combined footing with four pedestals

PROJECT: DOCUMENT NO. DATE

DESIGNED CHECKED PAGE

S B C of Soil 250 kN/m2

Unit weight of soil 18 kN/m3

Unit weigt of concrete 25 kN/m3

Unit weight of soil 18 kN/m3

Grade of concrete (N/sq.mm)= M25

Grade of steel (N/sq.mm)= Fe415

Depth of foundation 2.5 m

Thickness of footing 0.6 m

Depth of footing(up to base of foundation) 2.5 m

Factor 1.5

Size of footing

Length=L 4.000 m

Width =B 2.800 m

Area of raft 11.20 m2

Ixx= 14.93 m4

Zxx= 7.47 m3

Izz 7.32 m4

Zzz= 5.23 m3

Size of pedestal

Width =a= 1.800 m

Depth=b= 0.900 m

Centre to centre spacing in X-direction=L1 0.625 m

Centre to centre spacing in Z-direction=B1 1.335

tL= 0.788 m

TITLE : DESIGN OF COMBINE FOOTING


DESIGNED CHECKED PAGETITLE : DESIGN OF COMBINE FOOTING

tB= 0.283 m

Forces/Moments

SELECT SIDE: R/S

Pedestal Height 2.200 m

NODE NOPedstal

No Fx Fy Fz Mx My Mz

kN kN kN kN-m kN-m kN-M

384 P1 0.45 80 71 462 0 0

391 P2 0 0 0 0 0 0

392 P3 13 855 2.3 2 0 0

394 P4 0 0 0 0 0 0

Total 13.45 935 73.3 464 0 0

unit t-mPedestal

noVertical Load(P)

DX Pi Dxi DZ Pi DZi MX MZ

t m t-m m t-m t-m t-m

1 8 1.40 11.2 2.000 16 61.82 0.099

0 0 0 0 0

3 85.5 0.78 66.2625 0.665 56.86 0.706 2.86

0 0 0 0 0

TOTAL 93.5 77.4625 72.86 62.526 2.959

X-bar= 0.82848 m

Z-bar= 0.77922 m

ex=eccentricity= 0.57152 m

ez=eccentricity in z-direction= 1.2208 m

MxMy

Mz

X

Y

Z

SELECT



Additional Moment due to eccentricity=Mx' 114.143 t-m

Additional Moment due to eccentricity=Mz' 53.438 t-m

Total moments= Mx 176.67 t-m

Total moments= Mz 56.397 t-m

P/A= 8.3482 t/sq.m

Mxx/Zxx= 23.661 t/sq.m

Mzz/Zzz= 10.79 t/sq.m

BASE PRESSURE OF THE RAFT:

P/A + Mxx/Zxx + Mzz/Zzz 42.799 t/sq.m

P/A + Mxx/Zxx - Mzz/Zz z 21.219 t/sq.m

P/A - Mxx/Zxx + Mzz/Zzz -4.5226 t/sq.m

P/A - Mxx/Zxx - Mzz/Zzz -26.103 t/sq.m

Vertical pressure intensity:

Due to soil= 3.42 t/sq.m

Self weight of footing= 1.5 t/sq.m

Total= 4.92 t/sq.m

Net pressure

p1 47.719 t/sq.m

p2 26.14 t/sq.m

p3 0.397 t/sq.m

p4 -21.183 t/sq.m

Not safe



Analysis of footing along Longitudinal axis(LA) of footing

Considering max. intensity of load= 47.719 t/sq.m

Bending moments:

Reaction of sub grade= 47.72 t/sq.m

Max BM(x-direction at centre) 3.495 t-m

Max BM (z-direction at centre) 15.9462 t-m

Analysis of footing along X-axis in over hang(bottom)

BM at ped face/m (T-m)= 14.797 t-m

effective depth required(mm),d= 207.10 mm

say 210.00 mm

Provide footing thickness, D(mm)= 600 mm

clear cover to footing (mm)= 50 mm

Assume bar dia(mm)= 12 mm

Area=ast= 1.13 mm

effective depth available(mm),d= 532 mm

Mu/bd2 (N/sq.mm) = 0.523

P1

P2 P3

P4

L

B

a

b

B1

L1

tL=(L-L1)/2-a/2

tb=(B-B1)/2-b/2

MxMy

Mz

X

Y

Z



%pt req (from SP-16)= 0.211 %

Min steel= 0.120 %

%pt considered= 0.211 %

Astreq (sq.mm)/m width = 1120.866

use bar dia (mm) = 12

Area=ast= 1.131

Spacing of reinforcement required 100.902

Provide reinforcement= 100.00

Provide reinforcement= Y12-100c/c

%pt(provided)= 0.213 %

permissible shear strength,tcav(N/sq.mm)= 0.34 N/mm2

Check for one-way shear

Shear is checked at effective depth 'd' from the face of the pedestals and the maximum

shear is considered for the design.

Max SF(T) -factored 72.47 t

Shear stress,tvu(N/sq.mm)= Not safe 1.36 N/mm2

Analysis of footing along Z-axis in over hang(bottom)

BM at ped face/m (T-m)= 1.904 t-m

effective depth required(mm),d= 74.15 mm

say 75.00 mm

Provide footing thickness, D(mm)= 600 mm



Area=ast= 1.13 mm




Mu/bd2 (N/sq.mm) = 0.067

%pt req (from SP-16)= 0.026 %

Min steel= 0.120 %

%pt considered= 0.120 %



Area=ast= 1.131

Spacing of reinforcement required 177.16

Provide reinforcement= 170.00




Check for one-way shear

Max SF(T) -factored 44.20 t

Shear stress,tvu(N/sq.mm)= 0.83 N/mm2

Not safe

Reinforcement at Top(z-direction)

Max BM(T-m)/ width 15.95 t-m

Footing thickness, D(mm)= 600 mm




Mu/bd2 (N/sq.mm) = 0.563



%pt req (from SP-16)= 0.228 page no 49

Min steel= 0.12 %

Astreq (sq.mm)/m width = 1211.49 mm2


Area=ast= 1.13 mm2

Spacing of reinforcement required 93.35 mm

Provide reinforcement= 90.00 mm


Area of steel provided=Ast= 1256.64 mm2

%pt(provided)= 0.236

Reinforcement steel at top(x-direction)

Max BM(T-m)/m width= 3.50 t-m

Footing thickness, D(mm)= 600 mm




Mu/bd2 (N/sq.mm) = 0.13 mm2

%pt req (from SP-16)= 0.048

Min steel= 0.12 %


use bar dia (mm) = 20 mm

Area 3.14 mm2

Spacing of reinforcement required(mm)= 503.46 mm

Provide reinforcement= 200.00 mm







Check for one-way shear(X-direction)

Shear is checked at effective depth 'd' from the face of pedestal

The critical shear at 'd' from pedestal face

Efeective depth of footing= 520 mm

Pedestal width (mm) = 1800 mm

Footing projection beyond face of pedestal = 788 mm

length beyond d= 268 mm

SF (Factored) 19.147 t

Shear stress(factored) ,tvu 0.243 N/sq.mm

Permissible shear= 0.40 N/sq.mm

Safe

Reinforcement at Top (z-direction)

Max BM(T-m)= 15.95 t-m

%pt(as per IS:456-2000) = 0.228

Ast 1184.17 mm2


Area 2.01 sq.cm


Provide reinforcement= 160 mm




Steel provided

Distribution steel [ Min. 0.06 %] 360 mm2


Area 1.13 sq.cm


Provide reinforcement= 200 mm






Summary

Y16-160c/c Y12-200c/c

600 Y12-170c/c

SECTION IN Z-Z DIRECTION(THROUGH PIERS)

Y12-200c/c

Y20-200c/c 600

Y12-100c/c Y12-200c/c

SECTION-1-1

Y12-200c/c Y20-200c/c Y16-160c/c

SECTION 2-2 :REINFORCEMENT AT TOP

1

1

223 3



4.00m Y12-200c/c

2.80m

Y12-100c/c Y12-170c/c

SECTION 3-3 REINFORCEMENT AT BOTTOM

Footing thickness,D,(mm)= 600

check for punching shear

Critical vertical load for punching 85.5 t

Punching shear-length 2.400 m

Punching shear-width 1.500 m

Upward force= 154.08 t

Area for punching shear 9.6 m2

Punching shear force 68.58 t

Punching shear stress 0.14 N/mm2

Allowable shear stress 0.80 N/mm2

Permissible shear stress as per 456-2000 c = ks xc

Where Ks = (0.5+bc) but no greater than 1, bc being the ratio

of short side to long side of column

c = 0.7

Ks = 1



c = 0.25 x Sqrt(fck) (for limit state design)

= 1.25 N/mm2

ks xc = 1 x 1.25

c = 1.25 N/mm2

v < c

SAFE

Check for uplift:

Max.upward force= 0.000 t

Downward force

weight of foundation 17.125 t

Dead load of structure= 37.375 t

Net force= 54.500

safe

Check for Overturning

(a) Check for overturning in x-x direction:

Disturbing moment=Mz= 2.959 t-m

Stabilizing Moment=Mz' 77.4625 t-m

Factor of safety= 26.18

(b) Check for overturning in z-z direction safe

Disturbing moment=Mx= 62.5260 t-m

Stabilizing Moment=Mx'= 72.86 t-m

Factor of safety= 1.17

safe



Check for sliding:

Vertical load

Reaction from structure 93.50 t

Self weight of foundation= 17.13 t

Weight of pedestal= 31.38 t

Total 142.00 t

Disturbing load (resultant of x-x and y-y ) 7.45 t

Angle of internal friction 20 o

Coefficient of friction 0.36

Cohesive resistance of soil 0.00 t/sq.m

Restoring force 51.68 t

Factor of safety against sliding 6.94

safe

combined footing with four pedestals

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