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# Designing the Foundation

## load, footings, live, ft, sq, lb, weight, walls, columns and building

DESIGNING THE FOUNDATION.

The first step is to ascertain the load to be supported by the foundation. This load consists of three parts: (1) the building itself, (2) the movable loads on the floors and the snow on the roof, and (3) the part of the load that may be transferred from one part of the foundation to the other by the force of the wind.

The weight of the building is easily ascer tained by calculating the cubical contents of all the various materials in the structure. If the weight is not equally distributed, care must be taken to ascertain the proportion to be carried by each part of the foundation. For example, if one vertical section of the wall is to contain a number of large windows while another will consist entirely of solid masonry, it is evident that the pressure on the foundation under the first section will be less than that under the second.

In this connection it must be borne in mind that concentrated pressures are not transmitted, undiminished, through a solid mass in the line of application, but spread out in successively radiating lines; and hence, if any considerable distance intervenes between the foundation and the point of application of this concentrated load, the pressure will be nearly or quite uniformly distributed over the entire area of the base. The exact distribution of the pressure can not be computed.

Table 60, gives the weight_of different kinds of masonry.

Ordinary lathing and plastering weighs about 10 lb. per sq. ft. The weight of floors is approximately 10 lb. per sq. ft. for dwellings; 25 lb. per sq. ft. for public buildings; and 40 or 50 lb. per sq. ft. for warehouses. The weight of the roof varies with the kind of covering, the span, etc.; but a shingle roof may be taken at 10 lb. per sq. ft., and a roof covered with slate or corrugated iron at 25 lb. per sq. ft.

The movable load on the floor depends upon the nature of the building. For dwellings, it does not exceed 10 lb. per sq. ft.; for large office buildings, it is usually taken at 30 lb. per sq. ft., but is seldom if ever that high; * for churches, theatres, etc., the maximum load—a crowd of people—may, but seldom does,* reach 100 lb. per sq. ft.; for stores, warehouses, factories, etc., the load will be from 100 to 400 lb. per sq. ft., according to the purposes for which they are used.

The preceding loads are the ones to be used in determining the strength of the floor, and not in designing the footings; for there is no probability that each and every square foot of floor will have its maximum load at the same time. The amount of moving load to be considered as reaching the footings in any particular case is a matter of judgment.

## At Chicago

in designing tall steel-skeleton office buildings, hotels, and retail stores, it is the practice to assume that nearly all of the maximum live load reaches the girders, that a smaller per cent reaches the columns of the upper story and a decreasing amount the columns of the succeeding stories downward, and that no live load reaches the footings. In wholesale stores and warehouses a portion of the total live load is assumed to reach the footings, the exact amount being a matter of judgment and varying with the circum stances. In many cities the building law specifies the proportion of

live load to be assumed as reaching the footing.