That the walls and piers of a building may settle uniformly without producing cracks in the superstructure, it is not only essential that the area of the footings shall be in proportion to the load and the bearing power of the soil, but also that the centre of pressure (a vertical line through the centre of gravity of the weight) shall pass through the centre of the area of the foundation.
This condition is of the first importance, for if the centre of pressure does not coincide with the centre of the base, the ground will yield most on the side which is pressed most, and as the ground yields the base assumes an inclined position and carries the lower part of the structure with it, thus producing unsightly cracks, if nothing more.
A case in which a violation of this rule cannot well be avoided is the foundation under the side wall of a building, where the footing is not allowed to project beyond the lot line. In such a case the centre of pressure is indicated by the downward arrow, and the centre of base by the upward arrow, Fig. 4. It is evident that the intensity of the pressure is greatest on the portion of the footing to the right of the centre of base, and the footing will in consequence settle obliquely as shown in the figure, having a tendency to throw the wall outward. This tendency may be counteracted by tying the wall securely to the floor joist, but it would be much better if some arrangement could be made so that the footing would settle evenly. Where it is absolutely necessary to build the footing without projecting beyond the lot line, the footing should be carefully built of dimension stone, or of hard brick, well grouted in cement mortar, and the footing should be no wider than is absolutely demanded by the nature of the soil, and the offsets on the inside of the wall should be very slight. The footing shown in Fig. 4 is to be preferred to that shown in Fig. 5.
30. Fig. 6 illustrates another case where the centre of pressure comes outside the centre of base, consequently the wall inclines outward, producing cracks over the opening. This is a very common occurrence in brick and stone walls where wide openings occur. In such cases the footing under the opening should either be omitted entirely or made much narrower than under the pier, and the two should not be bonded together. Where several openings occur one above the other, as in Fig. 7, and the footing is continued under the opening, the unequal settlement of the footings will very likely produce cracks over all the openings, the side walls inclining slightly outward. Where the width of the opening is 8 feet or more, and the bottom of the opening is not a great ways above the footing, the footing under the wall on each side should be treated as under a pier, as shown in Fig. 8, and the space between the footings filled in with a dwarf wall only. If the bottom of the opening is twice its width above the foundation, the wall under the opening will distribute the weight equally over the footing and the settlement will be uniform.
As a rule the foundation of a wall should never be bonded into that of another wall either much heavier or much lighter than itself.
The footings should also be proportioned so that the centre of pressure will strike a little inside of the centre of the base, to make sure that it will not be outside. Any inward inclination of the wall is rendered impossible by the interior walls and the floors, while an outward inclination can be conteracted only by anchors and the bond of the masonry. A slight deviation of the centre of the pressure outside of the centre of the base has a marked effect, and is not easily counteracted by anchors.
At Chicago an omission of 1 to 2 per cent, of the weight (by leaving openings) usually causes sufficient inequality in the settlement to produce unsightly cracks.*
Where slight differences in weight occur, cracks may generally be prevented by building in hoop iron ties, rods or beams over the openings. It is also a wise precaution, where one wall joins another, either in the middle or at the corner of a building, to tie the walls together by long iron anchors built into the walls about every six feet in height.