SPREAD FOUNDATIONS 188. Distribution of Loads.—When bedrock is at considerable depth, it frequently becomes necessary to spread foundations ovOr large areas near the surface of the ground by the use of footings at the bases of columns or walls. The method to be employed in such work depends upon the area of soil required to support the loads and the extent of the footings necessary beyond the bases of the walls or columns. \Vhen the extensions are small, masonry footings may often be employed to advantage, and this is the most common type of foundations for light buildings upon firm soil. When footings must extend to greater distances beyond the bases of the walls or piers, grillage or reinforced concrete footings occupy less space and are more economical.
In foundations of this type some settlement ,is usually to be ex pected, and the object should be to make this settlement as small and as uniform as possible. The loads to be carried by the different parts of the foundation should be ascertained and the footings so proportioned as to bring uniform pressure upon the soil under the foundation. Inequalities in the settlement of the foundations of buildings are apt to crack the walls, injuring the appearance when not sufficient to impair the stability of the structure. To pro duce uniform pressure it is necessary that the center of pressure of the load pass through the center of arca of the base of the foundation.
In determining the loads which may come upon the footings in the foundation of a building, the dead loads and live loads are separately computed. The entire dead load is always upon the foundation, while the live load may vary, and only such portion as may reasonably be assumed usually to exist should be used in estimating the load distribution upon the footings, which will depend upon the character of the building. In hotels, office buildings, etc., while the floors of each portion should be designed to carry the maximum live load which could come upon it, only a small percentage of the total of this live Load can reach the footings at once, and it is common to neglect it altogether. In churches, theaters, etc., the maximum floor loads are
more apt to occur, and a larger percentage should be used in design ing the foundations. The building codes of the various cities corn monly prescribe the loads to be used in designing foundations for buildings.
When the exterior walls of a building carry much of its center of pressure should be somewhat, inside the center of the footing, thus avoiding any tendency to tip outward and crack the walls of the structure; a tendency to tip inward will be resisted by the interior walls and floors. The rigid connection of a lightly loaded interior wall with a heavily loaded exterior one often causes an eccentricity of loading in the foundation which produces serious cracks. When a series of openings one above the other through the wall of a building cause the loads to be brought to the foundation through piers between the openings, the footings should be disconnected and properly cen tered for each pier, unless the foundation has sufficient stiffness in itself to distribute the loads over its whole base. The walls of many buildings are cracked over the openings by the use of continuous foundations in such cases.
189. Masonry Footings.—For light loads, footings of brick or stone masonry or of concrete are commonly employed. Where suitable stone is available, stone masonry is often the most economi cal, but concrete is now usually preferred. Brick footings are less desirable on account of the likeli hood of the deterioration of the bricks when used under ground.
In placing stone footings, the stones must be carefully bedded so as to bear evenly upon the foundation soil. The projection of the footing, when of consider able extent, is stepped off as shown in Fig. 104. The width of a step should not ordinarily be greater than two-thirds of the height of the course, and a stone should not project more than one-third of its length beyond the course above. Footing stones under walls carrying heavy loads should be large and roughly squared, and should be set in a thick bed of mortar to give even bearing upon the soil beneath.