Isolated or single footings are structural elements which are designed to transmit and distribute loads of single columns to the soil without exceeding its bearing capacity, in addition to prevent excessive settlement, and provide adequate safety against sliding and overturning. ACI 318-14 provides guidelines regarding the design of isolated footing. For instance, minimum effective depth above reinforcements, concrete cover, embedments, anchorages, development length, provision of dowels, and footing strength both flexural and shear.
Isolated footings are used in case of light column loads, when columns are not closely spaced, and in case of good homogeneous soil. It is one of the most economical types of footings and is used when columns are spaced at relatively long distances. Isolated footings are generally square in plan, but they can just as well be rectangular or even circular or octagonal.
Isolated Footing Design Guidelines Based on ACI 318-14
- The concrete compressive strength shall meet both strength and durability requirements. That is why ACI 318-14 specify minimum concrete compressive strength of 17MPa for normal applications.
- According to ACI 314-14 section 188.8.131.52, the steel bars shall be of deformed type.
- Embedments shall neither impair strength nor harmful to concrete and reinforcements.
- Factored forces and moments at the base of columns are transferred to the foundation using reinforcement, dowels, anchor bolts, or mechanical connectors.
- Minimum dowel reinforcement should be placed even if the concrete bearing strength is not exceeded.
- Sufficient anchorage needs to be provided for tension reinforcement when reinforcement stress is not directly proportional to the moment such as in sloped, stepped or tapered foundation.
- The anchorage length of both flexural and dowel reinforcement needs to be adequate to avoid bond failure of the dowels in the footing and to prevent failure of the lap splices between the dowels and the column bars.
- According to ACI 318-14 section 13.3, depth of footing above reinforcement must not to be smaller than 150 mm.
- The depth of the footing must be set so that the shear capacity of the concrete equals or exceeds the critical shear forces (one-way shear and punching shear) produced by factored loads.
- In sloped, stepped, or tapered foundation, location and depth steps and angle of slope shall meet design requirements at each section.
- Concrete cover of 75 mm is required provided that concrete is cast against soil.
- Based on ACI Code specifications, base area of footing is determined from unfactored forces and moments transmitted by footing to soil and the allowable soil pressure evaluated through principles of soil mechanics. The required base area of the footing is obtained by dividing the column service loads by the allowable net soil pressure of the soil. The net factored soil pressure is equal to factored load column loads by the chosen footing area.