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.

Dead Load.

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.

Live Load.

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.

On a compressible soil it is very important that the live load assumed as reaching the footings shall be neither over- nor under estimated. The dead load can be estimated with suffici3nt accuracy, and as the load on the footings under the walls is chiefly dead load, this part of the foundation is likely to receive the assumed load. But the possible maximum on the footings of interior columns is made up largely of live load, and if the live load reaching these footings is taken too large, the footings are likely to be made too great and consequently the columns will not settle as much as the walls; and on the other hand, if the live load reaching the column footings is taken too small, the columns will settle more than the walls. Ex perience in Chicago—extended both in time and in number of build ings—in founding upon a compressible soil shows that the settle ment of the columns and walls of eight- and ten-story office buildings, hotels, retail stores, etc., are almost exactly the same when designed on the assumption that no live load reaches the footings.

In the larger cities the building laws specify the proportion of the maximum live load that is to be included in determining the area of the footings; but some of these laws entirely ignore the principle of the preceding paragraph. Of course, on a non-compressible foundation an error in the amount of live load assumed to reach the footings is of no consequence; but no soil is absolutely non-com pressible, and hence in all cases except when the foundation is on solid rock, the above principle should be applied.

Attention must be given to the manner in which the weight of the roof and floors is transferred to the walls. For example, if the floor joists of a warehouse run from back to front, it is evident that the back and front walls alone will carry the weight of the floors and of the goods placed upon them, and this will make the pressure upon the foundation under them considerably greater than under the other walls. Again, if a stone-front is to be carried on an arch or on a girder having its bearings on piers at each side of the building, it is manifest that the weight of the whole superincumbent structure, instead of being distributed equally on the foundation under the front, will be concentrated on that part of the foundation immediately under the piers.

Having determined the pressure which may safely be brought upon the soil, and having ascertained the weight of each part of the structure, the area required for the founda tion is easily determined by dividing the latter by the farmer. Then, having found the area of foundation, the base of the structure must be extended by footings of masonry, concrete, timber, etc., so as to (1) cover that area and (2) distribute the pressure uniformly over it. The two items will be considered in inverse order.