ISOLATED FOOTING WITH BIAXIAL MOMENT

DESIGN OF BI-AXIAL ISOLATED RCC FOOTING (IS 456, 2000)

Building Name
Footing Number:
Node number
COLUMN
WIDTH (Breadth) = 230 mm
DEPTH (Length) = 300 mm
Axial load from output (P1) = 725 KN
Moment about Z axis (Mz) = 5 KN-m
Moment about X axis (Mx) = 5 KN-m
Safe bearing pressure = 300 KN/m2
AREA= 2.658333333 mm2

1 x2 -0.07 x -2.65833333
Required length of the footing L 1.665815
Required width of the footing w -1.595815
Depth required 171.0741781
Over All Depth 226.0741781

PEDASTAL Z and X axis are assumed as two horizontal Glob

Hence, input the dimensions of footing easily by
Breath = 230 mm length parallel to global Z and footing breadth p
Length = 300 mm global
X
Breadth 1.59 m
FOOTING
Revised Footing length (L, dim. ) = 1.67 m global
Revised Footing Breadth (W, ) = 1.59 m global
Z Z
Depth of footing (t) = 500 mm
Clear cover of footing = 50 mm global
Main bar dia of footing = 10 mm X
Effective depth of footing = 445 mm Length 1.67 m
Selfweight of the footing = 33.19 KN
Footing Dimensions
Area of Footing(A) = 2.66 m2
Sect mod of foot about Z axis (Zz) = 0.70 m3
Sec mod of foot about X axis (Zx) = 0.74 m3

MATERIALS OF CONSTRUCTION
Grade of concrete fck = 25 N/mm2
Grade of steel fy = 500 N/mm2
Unit wt of soil = 18 KN/m3
CHECK FOR GROSS BEARING PRESSURE
Safe NET bearing pressure = 300 KN/m2
Safe gross bearing pr. = 327.00 KN/m3 (net pr. + depth of foot * soil unit wt)
Axial load from output (P1) = 725 KN
Moment about Z axis (Mz) = 5 KN-m
Moment about X axis (Mx) = 5 KN-m
Depth of top of foot. from ground = 1 m
Unit wt of soil = 18 KN/m3
Weight of soil retained above foot = 46.55 KN
P = (P1+soil+foot self wt) = 804.74 KN
Maximum bearing pressure = 316.94 KN/m2 P M y Mx
± ±
Minimum bearing pressure = 289.20 KN/m2 A Zy Zx
Hence footing is safe against max gross bearing pr.

DESIGN FORCES self wt of footing & soil wt

Axial load:(Pu) = is not considered in Pu.
1087.5 KN
Moment about Z axis (Muz) = 7.5 KN-m
Moment about X axis (Mux) = 7.5 KN-m
Maximum effective soil pressure pe max
( Pu/Area+ Muz/Zz + Mux/Zx) = 430.36 KN/m2
Minimum effective soil pressure pe min
( Pu/Area - Muz/Zz - Mux/Zx) = 388.75 KN/m2
Design of footing is done using above maximum effective soil pressure

Page 1
 ISOLATED FOOTING WITH BIAXIAL MOMENT

CALCULATION FOR BOTTOM STEEL

Mu about X1 X1 = ( pe max x length2/2)= 100.97 KN-m per meter If this moment is very high try to
reduce moment by changing
Mulimit = 658.56 KN-m per meter
footing size.
A st =
0. 5 f ck
fy [ √
1− 1−
4.6 Mu
f ck bd 2
bd
] The section is singly reinforced Higher moment will require
higher steel and section may be
doubly reinforced. This design is
done as singly reinforced
Hence, Ast = 534.712 mm2 section.
Min Ast = 534.000 mm2 (0.12 % for slab, cl 26.5.2.1)
Spacing required = 146.88 mm (considering max of above two calculated values of Ast)
Spacing provided = 125 mm
pt provided = 0.14 %
pt required = 0.12 %
provide a maximum st 0.14 %
Hence provide 10 mm dia bar @ 125 mm c/c parellel to length of footing ( || to Z)

Mu about N1 N1 = ( pe max x length2/2)= 99.50 KN-m per meter

Calc. Ast = 526.742 mm2 The section is singly reinforced
Min Ast = 534 mm2 (0.12 % for slab, cl 26.5.2.1)
Spacing = 147.08 mm (considering max of above two calculated values of Ast)
Spacing provided = 125 mm
pt provided = 0.14 %
pt required = 0.12 %
provide a maximum st 0.14 %
Hence provide 10 mm dia bar @ 125 mm c/c parellel to breadth of footing ( || to X)
Arrangement of bottom reinforcement as per above design is shown below

10 mm dia bar @ 125 mm c/c All drawings on this sheet

are updated automatically
as per design changes and
need not be edited.

10 mm dia bar @ 125 mm c/c

1 1

Footing Length 1670 mm Breadth 1590 mm

Sec 1-1

300 230
675
L1 X1 X

a a

Z Z
N1 N1
a a

L2 L2

240 X1 X
L1 Breadth 1590 mm
745
Footing Length 1670 mm 235
PLAN

Page 2
 ISOLATED FOOTING WITH BIAXIAL MOMENT

CHECK FOR ONE WAY SHEAR :

One way shear at critical section L1- L1
Distance of critical sec. from edge of footing = 0.24 m
Shear force Vs =pe max x 0.24 x 1m width of footing = 103.288 KN
Instead of calculating
tv = Vs/bd = accurate pressure
Shear stress 0.232 N/mm2 ordinate at L1-L1 we
tc = 0.28 N/mm2 have used "pemax" on
conservative side.
tv < tc hence O.K.

One way shear at critical section L2- L2

Distance of critical sec. from edge of footing = 0.235 m
Shear force Vs =pe max x 0.235 x 1m width of footing = 101.136 KN
Shear stress tv = Vs/bd = 0.227 N/mm2
tc = 0.28 N/mm2
tv < tc hence O.K.

CHECK FOR TWO WAY SHEAR

Ref. cl 34.2.4 and cl.31.6.3 of IS 456 : 2000
Allowable shear stress tv allowable = kstc
ks = ( 0.5 + bc) = 1.266666667 >1
Hence, ks= 1
tc = 0.25 (fck)0.5 = 1.25 N/mm2
tv allowable = ks x tc = 1.25 N/mm2
Shear force Vs = 430.364 ( 1.67 x 1.59 - 0.745 x 0.675) = 926.33 KN
Length of critical section = 2 x ( 745 + 675) = 2840 mm
Area of the critical section (length of critical sec x eff. d ) = 1263800 mm2
Hence shear stress tv = 0.733 N/mm2
tv < allowable hence O.K.

Page 3
Password = bhat