Footing Failures 1 Bearing failure 2 Serviceability failure
Footing
Failures 1. Bearing failure 2. Serviceability failure 3. Excessive total settlement
P M Centroidal Axis P/A My/I
Kern B L/6 L
Pc=0 240 lb/ft 2 300 lb/ft 2 540 lb/ft 2
Pc 240 lb/ft 2 2 ft of soil @120 lb/ft 3 2 ft of concrete @150 lb/ft 3 300 lb/ft 2 540 lb/ft 2 Gross soil pressure = 540+qn lb/ft 3 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. 4 xdead load+1. 7 xlive 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 q 1 and q 2 • Service load equation • Vertical equilibrium
Check for One 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 ) 1. find pressure at critical section 2. 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. 0018 bh • Max spacing=18 in Development length
Thank You
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