Foundation Design using SAFE

Aim:- To design the foundation by using a SAFE software

Introduction:-

There are 2 types of foundation

1. Shallow foundation
2. Deep foundation

Shallow foundation are of 5 types they are

1. Isolated footing
2. Combined footing
3. Strip footing
4. Strap footing
5. Raft or Mat footing

Deep foundation are of 2 types they are

1. Pile foundation
2. Drilled shaft or caissons

Isolated footing

• It is the simplest form of foundation used world wide depending upon the soil type
• These are independent footings designed to support single column. This footing is used when the distance of column large
• This footing is suitable where the safe bearing capacity of soil is good and it's economical

Pile foundation

• Pile foundation is a kind of deep foundation and it's used in a place where the water table of soil is above or if the soil is clayey type
• Pile foundation is a type of slender column below the soil which is used to transfer the load at desired depth either by end bearing or skin friction
• The depth of pile decided by the depth of the hard strata
• Each pile will have a pile cap for connecting the column

SAFE :-

• The safe software is used for design the foundation slab system and concrete floors
• Safe offers single user interface to perform modelling, analysing, designing and reporting
• Safe offers many templates for quickly start new model
• This software is developed by the computers and structures INC
• This software provides the details about deformation shape, soil pressure, reinforcement details, punnching shear etc.,

Data required

Square footing column size = 400x400mm

Compression axial load combination of (1.5DL + 1.5LL) = 2000KN

The safe soil bearing capacity 150KN/m^2 at a depth of 2m bellow EGL

Assume M-25 concrete grade

Data required for the safe analysis and design

Unfactored load = 2000/1.5 = 1333.3 KN

Dead load break up = 1333.3/2 = 666.7 KN

Live load break up = 1333.3/2 = 666.7 KN

Subgrade modulus of the soil = (Safe bearing capacity x Factor of safety) / Maximum settlement

K = (150 x 3) / 0.05

K = 9000KN/m^3

Here the maximum settelement is 50mm = 0.05m

Rebar yield strength Fy = 500N/mm^2

Rebar ultimate strength Fu = 575N/mm^2

Elastic modulus of M-25 grade concrete = 5000`sqrtfck`= 5000 x (25)^1/2 = 25000 N/mm^2

Procedure:-

Step1:-

• Open the safe software by double clicking on it
• Then click on the new model
• After that the model initialization dialouge box opens
• Then change the setting Display unit as metric SI , Region for default material as India, Steel section data based as Indian,Steel design code as BS 5950-1990, Concrete design code as IS 456:200
• Then click on OK

Step2:-

• Click on the isolated footing option (Which is a default template)
• Here change the x and y direction distance as 3m
• Assume the footing thickness as 450mm (If it is not satisfied in the software then change it - It's not constant value)
• Subgrade modulus is 9000KN/m^3 (Which is a calculated value)
• The dead and live load 666.7KN (Which is a calculated value)
• Then click on OK

Step3:-

• After the completion of the input data the model will look like as shown below
• Make sure that the footing and column placed correctly

Step4:-

• After the model then assign the material
• Click on the define then click on the material property
• Then click on the add new material
• Then create the M-25 concrete as shown
• Then click on OK

Step5:-

• Then click on the add new material
• Then create the Fe 500 steel as shown
• Then click on OK

Step6:-

• Click on the define then click on the seection property
• Then click on slab section
• Then click on the add new material
• Then create the Footing 450mm as shown
• Then click on OK

Step7:-

• Then click on the add new material
• Then create the Stiff 450mm as shown
• Then click on OK

Step8:-

• Here we are defining the spring property which is used to stiff the foundation
• The point spring is used to point loads and area spring is used for the providing unoform spring (load) throughout
• Click on the define then click on the spring properties
• Then click on the area spring

Step9:-

• After selecting the area spring property click on the modify property
• Edit the subgrade modulus as 9000KN/m^3
• Keep the load as compression
• Then click on OK

Step10:-

• After defining all these things then define the load patterns
• Click on the define then click on the load patterns
• Add the DL, LL and Soil surcharge load
• Except DL keep the self weight multiplier for others as 0
• Then click on OK

Step11:-

• After that go to the load cases
• Click on the define then click on the load cases
• The loads what we defined in the load pattern as visible in the load cases
• Then click on OK

Step12:-

• After defining the load patterns and load cases
• Then go to the load combinations
• Click on the define then click on the load combination
• Then click on the add new combo
• Add the comb 1 combination which includes a Dead load and soil surcharge as a 1.5 scale factor
• Then click on OK

Step13:-

• Then click on the add new load combo
• Add the comb 2 combination which includes a Dead load, Live load and soil surcharge as a 1.5 scale factor
• Then click on OK

Step14:-

• Then click on the add new load combo
• Add the Settlement combination which includes a Dead load, Live load and soil surcharge as a 1 scale factor
• Then click on OK

Step15:-

• After the load defining then assign the strips these strips are used to identify the longitudinal rebars
• click on the lines which is a diaphragm lines
• Then click on right button of your mouse

Step16:-

• After that strip object information dialouge box opens
• Then make sure that the all the input datas are correct

Step17:-

• The strips which is assigned to footing is as shown below

Step18:-

• After assigning the design strips then assign the loads to the column
• Assign the dead load as well as live load in to the column
• Click on the assign then click on the joint loads then click on the force

Step19:-

• After selecting the joint loads then joint load assignment dialouge box opens
• Then apply the dead load as -666.67 in the force global z
• Then click apply
• Then apply the live load as -666.67 in the force global z
• Then click apply
• Then click OK
• Now the loads are assigned to the column

Step20:-

• To make sure the loads are assigned or not
• Click on the point where you mentioned the loads
• Then click right button of your mouse
• Then you will see the loads in the table as shown

Step21:- After defining and assigning the property then run the analysis

When I check the model it fails in the punching shear so I have changed the thickness of footing and thickness of stiff as 600mm

Check the need to be performed in safe analysis

Safe bearing capacity under service load:-

• Click on the display soil pressure then select the settlement combo
• Then click on apply
• The maximum displacement value is 150KN/m^2 but in the software it is showing 58.51KN/m^2
• Hence it is safe

Settlement under service load:-

• Click on the show deformed shape then you will get a deformed shape dialouge box
• Then click on combo then click on settlement then click apply
• The deformed shape is shown in the below figure

• This is the deformed shape of the model
• The allowable deflection is 50mm but I'm getting the 6.501 as the deflection value
• Hence safe

Two way shear check :-

• To check the punching shear just click on the show punching shear icon
• The maximum punching shear may go upto 0.9 or 0.95 but I'm getting the punching shear as 0.718
• Hence it is safe

Ultimate bending moment

• Click on the display strip force then the strip force dialouge box opens
• Then click on the combo then select the DconS1
• Then select the moment click on apply then it will show the ultimate bending moment
• It's taking the maximum value 628KN/m^2

Flexural reinforcement design for footing slab

• Click on the show slab design then it shows the slab design dialouge box
• make sure that the impose minimum reinforcement is selected and layer A and layer B are selected
• Then click on OK

Result:-

• Safe bearing capacity under service load - 58.51KN/m^2 - Safe
• Settlement under service load - 6.501mm - Safe
• Two way shear check - 0.718 - Safe
• Ultimate bending moment - 628mm^2
• Flexural reinforcement design for footing slab - 1050mm^2

By using SAFE software we came to know that The isolated footing design is safe

Hence the footing design is safe

2.

Column size = 600x600mm

Load carrying capacity = 200KN

Data required for the safe analysis and design

Unfactored load =1175 KN

Dead load break up = 1175/2 = 587.5 KN

Live load break up = 1175/2 = 587.5 KN

Point spring of pile = (A x E / L)

K = (196349x25000) / 16000

K = 306796KN/m

K = 306.8KN/mm

Rebar yield strength Fy = 500N/mm^2

Rebar ultimate strength Fu = 575N/mm^2

Elastic modulus of M-25 grade concrete = 5000`sqrtfck`= 5000 x (25)^1/2 = 25000 N/mm^2

Number of piles required = unfactored load with self weight addition / individual pile capacity

Number of piles required = 1175 x 1.1 /200

Number of piles required = 6.5 = 8Nos

Consider 8 pile for symmetric placement of pile under pile cap

Take center to center distance between the pile = 3 x pile diameter

Take center to center distance between the pile = 3 x 500 = 1500mm

Edge distance from the pile center = 1 x pile diameter

Edge distance from the pile center = 1 x 500 = 500mm

Assuming pile cap depth as 500mm

Procedure:-

Step1:-

• Open the safe software by double clicking on it
• Then click on the new model
• After that the model initialization dialouge box opens
• Then change the setting Display unit as metric SI , Region for default material as India, Steel section data based as Indian,Steel design code as BS 5950-1990, Concrete design code as IS 456:200
• Then click on OK

Step2:-

• After that click on Grid only then new model quick templates diaalouge box opens
• Then change the number of grids in X and Y direction as 2
• Then change the spacing if grids as 4m in both X and Y direction
• Then click on OK

Step3:-

• after the mentioning all the information then the model grid look like as shown below

Step4:-

• After the model then assign the material
• Click on the define then click on the material property
• Then click on the add new material
• Then create the M-25 concrete as shown
• Then click on OK

Step5:-

• Then click on the add new material
• Then create the Fe 500 steel as shown
• Then click on OK

Step6:-

• Click on the define then click on the seection property
• Then click on slab section
• Then click on the add new material
• Then create the Pile cap 500mm as shown
• Then click on OK

Step7:-

• Then click on the add new material
• Then create the Stiff 500mm as shown
• Then click on OK

Step8:-

• Then click on the add new material
• Then create the Pile 500mm as shown
• Then click on OK

Step9:-

• After assigning all the materials
• Click on the Draw rectangular floor /wall to draw the slab

Step10:-

• After selecting the Draw rectangular floor /wall tool
• Select the one corner and drag into diagonal corner then the slab is created
• Here make sure that the pile cap is selected

Step11;-

• After creating the pile cap then draw the reference points to place the piles
• Calculate the points properly then give the dimension in the box which is appearing in the left corner

Step12:-

• After completion of the points assignment
• The model looks like this

Step13:-

• To draw the pile with reference to the points
• Click on the Quick draw area around points
• This will helps to create the pile by clicking it once

Step14:-

• After creating the piles then create the rectangular column which has 600x600mm column
• Click on the Quick draw area around points
• This will helps to create the stiff by clicking it once

Step15;-

• Assign the spring to the piles by selecting the middle points in the piles
• Click on the assign then click on the joints
• Then click on the spring

Step16;-

• After clicking springs then joints assignment dialouge box will opens then click on modify/show deformation
• Then click on add new property
• Then the point spring property data dialouge box opens
• Ener the point spring as 306796KN/m
• Then click on compression
• Then click on OK

Step17:-

• After providing the spring value to the piles then assign the load to the column
• Assign a 587.5 KN load at the mid point of the column
• Then click on apply then click OK

Step18:-

• After assigning the loads to it then run the analysis

Step19;-

• In this step I'm getting a warning message from the software when I click Run analysis
• The warning message is about the mesh
• So provide the proper mesh to the footing
• First select all the point by draging the mouse
• Go to the analyse then go to the automatic mesh setting for floor
• Before it is in General mesh then change it into rectangular mesh and provide the approximate maximum size as 0.1

Step20:-

• After making all the changes then click on Run analysis

Step21:-

Check needs to be performed in SAFE analysis

Individual pile capacity under service load

• Here check the reactions of the piles
• Select the reaction icon then select the service combo, the load combination shoul dbe combo
• Then click on OK
• The maximum load is taken by the piles are 195 KN almost 200KN
• The minimum load taken by the piles are 152KN
• These loads snap shot is inserted below
• Hence safe

Settelment under service load

• This is the deformed shape of the model
• The load is combo, then type of load is service combo
• The allowable deflection is 50mm but I'm getting the 1mm as the deflection value
• Hence safe

Two way shear check (Punching shear)

• To check the punching shear just click on the show punching shear icon
• The maximum punching shear may go upto 0.9 or 0.95 but I'm getting the punching shear as 0.245 is minimum and 0.388 is maximum
• Hence it is safe

Flexural reinforcement design for footing slab

• Click on the show slab design then it shows the slab design dialouge box
• make sure that the impose minimum reinforcement is selected and layer A and layer B are selected
• Then click on OK

• In this the top and bottom reinforcement are calculated by the software itself
• The maximum reinforcemnt is required at the mid of the column, it is a 1813mm^2

Result:-

• Individual pile capacity under service load -The maximum load is taken by the piles are 195 KN almost 200KNThe minimum load taken by the piles are 152KN - Safe
• Settlement under service load - 1mm - Safe
• Two way shear check - 0.245 is minimum and 0.388 is maximum - Safe
• Flexural reinforcement design for footing slab - 1813mm^2

By using SAFE software we came to know that The pile foundation design is safe

This design is done according to the IS 2911 (part-2) code book and SP 34 code book

Hence the foundation design is safe