Footing

Failures 1.Bearing failure 2.Serviceability failure 3.Excessive total settlement

P M Centroidal Axis P/A My/I

L/6 Kern L B

Pc=0 240 lb/ft2 300 lb/ft2 540 lb/ft2

Pc 240 lb/ft2 2 ft of soil @120 lb/ft3 300 lb/ft2 2 ft of concrete @150 lb/ft3 540 lb/ft2 Gross soil pressure = 540+qn lb/ft3 qn=Pc/A

P M Net soil pressure qn=Pc/A

Only Dead load and Live load Area of footing Including Wind load also Area of footing or

Isolated footing with axial load and moment Design Input Grade of concrete Grade of steel Allowable soil pressure Column load Moment Condition about overburden of soil

Design procedure Checking the footing area Assume column size Assume footing size Total load=wt of footing+column load Average soil pressure q= total load / footing area Allowable soil pressure > Average soil pressure then footing are is ok

Finding factored loads Factored column load=1.4xdead load+1.7xlive load Average factored soil pressure qnet=Factored column load/footing area Average dv=total thick – cover - bar dia Eccentricity e=Mu/Pu Solving two equations to get q1 and q2 Service load equation Vertical equilibrium

Check for One way shear Check for Two way shear Take the critical section as ‘d’ distance from the face of the column (as per ACI 318 ) 1.find pressure at critical section 2.find Vu 3.find Vu If Vu>Vu Check for Two way shear Take the critical section as ‘d/2’ distance all around the face of the column (as per ACI 318 ) find pressure at critical section Find the value of bo 2.find Vu 3.find Vu is smallest of these

Safe against two way shear Otherwise increase the depth

Check for flexural reinforcement Max moment occurs at the face of the column Find average pressure Find required moment Mn=Mu/ Find required Ast Min Ast=0.0018bh Max spacing=18 in Development length

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