OFFICE BUILDINGS 60. An office building may be defined as any building which is of such height as to necessitate a steel frame. Steel frames become a necessity when the height of the building becomes such as to require a masonry wall to be so thick as to take up valuable room which could otherwise be used for renting purposes.
The columns, and the joining and spacing of the floor-beams and girders, have already been discussed.
The great advantage of this type of building is that the walls may be made very thin-8 to 12 inches—since they extend only one story. This is made necessary by the fact that if they did extend more than one story, the expansion of the steel and the masonry would cause cracks in the walls, as these substances expand and contract at different rates under changes of temperature.
Another advantage is that the work of build ing the walls may begin at any story without in the least affecting the work at the other stories (see Plates 21 and 22). Still another advantage lies in the fact that since the steel is so strong, the columns may be placed far apart, thus leav ing large, open spaces which may be divided up into rooms of any size after the building is com pleted; or, after they have been once divided, the old. partitions may be taken out and a new division made to suit the new tenant.
The beams or combinations of beams which go between the columns at the outer walls, and which carry the outer walls, are called spandrel beams. The wall for the next story above is built into angles and various other shapes which are attached to the spandrel beams. Plate 23
shows spandrel beams.
61. Bents and Towers. When the end of a structure rests upon a two-legged tower, the structure is said to be supported by a bent. A familiar example of this is found in the case of elevated railroad structures. The legs of this bent are columns, and carry the reactions of the structure and its loads.
If there is no wind, the above-mentioned loads are the greatest that come upon it; but if the wind blows sidewise on the structure, addi tional stresses occur. The stress in the leg on the side where the wind blows is decreased, while that in the opposite leg is increased by a like amount. If the bent is high, this effect may be such as to lift one side; and in such cases, the ends of the columns must be bolted down to a large block of masonry or fixed in iron blocks. The columns must also be designed to withstand this additional stress.
Likewise, in towers, great stresses occur from the wind; and the columns must be de signed for the extra stresses brought upon them, and must also be fixed so as to prevent the tower blowing over.
62. Steel Grillage Foundations. In order that the wall or the end of a column may not sink into the soil, steel beams are laid in con crete as shown (C 165). If the pressure comes from a column, a masonry footing may be built on top of the beams, and the same method used.
On (C 165-166) are instructions, examples, and tables illustrating the design of such footings.