generally the best arrangement also for the framing for, in the upper sections of columns, the distance between the flanges of the columns might not be sufficient to allow the girder to frame into the web, while the beams, having a smaller flange, would take less room. An exception to the above consideration would be the case of double-beam girders, as will be explained later.