20. Knowing the character and supporting power of the soil on which he is to build, the architect is prepared to design his foundation plans, but in no case should this be done when the preceding information is wanting.
In designing the foundations the first point to be settled will be the depth of the foundations; second, whether they shall be built in piers or in a continuous wall; and, third, the width of the foundations.
For isolated buildings on firm soil the depth of the foundations will generally be determined by the depth of the basement or by the frost line. Even where there is no frost, and the ground is firm, the footings should be carried at least 2 feet below the surface of the ground, so as to be below the action of the surface water. In very few soils, however, is it safe to start the foundations at a less depth than 5 feet. (See Section 9.)
22. The depth of the foundations for city buildings, built near the lot line, should be governed by the local laws bearing on the subject, the character of the soil, and probable future action of the owners of the adjoining property.
In most cities the law provides that the owner of any lot excavating below a certain depth (usually about 10 feet) shall protect the wall of the adjoining property at his own expense, but if he does not excavate below that depth (10 feet) then the adjoining owners must themselves protect their property from falling in.
It is, therefore, always wise to provide against any such future expense and trouble by carrying the footings - at least those of the side walls - to the prescribed limit, above which the owner will be responsible, even if the requirements of the soil or building do not necessitate it. This precaution is especially important when the building is erected on sand.
It has been found that when heavy buildings are to be erected on soft or compressible soils, greater security from settlement may be obtained by dividing the foundation into isolated piers, as described in Chapter II (Foundations On Compressible Soils).
When building on firm soils, however, no advantage is gained by pursuing this method, unless the walls of the building are themselves composed of piers with thin curtain walls between, in which case the foundations under the piers and walls should be built of different widths, and not bonded together, as described in Section 30.
When the walls are continuous, however, and of the same thickness throughout, the foundation should be continuous. The architect should constantly bear in mind that in all kinds of building construction the simplest methods are almost always the best, and complicated arrangements and the use of iron, etc., in foundations should be avoided, at least on firm soils.
Whether the foundations are continuous or divided into piers the area of the footings should be carefully proportioned to the weight which they support and the bearing power of the soil. The former is perhaps the most important of all considerations in designing the footings. While the safe bearing power of the soil ought not to be exceeded, this is, on most soils, not of so much importance as the proportioning of the footings, so that the pressure on the soil from every square foot of the footings will be the same. If this condition were always obtained there would be few cracks in the mason work of buildings, as such cracks are caused not by a uniform settlement of an inch or two, which with most buildings would not be noticed, but by unequal settlement.
25. In proportioning the area of the footings the architect should carefully compute the weights coming upon each pier, and the weight of and loads supported by the walls, and record the same in a memorandum book for reference.
He should then decide, by means of Section 16 and from an examination of the ground, or, if necessary, from actual tests, the bearing weight which it appears advisable to assume, and dividing the load on the various footings by this assumed carrying load will give the proper area of the footings.
The pressure under piers supporting a tier of iron columns may be made 10 per cent, more than under a brick wall, so that the pier may settle a little more to allow for the compression in the joints of the mason work.
26. In computing the weight to be supported by the footings the live (or movable) load and dead load should be computed separately. In building on any compact soil the object in carefully proportioning the footings, as has been stated, is not so much to prevent any settling of the building as a whole, but to provide for a uniform settling of all portions of the building, so that the floors may remain level and no cracks be developed in the walls. In order to secure this, it is necessary that the loads for which the footings are proportioned should be as near the actual conditions as possible.* Thus the dead load under the walls of a five-story building would be a considerable item, while the dead load under a tier of iron columns would be much less in proportion to the floor area supported, and, as the dead load is always constant and the live load may vary greatly, only the amount of live load that will probably be supported by the footings most of the time should be considered.
For warehouses, stores, etc., about 50 per cent, of the live load for which the floor beams are proportioned should be added to the dead load supported on the footings.
For office buildings, hotels, etc., the weight of the people who may occupy them should be neglected altogether in proportioning the footings, and only about 15 pounds per square foot of floor allowed to cover the weight of furniture, safes, books, etc. [Actual statistics show that the permanent average loads in such buildings do not exceed the above limit.]
For theatres and similar buildings some allowance should probably be made for the weight of people, the actual amount depending upon the arrangement of the plan and the character of the soil.
Almost any soil, after it has been compacted by the dead weight of a building, will carry a shifting load of people without further settlement, while if the footings were computed to carry the full live loads for which the floor beams were designed, it would be found that when the building was finished the actual loads on the footings under the walls would be much greater than under the interior piers, and if the ground had settled at all during building, the probabilities would be that the floors of the building would be higher in the centre than at the walls.
* Foundations shall be proportioned to the actual average loads they will have to carry in the completed and occupied building, and not to theoretical or occasional load*. - Chicago Building Ordinance.