A10 Foundations RA1010-140 Strip Footings Table A1010-141 Section Through Typical Strip Footing General: Strip (continuous) footings are a type of spread footing used to convert the lineal type wall into an allowable area load on supporting soil. Many of the general comments on isolated spread footings (Section A1010 210) apply here as well. Strip footings may be used under walls of concrete, brick, block or stone. They are constructed of continuously placed concrete, 2000 psi or greater. Normally they are not narrower than twice the wall thickness, nor are they thinner than the wall thickness. Plain concrete footings should be at least 8″ thick, and reinforced footings require at least 6″ of thickness above the bottom reinforcement. Where there is no basement, the bottom of footing is usually 3′ to 5′ below finished exterior grade, and 12″ below the average frost penetration (see Figure A1010-141), resting on undisturbed soil. In some cases strip footings serve as leveling pads (stone walls, etc). Method of construction is affected by reinforcement and soil type. If steel mats and/or dowels are present and require accurate placement (frequently for reinforced concrete or reinforced masonry walls), then side forms are required, as they are in wet or sandy soil. If there is no reinforcement or only longitudinal reinforcement, and the soil will ‘‘stand’’ forms might be eliminated and economically should be. Preferably, strip footings are used on sand and gravel. The presence of a small amount of clay or dense silty sand in gravel is acceptable, as is bearing on rock or chalk. They are sometimes used in uniform, firm or stiff clays with little nearby ground vegetation, and placed at least 3-1/2′ into the clay. If the clay is sloped, there is potential for downhill creep. Footings up to 3-1/2′ wide are sometimes used in soft or silty clays, but settlement must be expected and provision made. Sloping terrain requires stepping the footing to maintain depth, since a sloped footing will create horizontal thrust which may distress the structure. Steps should be at least 2′ long horizontally and each vertical step no greater than three-fourths the horizontal distance between steps. Vertical risers must be at least 6″ thick and of footing width. Potential for erosion of surrounding soil must be considered. Strip footings should be used under walls constructed of unreinforced concrete, or of units bonded with mortar. They are also used when bearing capacity of soil is inadequate to support wall thickness alone. Footings require horizontal reinforcement at right angles to the wall line when side projection of footings beyond the face of the wall is greater than 1/2 to 2/3 footing thickness. Horizontal reinforcement parallel to the wall line should be used if soil bearing capacity is low, or soil compressibility is variable, or the footing spans pipe trenches, etc. Vertical reinforcing dowels depend upon wall design. Alternate foundation types include bored piles, grade beams, and mat. Short, bored compacted concrete piles are sometimes competitive with strip footings, especially in shrinkable clay soils where they may provide an added factor of safety. When soil bearing capacity is inadequate, grade beams can be used to transfer wall loads across the inadequate ground to column support foundations. Mat footings replace both strip and isolated spread footings. They may eliminate differential settlement problems. Figure A1010-141 Section Through Typical Strip Footing “SLAB-ON- GRADE”