Owner-Built Home & Homestead
(Page 4 of 15)
The use of concrete piers also goes with "grade-beam foundation," which is fairly new and known to have many economic advantages over the traditional "continuous-footing" type. According to foundation cost comparisons made at Pennsylvania State University, a continuous-footing foundation, using concrete block or poured concrete walls, will cost about 25% more than a slab-on-ground floor built with a grade-beam and pier foundation (a saving in cost by grade-beam is especially apparent in homes built in northern climates).
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The first step in building a grade-beam foundation is to dig pier holes, 6 to 8 feet apart, along the perimeter of the house. The holes should be dug below the frost line, with bottoms in good load bearing soil. Concrete piers are made, 10-12 inches in diameter with a 5/8-inch reinforcing rod that protrudes 11 inches from the top of a pier into the space for the poured concrete grade-beams. The strength of a grade-beam depends upon well-placed reinforcing rods, and two 1/2-inch rods should be placed at the top edge and bottom edge of a beam. Though poured on the ground between forms, a rigid beam is really supported by the piers.
Some simple construction practices can be used in laying out any type of foundation. By erecting "batter boards," building corners and edges and floor levels can be preserved during excavation. Floor levels are set on the batter boards by the use of a garden hose filled with water (the water level establishes equal elevation). The "square of the hypotenuse theorem" provides a simple test for determining exactly square angles in the foundation (lay off 16 feet along the cord on one side of the stake, 12 feet along the other cord, and the line connecting the two markers is the hypotenuse of a right triangle and should be 20 feet.)
At times special ingenuity is required to solve foundation problems on unusual sites. For instance, in areas where only muck or silt exists as bearing soil, a "raft" type of footing becomes necessary. In this case the entire building is supported on a concrete "raft" floating on the wet soil. The building is designed so its load equals the displacement of muck.
Hillside sites always require special attention to the footing and foundation system (concrete retaining walls may be needed). A cantilever foundation is often used to advantage on hillsides. Wright called the cantilever "the most romantic, most free, of all principles of construction," and he used the cantilever in most of his famous buildings, including the Imperial Hotel in Tokyo, Japan, and the Johnson Wax Company building (tower). The tower has been likened to a tree form: In structural terms, a tree is a vertical beam cantilevered out of the ground. It maintains stability against wind pressure and snow load by means of restraints applied to its roots by the earth; there is a subtle balance of forces between the bearing earth and the tree. As with Wright's tower, a central, vertical, foundation core can function as the only structural support.
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