DESIGN OF FOOTING

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DESIGN OF FOOTING

DATA:
Load from column      =          500 kN
Concrete grade                        =          20 N/mm2
Steel grade                              =          415 N/mm2
SIZE OF FOOTING:
Load from column                  =          500 KN
Assume self weight of footing as 10% of column load
 self weight of footing        =          0.1  500
                                         =     50 KN
Total  load on soil                   =          500 + 50
                                         =     550 KN
Safe bearing of the soil           =          200 KN / m2
AREA OF FOOTING:
Area of footing                       =         
B2                                            =          2.75 m2
B                                             =          1.65 m
Say square footing of 1.7 m  1.7m



NET UPWARD PRESSSURE:
Net upward design pressure at the base
W                                =         
                                    =         
W                                =          259.71 KN/m2



BENDING MOMENT:
Projection of  footing on  the column =
                                                              = 0.70 m
Maximum Bending moment @ section
Mn          =        259.51  1.7  0.7
Mn          =        108. 08 KNM
Mn          =        108. 08  106 Nmm
DEPTH OF FOOTING:
For M20 grade of concrete Y Fe415 steel
M.R                 =          2.07 bd2
D           =       
D           =        175.25 mm = 200 mm
Considering the effect of shear, provide an effective depth of 400 mm for top layers of bars
Assume  25 mm  bars
Nominal cover = 50 mm
Total thickness            =          400 + 12.5 + 25 + 50
                               =   487.5 mm  = 490 mm
                    D    =       490 mm






TENSION REINFORCEMENT:
Maximum BM             =          108. 08  106 N mm
                                     MU = 0.87 fy Ast
108.08  106 = 0.87  415  Ast   
Ast = 732. 03 mm2

Minimum Area of Steel =
                                         = 816 mm2
Provide 4 nos of 25mm diabars (ast) = 1963.49 mm2 in each direction
Percentage of steel = 0.3%
DEVELOPMENT LENGTH:
      =
Ld = 1880.46 mm
The projection of footing from the face of column = 700 mm
Providing  on end cover of 50 mm
Length of bar of column face 650 mm < 1880 mm
Hence OK
CHECK FOR TRANSVERSE SHEAR:
Effective depth (d)                 =          400 mm
Length of footing                   =          700 – 400
                                         =     300 mm
Tranverse shear (VY)   =          259. 51  1.7  0.45
                             VY  =          198. 52 KN
Nominal Shear Stress                 =         
                                                         =       0.291 N/mm2
From Table 19 of IS 456 – 2000 Permissible shear stress in concrete (M20 grade)
        =        0.379 N/mm2
 
 Hence OK


CHECK FOR PUNCHING SHEAR:
In the distance of (½  eff. Depth)
Distance = = 200 mm
 side of section = 300 + 2(200) = 700 mm
VZ          =        259. 51 ( 1.7  .7 – 0.7  0.7)
            =          622. 82 KN
VZ          =        622. 82  103 N
Nominal Shear Stress (
As per clause 31.6.3.1 of Is 456 – 2000 permissible shear stress in concrete =  for square column = KS = 1
          = 0.25 = 1.118 N/mm2
Hence OK
CHECK FOR SAFE BEARING CAPACITY OF SOIL:
Column load = 500 KN
Self weight of footing                 =     1.7  1.7  0.40  25
                                              =            28.9 KN
Total Load  on Soil          =     500 + 28.9  =  528. 9 KN                                                   
Pressure on the soil          =    
                                              =            183.01 KN/mm2
183.01 KN /m2 < 200 KN/m2
Hence  it  is  Safe
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