DESIGN OF FOOTING

By
Advertisement Flower
DESIGN OF  FOOTING
        Size of the column 600mm×600mm
     Axial  Load  =1125kN
Add  10% for  self  wt of footing
         =131125kN  
     Total load        =1250
    Assume SBC  of soil  =200kN/M²
                    Area =P/SBC
                   =1250/200
                   =6.25m²
Select 2.5×2.5 square footing
Side of the increase bed square in column
                 = 600/√2.5
                 =379mm
Net upward pressure    =(1125/2.5×2.5)
             =180 kN/m²
Max.B.M at the side  of the inscribed square
                                 =2.0×180×1.9×1.9/2
            =649.8kN.m
        M=0.138bd²
           Effective depth req= √M/fckbd2
  =433.72mm
Considering effects of shear provide the depth of 600mm for top of layers .assume  16mmdia bars with cover 50mm
Total thickness
Eff depth  provided  = 400+50-16/2+16
  =658mm
                            M =0.87 ×fyAstd[1-(fyAst/fckbd)]
                      649.8 =0.87×415×Ast×600[1-(415×Ast/25×1000×600)]
                           Ast=2327mm2
Provide 10 nos of 16mm φ bars
           Minimum Ast=201 mm2.
DEVELOPMENT  LENGTH:
                           Ld=φσs/4τbd
                              = 16×0.87×415/4×1.4
                              =1031.57mm
The projection of footing  from the face of column =1900mm
Length of bar beyond the column face  =1900mm>1031.57mm
                                  Hence ok
CHECK FOR  TRANSVERSE SHEAR:
The Crtical section  for transverse shear will  be at a distance d=658mm  from the face of column
Length of footing beyond the critical section,
                          yy   =1900-658
                                 =1242mm
Transverse shear,
                            Vy=180×2.5×1.24
                               =558kN
Nominal shear stress,
              τvy=0.37N/mm²
Permissible shear stress
                         Pt=100Ast/bd
                            =1.03
                        τcy=0.39N/mm²
                               τv< τc
                        Hence safe
 CHECK FOR PUNCHING  SHEAR :
The critical section  for punching shear is at a distance of 300mm  {1/2 effective depth around  the face of the column }
                                       = 600+2×300
                                       =1200mm
Punching shear across section -Z
  Net up ward pressure  = 180kN/M²
            Punching  shear= 180[2.5×2.5-{0.72×0.72}
            Vz = 865.8kN
             τvz =Vz/bd
                 =865.8×10³/4.0×1200×600
                 =0.3 N/mm²
As per IS 456-2000 - 31.6.3.1
Permissible shear stress in concrete =ks τcz
               For square ks =1
       ks τcz =0.2√fck
                                                =1×0.25×√25
                                   =1.25 N/mm²
                        τvz <  ks τcz
                                Hence ok
              
CHECKING FOR BEARING STRESS:
Base of frustum of pyramid
                             =700+4×600
                               =2900mm
Supporting area of bearing of footing
                         A1= 6.25m²
Loaded area at the column base
                        A2= 0.6×0.6
                             =0.36 mm²
               √ A1 /A2 =√6.25/0.36
                            =4.16 restricted to 2
Permissible bearing stress
                              = 0.45 fck ×2
                             =22.5 N/mm²
Actual bearing stress on the loaded area
                           =6.02 N/mm²  < 18 N/mm²
                   Hence ok

CHECK FOR  SAFE BEARING CAPACITY OF SOIL:
        Column load = 1125 kN
Self wt of footing = 2.5 ×2.5× 0.7× 25
                           =109.37kN
Total load on soil = 1234kN
Pressure on soil = 1234/2.5× 2.5
                         = 197kN/m² < 200 kN/m²
                   Hence safe.
Post a Comment
Powered by Blogger.
Don't Forget To Join US Our Community
×
Widget