In designing a foundation, the engineer and contractor must realize that the load to be distributed by the footing is made up of a combination of forces. The primary force is the dead load, that is the weight of the structure and all material permanently attached to it. The live load includes all vertical loads that are not a permanent part of the structure but can be expected to be superimposed on it during its useful life. Note that the dead load of a structures remains constant, while the live load will vary and must be assumed as a maximum expected. The live and dead loads are carried to the footing and foundation bed throughout the walls of a building. The proper design of a foundation can become very complex and if a major project is involved, the design must be carried out by a qualified structural engineer. The following interpretation of the bearing formula can be applied to a lightly loaded building: "the footing shall be twice as wide as the foundation wall and as thick as wide as the foundation wall is high." This means, for example, that an 8 by 16 inch footing would be needed for an 8 inch wide wall. If larger footings are involved and the bearing area has been established with reasonable accuracy, it is generally safe to make the thickness of the footing twice the projection from the edge of the foundation wall. There are two types of foundations. A continuous foundation is probably the one type most commonly used for houses and other small buildings. This design calls for a foundation wall extending above a footing slab. However, continuous foundations are the least suitable for most soil conditions. They achieve their maximum effectiveness and economy in dense sands and gravels, or on bedrock. For most types of soil, and in particular where a basement is not required, grade-beam foundations will usually prove to be the most economical and effective for houses and similar buildings. A grade-beam foundation consists of a series of concrete piers that support a reinforced beam around the perimeter of the building. The grade beam carries the weight of the building and its superimposed loads. The piers in turn support the grade beam and distribute the loads over the foundation bed. All loads are absorbed by direct bearing of the bottom of the pier on the soil; hence the base of a pier must have a sufficiently large area to distribute these loads safely. A certain amount of skin friction is exerted on the face of the piers, but a shallow depths this has a negligible influence on bearing capacity and can be ignored.