Projection of footing beyond the column face, l = (2250 – 450)/2 = 900 mmīending moment at the critical section in the footing is given by: The projection of footing beyond the column face is treated as a cantilever slab subjected to factored upward pressure of soil.įactored maximum upward pressure of soil, pu,max= 375.315 kN/m2įactored upward pressure of soil at critical section, pu= 312.1 kN/m2 The critical section for flexure occurs at the face of the column (Fig. For our example, we will continue to use D = 500mm. Please note, there is much difference between allowable and actual values of shear stress, so depth of footing can be revised and reduced. Hence, the assumed thickness of footing D = 500 mm is sufficient. Since the punching shear stress (0.802 N/mm2) is less than the allowable shear stress (1.369 N/mm2), the assumed thickness is sufficient to resist the punching shear force. Therefore, allowable shear stress = 1×1.369 = 1.369 N/mm 2 Therefore, nominal shear stress in punching or punching shear stress is calculated as below: Perimeter along the critical section = 4 (a+d) = 4 (450+ 442) = 3568 mm Punching shear force = Factored load – (Factored average pressure x punching area of footing) 1), where a and b are the dimensions of the column.įig 1: Critical section for Two Way Shear (Punching Shear) The critical section for the two way shear or punching shear occurs at a distance of d/2 from the face of the column (Fig. Hence, factored upward pressure of soil = pumax = 375.315 kN/m2įurther, average pressure at the center of the footing is given by Pu,avg= 296.3 kN/m2Īnd, factored load, Pu= 1500 kN, factored uniaxial moment, Mu= 150 kN-m.Īssume an uniform overall thickness of footing, D =500 mmĪssuming 16 mm diameter bars for main steel, effective depth of footing ‘d’ is Now the pressure on isolated footing is calculated asīut pmax is greater than SBC of soil, so we need to revise the size of footing so that Pmax is below 300 kN/m 2.Ĭonsider width and length of footing = L =B =2.25m Therefore, length of footing (L) = Width of footing (B) Here we will consider square isolated footing. Size of footing to be designed can be square, rectangular or circular in plan. For simplicity, self weight of footing and weight of soil on footing is considered as 10 to 15% of the vertical load.Įxtra load at 10% of load due to self weight of soil = 1000 x 10% = 100kN
Loads on footing consists of load from column, self weight of footing and weight of soil above footing. Step by Step Procedure of Isolated Footing Design: The grade of concrete to be used is M30 and grade of steel is Fe415.
The safe bearing capacity (SBC) of soil is 300 kN/m2.