OENTEB OF PBESSIIBE AND CENTER OF BASE. In construct ing a foundation the object is not so much to secure an absolutely unyielding base as to secure one that will settle as little as possible, and uniformly. All soils will yield somewhat under the pressure of any building, and even masonry itself is compressed by the weight of the load above it. The pressure per square foot should, therefore, be the same for all parts of the building, and particularly of the foundation, so that the settlement may be uniform. This can be secured only when the axis of the load (a vertical line through the center of gravity of the weight) passes through the center of the area of the foundation. If the axis of pressure does not coincide exactly with the axis of the base, the ground will yield most on the side which is pressed most; and, as the ground yields, the base assumes an in clined position, and carries the lower part of the structure with it, thus producing unsightly cracks, if nothing more.
The coincidence of the axis of pressure with the axis of resistance is of the greatest importance. The principle is almost self-evident, and yet the neglect to observe it is the most frequent cause of failure in the foundations of buildings.
Fig. 78 is an example of the way in which this principle is violated. The shaded portion represents a heavily loaded exterior wall, and the unshaded portion a lightly loaded interior wall. The dations of the two walls are rigidly connected at their section. The center of the load is under the shaded section, and the center of the resisting area is at some point farther to the left; consequently the exterior wall is caused to incline outward, producing cracks at or near the corners of the building. The two foundations are connected in the belief that an increase of the bearing surface is of advantage; but the true principle is that the coincidence of the axis of pressure with the axis of resistance is of more importance.
Fig. 79 is another illustration of the same principle. The foun dation is continuous under the opening, and hence the center of the foundation is to the left of the center of pressure; consequently the wall inclines to the right, producing cracks, usually over the opening.
One conclusion to be drawn from the above examples is that the foundation of a wall should never be connected with that of another wall either much heavier or much lighter than itself, as both are equally objec tionable. A second conclusion is that the
axis of the load should strike a little inside of the center of the area of the base, to make sure that it will not be outside. Any inward in clination of the wall is rendered impossible by the interior walls of the building, the floorbeams, etc.; while an outward inclination can be counteracted only by the bond of the masonry and by anchors. A slight deviation of the axis of the load outward from the center of the base has a marked effect, and is not easily counteracted by anchors.
The center of the load can be made to fall inside of the center of foundation by extending the footings outwards, or by curtailing the foundations on the inside. The latter finds exemplification in the properly constructed foundation of a wall con taining a number of openings. For example, in Fig. 80, if the foundation is uniform under the entire front, the center of pressure must be outside of the center of the base; and con sequently the two side walls will incline out ward, and show cracks over the openings.
If the width of the foundation under the open ings be decreased, or if this part of the founda tion be omitted entirely, the center of pressure will fall inside of the center of base and the walls will tend to incline inwards, and hence be stable.
The two conclusions above may be sum marized in the following important principle: All foundations should be so constructed as to compress the ground slightly CONCAVE upwards, rather than CONVEX upwards. On even compressible soils, a small difference in the pressure on the foundation will be sufficient to cause the bed to become convex upwards. At Chicago, in buildings founded upon soft clay an omission of 1 to 2 per cent of the weight (by leaving openings) usually causes sufficient convexity to produce unsightly cracks. With very slight differences of pressure on the foundation, it is sufficient to tie the building together by careful bonding, by hoop-iron built in over openings, and by heavy bars built in where one wall joins another.