DESIGN OF FOOTING
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,
Ï„vy=0.37N/mm²
Permissible shear
stress
Pt=100Ast/bd
=1.03
Ï„cy=0.39N/mm²
Ï„v< Ï„c
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
Ï„vz =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 Ï„cz
For square ks =1
ks Ï„cz =0.2√fck
=1×0.25×√25
=1.25 N/mm²
Ï„vz < ks Ï„cz
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.
About Author:
I am Thomas Britto here to share my experiences in the civil engineering field to all my readers.Today many students are struggling to buy books at high prices. So I decided to start a blog and share my experience and knowledge with all my readers.
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