Week 12 Challenge
1)
AIM:
To design Isolated Footing using STAAD Foundation for the given data:-
Size of column : 300 mmx 450 mm
Axial load : 1500 KN
Moment MuMu : 150 KNm ( Reversible)
SBC of soil : 200 KN/m2m2 at a depth of 1 m from ground level
Grade of concrete : M25
Grade of steel : Fe 415
Design should also be validated through manual calculation
Procedure:
- Open the Staad Foundation and click on General Design to design the combined footing.
- A new window will appear as shown below.
- Now, enter the support coordinates for the Isolated Footing. The footing coordinates be at origin.
- To add the footing, go to Column Position and enter the coordinates as (0,0,0) as shown in below figure.
- Now, set the column dimensions as 300 mm x 450 mm as shown in below figure.
- Now, add the load, so go to Load and Factors Option on the Main Navigator panel.
- Choose the Create New Load option to create the load cases for Dead Load and Live Load
- Also add the given load condition of Fy = -1500 kN and Mx = 150 kNm as shown in the below figure.
- Now we will generate the load combination cases as per Indian Code standards.
- Now create the job info to analyse and design the footing.
- A new job has been created for the isolated footing as shown in below figure.
- Now we will fill the data for the design of the footing as given.
Safe Bearing Capacity = 200 kN/m2 at a depth of 1.0 m from Ground Level.
Grade of Concrete = M25
Grade of Steel = Fe415
- After entering the data, click on the Design option to analyze and design the Footing as shown in below figure.
- Click on YES to start the Design Process.
- Design of the Footing has been done succesfully.
Following design results has been found out after the analysis:
- Dimension of footing:3 mx 3 m
- Over all depth of footing D = 650 m
- Depth of footing is checked for BM, Single shear, Double shear and is found safe for all of them.
- No tension steel is needed as there is no negative pressure.
- Development length = 648 mm
Now, validation of design of footing using hand calculation:
- VALIDATION OF DESIGN OF FOOTING THROUGH HAND CALCULATION :
Footing should be symmetric about the column, as the moment is reversible( Mx&MyMx&My are same ,that is square footing is to be designed.)
Service load = 1500KN
Considering Self weight of footing & weight of backfill soil as 10% of service load
P = 1.1x 1500= 1650 KN
Pu= 1.5x1650KN = 2475 KN
Area required = 1.1P/SBC = 1.1x1500/200 = 8.25 sqm`
Acal=8.25m2
Therefore L=B=√A=√8.25=2.87m
`provide square footing of size 3.0 m x 3.0 m
Aprovided=9m^2 > Area required which is 8.25 sqm (hence OK)
STEP-2 : NET UPWARD SOIL PRESSURE
σ(max,min)=Pd/Aprovided+Mu/Z
= (1.5x 1.1x1500/9) ±( 1.5x150/(B×L^2/6) ( B=L=3 M)
= 275±50
σ(max)= 325 KN/ sqm < 370 KN/sqm ( Gross factored Bearing capacity) ( ok)
σ(min)= 225 KN-m < 340 KN/sqm
Eccentricity e = M/P = 150/1500 = 0.1M = 100 mm
L/6= 3000/6 =500 mm
e < L/6 ,therefore no negative soil pressure will develop. Hence compression steel is not required.
STEP-3 DEPTH OF FOOTING ON THE BASIS OF B.M.
The maximum bending moment at the face of column (M)
= Po×(B−b)^2/8= 275×3(3−0.3)2/8= 751.78 KNm
Mu=0.138fck/bd^2
751.78×10^6= 0.138x20x3000xd2
d = 301 mm Say d = 305 mm
Assume d = 600 mm due to shear consideration
Add effective cover = 50 mm
D = 650 mm
Depth of footing from one way shear-
d= Pu(L−a)/2(P+τc×L^2)
= 1.5X1.1X1500( 3-0.3)/2
= 6682.5/11250 =0.594 m = 594 mm < 600 ( OK)
Depth required for two way shear ( punching shear)
P/L^2[L^2−(a+d)^2]=4(a+d)d×τp
[1.5x1.1x1500/9][(9−(0.3+0.6)^2]= 4x0.9 x0.6τp
τp= 1043.98 KN/m^2 = 1.044 N/mm2
Permissible τp= 0.25√fck
=0.25 √2020
= 0.25x4.472 = 1.118 N/mm^2 > 1.044N/mm^2 ( OK)
STEP-4: Reinforcement calculation
Ast=0.5fck/fy[1−{1−(4.6×Mu/fckBd2)
Ast= 5081 sqmm
p = Ast ×100 /B×d
= (5081/3000x650)x100 = 0.26% > 0.12% which is min requirement , hence OK
xu/d=1.2−[1.44−6.6Mu/fck×b×d^2]^1/2
=1.2−[1.44−(6.6x751780x1000/20x3000x600x600)^0.5
= 0.0998
Lever Arm Z = d(1-0.416xud)
= 600( 1-0.416x0.0998)
= 575.089 mm
Ast=Mu/0.87×fy×Z
= 751780X1000/0.87X415X575.089 = 3620.66 mm2
STEP-5 : DEVELOPMENT LENGTH
As per IS 456: 2000,Cl 26.2.1.1
σs=415N/mm2
τbd for deformed bar and M20 Grade of concrete = 1.92 N/mm2
Ld=ϕ×σA/4τbd
415ϕ/4x1.92 = 54ϕ = 54x12 = 648 mm for 12 mm dia bar.
Result:
Isolated Footing for the given data has been designed succesfully using STAAD Foundation and results of the same design are as follow:
- Dimension of footing: 3 mx 3 m
- Over all depth of footing D = 650 mm
- Depth of footing is checked for BM, Single shear, Double shear and is found safe
- No tension steel is needed as there is no negative pressure.
- Development length = 648 mm.
2)
Aim:
To design a Combined footing for the given data:-
1. Two columns C1 & C2 having size 400 mmx 400 mm & 300 mmx 500 mm respectively.
2. Service load on C1 is 800 KN & on C2 is 1200 KN
3. Column C1 is flushed with property line
4. C/C distance between columns = 3.0 m
5. SBC of soil = 200 kN/m2 at a depth of 1.5 m
6. Grade of Concrete : M20
7. Steel grade : Fe 415
Procedure:
- Open the Staad foundation software
- Click on general mode.
- Now enter the Support Coordinates to add the columns.
- Column C1 & C2 are placed at a distance of 3 m and their dimensions are 400 mmx 400 mm & 300 mm x 500 mm respectively as shown in below figure.
- Two primary load cases of Live load as per given input have been created as shown in below figure.
- Assign load of 800 KN on C1 & 1200 KN on C2 as shown in below figure.
- Now, we will generate load combination as shown in below figure.
- After assigning the Load Combinations, we will create the Job Setup.
- Make sure the following things i.e, Design Code should be Indian, and Units should be in SI unit.
- Edit current job under job set up then create strip footing after selecting both the columns as shown in below figure.
Under combined footing job we have to assign the following parameters:
- Unit weight of concrete
- Max. and min. spacing of bar
- Strength of concrete = M20
- Yield strength of steel= 415 N/mm^2
- Min. & Max. bar size
- Clear cover of pedestal & footing
- Unit weight and Safe Bearing Capacity(SBC) of soil
- Depth of soil above footing
- Min % of contact area for service load
- Footing geometry such as overhang length, min width, max length, width, max- min thickness, length and thickness increment etc. as shown in below figure.
- Now, click on Design command to obtain the design of the Combined Footing.
RESULT:
The points that can be concluded after the above design are :
- Design is successful with footing dimension as 9.2m x 7.5 m and thickness 0.5 m.
- C-1 column is flushed with property line that has its left over hang as 0.
- Main steel top 8mm dia bar @ 50 mm c/c
- Main steel bottom 8 mm dia bar @ 55 mm c/c
- Secondary steel bottom 12 mm dia 60 mm c/c and top 8 mm dia 80 mm c/c
- Design is safe against sliding , single shear , punching shear, soil pressure and overturning.
Hence, design of Combined Footing has been done succesfully.