ACI 318-14 approved design guidelines of isolated footing
Isolated alias single footing is utilized to provide support to single RCC columns. It is cost-effective and suitable when the columns are placed at comparatively long distances, loads operating on footings are less and the safe bearing strength of the soil usually remains high.
Isolated footing is categorized as pad footing and sloped footing.
The isolated footing is arranged underneath the column to disperse the loads securely to the bed soil.
The design of isolated footing is made for the following purposes :-
Area of footing
Thickness of footing
Reinforcement details of footing with a satisfactory moment and shear force review.
The design of isolated footing is made on the basis of the guidelines set by ACI 318-14.
1. The compressive strength of concrete should satisfy the needs for both strength and stability. As per ACI 318-14, least concrete compressive strength should be 17MPa for normal applications.
2. With adherence to ACI 314-14 section 188.8.131.52, the deformed type steel bars should be used.
3. Factored forces and moments provided at the base of columns are transmitted to the foundation with reinforcement, dowels, anchor bolts, or mechanical connectors.
4. There should be least reinforcement even if the concrete bearing strength is not crossed.
5. Adequate anchorage should be arranged for tension reinforcement if reinforcement stress is not directly relative to the moment like in sloped, stepped or tapered foundation.
6. There should be sufficient anchorage length of both flexural and dowel reinforcement to get rid of bond failure of the dowels in the footing and to resist failure of the lap splices among the dowels and the column bars.
7. As per ACI 318-14 section 13.3, depth of footing over reinforcement should not remain under 150 mm.
8. The depth of the footing should be in such a manner that the shear strength of the concrete remains equivalent or surpasses the critical shear forces (one-way shear and punching shear) developed with factored loads.
9. In sloped, stepped, or tapered foundation, location and depth steps and angle of slope should satisfy design requirements at each section.
10. Concrete cover of 75 mm is necessary when the concrete is cast against soil.
11. With adherence to ACI Code specifications, base area of footing is set from unfactored forces and moments transferred by footing to soil and the permissible soil pressure evaluated through principles of soil mechanics. To get the necessary base area of the footing, the column service loads are divided with permissible net soil pressure of the soil. The net factored soil pressure is equivalent to factored load column loads by the selected footing area.