The value of d is the effective thickness of footing. Increase depth for 1.75 to 2 times more than calculated value for shear considerations Īdopt thickness of footing equal to 340 mm. The thickness of footing is calculated from the maximum critical bending moment. L and B is for footing and l and b for column But for design it okey to take any one side of the footing because there may be case of moment reversal in the building.įind the maximum soil pressure P u,max and soil pressure at the edge of column P ux or y, then the moment should be calculated on both axis. Ultimate soil pressure P u = design load / area of footing * (1± 6e/(B or L))įor one edge, + sign is used and for other side – sign is used depending upon position of eccentricity. In Non-Shifting case, the ultimate soil pressure varies uniformly over the area of footing. The critical section for the bending moment is taken at the edge the column in case of column footing. Step 3: Calculate critical bending moment: Take size of isolated footing = 2200 mm * 2200 mm Safe bearing capacity of soil = 150 kN/m 2 Service load = factored load / factor of safety = 950.8 / 1.5 = 633.87 KN Step 2: Calculate an area of footings based on the factored axial loadĪrea of footing = service load / safe bearing capacity of soil Here we discuss about the only non-shifting case of isolated footing. It is compulsory to go for non-shifting case if there is possibility of moment reversal. Add 10% for self-weight of footing and soil above the footing.įactored axial load on footing = factored axial load from column + self-weight of footingĮccentricity on x-axis = M x/P = 54.042/864.37 = 0.0625 m = 62.5mmĮccentricity on y-axis = M y/P = 63.17 / 864.37 = 0.073 = 73 mmįor simplicity in construction and supervision, it is better to increase the thickness of footing rather than shifting but it makes the structure little bit uneconomical. The factored axial load and factored bending moment from the column is obtained from the structural software. Step 1: Calculate factored loads and bending moment The footing 8 is critical since it has highest axial force and high value of moment.įrom column, factored axial load = 864.37 KN and M x = 54.042 KN-m and M y = 63.1715 KN-m The structural design output value for isolated footing are: Footingįor design example, take the critical one. Grade of concrete = 25 N/mm 2 Grade of steel = 415 N/mm 2 Design an isolated footing of the given building as shown:
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