Isolated footings are a type of structural element that are designed to transmit and distribute loads of single columns to the soil without exceeding its bearing capacity. This type of footing is used to prevent excessive settlement and provide adequate safety against sliding and overturning. ACI 318-14 provides guidelines for designing isolated footings, including minimum effective depth above reinforcements, concrete cover, embedments, anchorages, development length, provision of dowels, and footing strength both flexural and shear.
Isolated footings are most suitable for light column loads, when columns are not closely spaced, and when the soil is good and homogeneous. This type of footing is also one of the most economical and is used when columns are spaced at relatively long distances. Although isolated footings are generally square in plan, they can also be rectangular, circular, or even octagonal.
Isolated Footing Design Guidelines Based on ACI 318-14
The ACI 318-14 specifies that the concrete compressive strength must meet both strength and durability requirements. For normal applications, the minimum concrete compressive strength should be 17MPa. Steel bars used for construction should be of deformed type, and embedments must not impair strength or harm the 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. Even if the concrete bearing strength is not exceeded, minimum dowel reinforcement should be placed. Adequate 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 prevent bond failure of the dowels in the footing and to avoid failure of the lap splices between the dowels and the column bars. The depth of the footing above reinforcement must not be smaller than 150 mm, according to ACI 318-14 section 13.3.
The depth of the footing must be set so that the shear capacity of the concrete equals or exceeds the critical shear forces produced by factored loads, including one-way shear and punching shear in isolated footing, which are illustrated in figures 2 and 3 respectively.
In sloped, stepped, or tapered foundation, the location and depth of steps and the angle of slope must meet design requirements at each section. The required base area of the footing is obtained by dividing the column service loads by the allowable net soil pressure of the soil, based on ACI Code specifications. The net factored soil pressure is equal to factored load column loads by the chosen footing area.
Concrete cover of 75 mm is required provided that concrete is cast against soil. The dowel arrangement must also be considered, as illustrated in figure 1, to ensure proper placement and prevent bond failure.
