This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The types of foundation in general use may be classified roughly under the following heads: -
1. Natural Foundations.
2. Artificial Foundations.
1. Natural Foundation is the name applied to such as are formed on the soil itself, and it is applicable when the soil is practically incompressible.
If the site be fairly level it is only necessary to make it thoroughly so, all loose and decayed portions being cut away and filled up with cement concrete. Sometimes fissures occur which, if not too deep, must be filled up with concrete or spanned by an arch as shown in Fig. 56.
If the site is on a slope or in a hollow the loose and decayed portions are cut away and a series of blasts made where the walls are designed to come. The holes formed thereby are filled with concrete, which is then benched up into level steps, upon which the walls are raised. The object of the blasting is to roughen the surface and thus prevent the concrete from sliding. If the soil is likely to be shaken by blasting, then the soil itself must be cut into steps as shown in Fig. 57. A drain must always be cut from the lowest point in the benching, to prevent the foundation from becoming water-logged.
When the foundation is benched the walls must be brought up level with large stones with fine bed joists, as at A, Fig. 57, otherwise the settlement of the more numerous joists in the deeper trenches will cause unsightly cracks to appear in the superstructure.
2. Artificial Foundation is the term applied to any artificially formed support for a building, and may be classified as -
Spread Foundations is the name given to foundations used for buildings upon compressible soil. These, consequently, require to extend considerably on each side of the walls, so as to reduce the load per square inch to that which will be safely borne by such soils. Foundations of this class are formed of concrete or large stones and sometimes of wood.
For small buildings on comparatively firm ground concrete foundations are almost invariably used. For unimportant work the depth or width is rarely calculated, the width being simply made wide enough on each side of the footing to give a man foot-room to work in, and the depth is made at least equal to the thickness of the wall. The footings are usually stepped out to the width of the wall on each side.
In important works the load brought upon the foundations must first be determined, and the concrete must be wide enough to reduce the load per square foot of foundation-bed to that which it will safely bear. This may be illustrated by the following example: Suppose that a 3-brick wall (Fig. 58), which with the usual footings weighs 9 tons per foot run, is to be supported by a concrete foundation upon a soft clay soil which will safely bear 1 1/2 ton per square foot. What width of concrete will be required ?
Consider one foot of the wall only, the weight of which is 9 tons. Then -
Width of concrete = Total load / safe resistance of soil
= 9 / 1.5feet
= 6 feet.
The Depth of Concrete is calculated by regarding the concrete as an inverted cantilever whose length is taken as half the width of the projecting portion of the footings plus the projection of the concrete beyond the footings, - in this case 1 ft. 3 ins., as shown in Fig. 58. The load upon this cantilever is 1 1/2 ton per foot run. The formula for calculating such a cantilever, as obtained by equating the bending moment with the moment of resistance, is
Wl/2 = cbd2/6
Whence d =
Where W' = ultimate total load on cantilever, in cwts. ,, /= length of cantilever, in inches. ,, b = breadth of cantilever, in inches. ,, d= depth of cantilever, in inches, and ,, C = a constant for the particular material, called the coefficient of rupture, in cwts.
The value of C is about 8 cwts. for Portland cement concrete; but if, as is more usual, it be taken to represent the actual load (in this case 1 1/2 ton, or 30 cwts.) and not the ultimate load, the value ofC is 1 cwt. The equation, in the case exemplified, thus becomes
= nearly 11 inches.
It is customary where very wide foundations are necessary to bed rolled steel joists or rails in the concrete, but on this subject more information will be found in Volume IV.
A rough - and - ready rule for finding the depth of concrete foundations is shown in Fig. 5g. Having found the width by calculation, draw lines at 45 degrees to the horizontal from the points where the faces of the wall cut the uppermost course of footings, and where the line cuts the line representing the width of the concrete indicate the depth thereof.
This rule is fairly satisfactory for thin walls on firm soil, but fails when applied to large walls, especially when on soft soil; but it gives a greater depth than that found by calculation for thoroughly good Portland cement concrete, for it was devised for weak lime concrete.
When a building is supported upon isolated piers the footings should be stepped out until they meet, and should rest upon a continuous concrete foundation as in Fig. 60.
Where there is not sufficient depth for the above method of supporting piers, inverted arches should be formed from pier to pier as shown in Fig. 61. So as to distribute the pressure along the foundations, care should always be taken to give the end pier sufficient abutment or to tie it in with iron tie rods, otherwise it will thrust outward as shown by the dotted lines.