Design Of Rectangular Footing 5i5a5e

DESIGN OF RECTANGULAR FOOTINGS(5,6,7,8,9,10) Design Parametres:Footing designation = Concrete mix Steel Cover to Reinforcement Safe Bearing Capacity(From soil Report)= Unit weight of RCC = Width of Column b = Depth of Column d = Characteristic compressive strength of concrete = Yield strength of steel =

F1 M20 Fe415 50mm 196.00KN/sqm 25.0KN/cum 0.23m 0.45m 20.00N/sqmm 415.00N/sqmm

Load calculations:Factored Load on the column from analysis = Add self weight of the footing&Weight of back fill (15%) Total Load =

1198.00KN 1198.00KN 179.700KN

1377.70KN

Ultimate Bearing Capacity of the soil qu = 1.5xSBC =

294.00KN/sqm

Size of the footing required:Area of the footing required =

4.69Sqm

Provide rectangular footing of size 2.40mx3.30m,the area comes to Width = 2.10m Depth = 2.70m The net ultimate bearing pressure acting on the footing due to direct load = Design moment =

5.67Sqm

242.98KN/sqm

30.00KN-m

Section Modulus for the above section =

(1/6)xbd2 =

Soil pressure due to moment = M/Z =

2.55cum 11.76KN/sqm

Max.Soil pressure =

pu + mu =

254.74KN/sqm Hence O.K

Min.Soil pressure =

pu - mu =

231.22KN/sqm Hence O.K

Hence,the design soil pressure =

254.74KN/sqm

The maximum bending moment at the face of the column wrt the section along width ML = (Considering 1m width of footing) The maximum bending moment at the face of the column wrt the section along length MB = (Considering 1m width of footing)

Depth of the footing required:i)Bending moment criteria:The critical section for bending is at the face of the column.Hence,the Maximum factored bending moment = ML = (Considering 1m width of footing)

161.21KN-m

Adopting Limit state method of design Mu = 0.138 fckbd2 The effective depth of footing required = d =

[Mu/(0.138fckb)]0.5 = 241.68mm

Over all depth required assuming 16mm dia bars =

= 299.68mm

However assume 550mm overall depth,then the effective depth comes to

Longitudinal direction:Calculation of reinforcement :The actual depth of neutral axis =

47.43mm

Area of steel required =

945.82sqmm

Assuming 12mm dia bars,the spacing comes to

120mm

Provide 12mm bars at a spacing of

100mm

Then the area of reinforcement provided =

along width

1130.40Sqm

Percentage of reinforcement provided =

0.23

Check for one way shear:The critical section of one way shear is at a distance of 'd' from the face of the column Hence,the factored design shear force VFd = Nominal shear stress Tv =

161.25KN

0.328N/sqmm <2.8 N/sqmm

(As per Table 20 of 1S 456) Hence,the section is safe from shear point of view

The percentage area of the tensile reinforcement provided =

0.230%

The design shear strength of concrete for the above steel percentage from Table 19 of IS 456 is 0.344 N/sqmm Hence Vuc = 169.25KN 0.344 >0.328 Hence,no shear reinforcement is required. Transverse direction:Maximum factored bending moment = MB = (Considering 1m width of footing)

111.35KN-m

Over all depth provided =

550.00mm

Effective depth assuming 10mm dia bars

Calculation of reinforcement :The actual depth of neutral axis =

32.12mm

Area of steel required =

640.50sqmm

Reinforcement in central band Total reinforcement

=

0.88

Reinforcement to be provided in central band =

560.44sqmm

Reinforcement to be provided in each edge band =

40.03sqmm

However provide 10mm dia bars at a spacing of

125mm

Then the area of reinforcement provided =

628.00Sqm

Percentage of reinforcement provided =

along length

0.127

Check for one way shear:The critical section of one way shear is at a distance of 'd' from the face of the column Hence,the factored design shear force VFd =

112.09KN

Nominal shear stress Tv =

0.226N/sqmm <2.8 N/sqmm (As per Table 20 of 1S 456)

Hence,the section is safe from shear point of view

The percentage area of the tensile reinforcement provided =

0.127%

The design shear strength of concrete for the above steel percentage from Table 19 of IS 456 is 0.280 N/sqmm Hence Vuc = 138.60KN 0.280 >0.226 Hence,no shear reinforcement is required.

iii)Two way shear criteria:The critical section for two-way shear is along the perphery of the square at a distance d/2 from the face of the column Hence perimetre of the preriphery b0 = Hence,the factored shear force VFd = qu(B2-AB2)= Nominal shear stress Tv = VFd/b0d =

3328mm 1271.14KN

0.78 N/sqmm

Permissible shear stress in concrete for two-way shear for M20 grade concrete Tc' =ks . Tc ks = (0.5+l/b)> 1 Hence ks = 1 Tc = 0.25(fck)1/2 =

1.12 N/sqmm

Hence,Tc' =

1.12 N/sqmm 1.12 >0.780

Hence,the section provided is safe from two-way shear point of view

iv)Check for transfer of bearing stress:Pu =

1377.70KN

Compressive bearing resistance = 0.45 fck(A1/A2)1/2. For the column face A1/A2 = 1 and for the other face A1/A2 > 2 but should be taken as 2. In any case, the column face governs.Force transferred to the base through column at the interface =

931.50KN >1377.70KN Hence O.K

There is no need to design separate dowel bars to tranfer the load to the base of the footing

10)

<294.00KN/sqm

>0

161.21KN-m

111.35KN-m

492.00mm

495.00mm

ase of the footing