Buildings having storerooms in the first story, and rooms or offices above, generally require an area on one or both sides to furnish light and ventilation for the inside rooms. As in such buildings the ground floor commands the greatest rental per square foot, it is essential to utilize the full area of the lot, and to do this it is necessary to start the light area at the level of the second floor, the first story being lighted by a skylight, which forms the bottom of the area.
A very common arrangement of such buildings when erected on an inside lot is shown in Fig. 476, which is a plan of one side of the second floor, the stories above being divided in the same way. For city buildings of the ordinary construction the area and party walls are usually of brick. As the area walls start from the second floor level they must, of course, be supported on girders of some kind. There are two methods of construction commonly employed in fram- , ing and supporting these girders. The cheapest method is that shown in Fig. 477. Girders of suitable size are placed directly under the area walls, and are themselves supported by columns or posts spaced at economical distances, as shown in the figure. Constructionally this method is probably the best, as it economizes material and carries the loads more directly to the foundation, but the posts are usually considered as objectionable in a store, and are apt to decrease the rent. It is, therefore, generally desirable to frame the girders supporting the area so that the columns may be omitted. The method of accomplishing this is shown in Fig. 478, which represents the framing of a portion of the second floor in a building 50 feet wide.
Girders A, A, A are placed crossways of the building and from 10 to 14 feet apart, the outer ends being supported on the first story wall and the inner ends on the columns which support the centre tier of girders.
The girders A, A, A support the girders B, B, which carry the wall above. The latter girders may either be framed between the former or may rest on top of them; in the latter case it will be necessary to drop the girders
A, A below the ceiling line. It would probably be more economical to frame the girders flush and drop both beneath the ceiling, as in that case the floor joists would rest on top of the girder
B, B. When the latter is flush with the ceiling the joists must be supported either on an angle bar riveted to the girder or be hung in some form of joist hanger. Keeping the girders
B, B flush with the ceiling, however, obstructs the light from the skylight much less than when they are dropped. Fig. 479 shows a section through the bottom of the light area with the girders B, B flush with the ceiling.
In brick buildings over two stories in height the girders A and B should be made of steel beams, used in pairs; or, if these cannot be obtained of sufficient size, riveted box girders should be used.
The floor joists in the third and upper floors, and also the rafters, are supported by the area wall, hence the load on the girders A and B is very considerable and must be computed with great care. It must also be remembered that the girders B, B transmit a concentrated load to the girders A, A, and the size of the latter must be computed accordingly.
The depth of the girders should be such that the deflection shall not exceed 1/30 of an inch per foot of span. If either of the girders are dropped beneath the ceiling they should be protected from fire by metal lathing or tile.
The load which the beams A, A transmit to the wall should also be carefully computed and a bearing plate of proper size placed under them. It will also probably be necessary to reinforce the wall by a pilaster, as shown in Figs 477 and 478.
If the side wall is a party wall, or comes close to the lot line, it will be necessary to convey the rain water through soil pipes placed inside the building and a gutter provided at the foot of the skylight to collect it. Fig. 479 shows the usual method of forming the gutter and finishing the wall.