Foundation Design using SAFE

Question 1.

 Model the isolated footing provided in week 11 challenge on SAFE. Analyze and design it to verify the size, depth and steel provided as per calculations.

Data from challenge 11:

• A square footing for a column size of 400x400.
• The compression axial load for the load combination of (1.5 DL + 1.5 LL) is 2000 KN.
• The safe soil bearing capacity is 150 KN/m2 at a depth of 2 meters below E.G.L.
• Concrete grade: M25.
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Objective of the challenge:

• To know the general design procedure of spread footing with SAFE software.
• To know the general design procedure of pile foundation with SAFE software.

Calculation:

• Data required for SAFE analysis and Design.
• Unfactored load = 2000/ 1.5 = 1333.3 KN
• As the exact values are unknown splitting the load into two equal halves.
• Dead load = 1333.3 /2 =666.7 KN
• Live load =1333.3 /2 = 666.7 KN
• Sub grade modulus for soil= Safe bearing capacity x Factor to ultimate/ Maximum settlement.

           =150 x 3 / 0.05= k= 9000 KN/m3.

• Rebar yield strength (Fy)= 500 N/mm2
• Rebar ultimate strength (Fu) = Say 545 N/mm2
• Elastic modulus of M25 concrete = 5000x√fck= 25000 N/mm2

Procedure:

Modeling in SAFE:

• Open the SAFE software with the below setting.
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• Click ok and select single footing and enter the below settings.
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• Click ok and the following screen will appear.
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• Defining material:
• For concrete:
• Go to Define -> Material properties -> Add new material. And enter the data in the dialog box.
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• Click ok.
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• Change appropriate value and click ok.
• For Rebar:
• Click Add new material.
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• 
• New rebar is added.
• For footing slab:
• Go to Define -> Section properties -> Slab section -> Add new properties
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• For STIFF Slab:
• Click Add new properties
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• Load combination:
• Go to Define -> Load combination -> Add default design combos and select the followings.
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• Service combo:
• Click Add new combo and enter the following combination.
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• Run the analysis:
• Click Run button to run the analysis.
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• Strip design:
• Select the horizontal, vertical strip and right click to see the information.
• Change the rebar material type to Fe500 .
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• Click Set display option-> show width to view design strip
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• Assign slab material:
• Select the slab and right click and change the change material to footing 1.
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• Result:
• Checks need to be done in SAFE analysis:

1. Safe bearing capacity under service load:

• Go to Display -> Show force/ stress diagrams -> Soil pressure.
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• Maximum safe bearing capacity is 150 KN/m2
• It is under the limit. Hence it is safe

2. Settlement under service load.

• Go to Display -> Show deformed shape.
• Select the combination as service
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• 
• The maximum settlement is with in the limit of 50mm

3. Two-way shear check (Punching shear):

• Click Display-> Punching shear design.
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• It is with in the limit 1. Hence safe.

4. Ultimate bending moment.

Bending moment:

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Shear force:

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5. Flexural reinforcement design for footing slab

• Click Display -> Show slab design.
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2. A column of size 600x600 carrying an unfactored gravity load (DL+LL) of 1175 KN is to be supported on pile cap, which ultimately transfers load to the piles underneath. Piles of 16 meters length and 500 mm diameter are to be used. Each pile has a load carrying capacity of 200 KN. Model, analyses and design the pile cap on SAFE to evaluate the following:

• Pile group to be used
• Depth of pile cap
• Longitudinal steel on top and bottom face in both directions.

Calculation:

Data required for SAFE analysis and Design.

• Unfactored load = 1175KN

• As the exact values are unknown splitting the load into two equal halves.
• Dead load = 1175 /2 =587.5 KN
• Live load =1175 /2 = 587.5 KN

• Point spring of pile = AX E/ L =306796.09 N/mm
•  A= π/4 xd2= 196349.5 mm2
• E = 5000x√fck =5000x √25 = 25000 N/mm2
• L =16000mm
•  Hence K = 306796.09 KN/m.
• Rebar yield strength, Fy =500 N/ mm2.
• Rebar ultimate strength, Fu = 545 N/ mm2.
• Elastic modulus of M25 concrete = 25000 N/ mm2.

• Number of piles required = Unfactored load with self-weight / Individual pile capacity.
• =1175 x 1.1 / 200 = 6.5.
• Consider 8 piles for symmetric placement of piles under pile cap.
• Center to center distance between the piles = 3 x Pile diameter = 3x500= 1500mm
• Edge distance from the pile center =1x500= 500mm.
• Assume pile cap depth = 500mm

Pile foundation-Plan:

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Procedure:

Modeling in SAFE:

• Open the new file
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• Select Grid only option.
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• Click ok
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• Defining material:
• For concrete:
• Go to Define -> Material properties -> Add new material. And enter the data in the dialog box.
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• 
• Change appropriate value and click ok.
• For Rebar:
• Click Add new material.
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• 
• New rebar is added.
• For slab:
• Go to Define -> Section properties -> Slab section -> Add new properties
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• Repeat the same procedure to create two more slab types.
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To place points:

• Go to Draw -> Draw joints objects
• In the properties of object dialog box enter the distance from the edge in X and Y direction.
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• All the points are placed as per the dimension in the plan.
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Drawing pile cap:

• Select Draw rectangular floor or wall icon.
• Feed the properties in the dialog box and click and drag diagonally from one end to the other of the grid.
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Drawing the pile around the points:

• Select Quick draw area around the points.
• Select the following properties in the dialog box and click on the points.
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• To draw Stiff slab for column:
• Select Quick draw area around the points.
• Select the following properties in the dialog box and click on the points.
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To draw spring in the pile:

• Go to Define -> Spring properties -> Point spring.
• Add new property a below with the calculated value.
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• Select all 8-pile joint point, Go to Assign -> Joint -> Spring
• Select the defined spring and click ok
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Assigning load to the column:

• Select the column point
• Go to Assign ->Joint loads -> Force.
• Add both live and dead loads in Z direction as shown below.
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Add Design strip:

• Go to Draw -> Draw Design strip.
• Enter the value in the dialog box.
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• Right click on the strip to assign HYSD500.
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Load combination:

• To add ultimate limit state and service combination, go to Define -> Load combination.
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Run the analysis:

• Click Run button to run the analysis.
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Result:

• Checks need to be done in SAFE analysis:
• Two-way shear check (Punching shear):
• Click Display-> Punching shear design.
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• It is less than 1, hence ok.
• Reaction check:
• Click Reaction icon and select combination as Service.
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• Change combination to ULS
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• Diagonal piles are carrying less force than the nearby piles. Hence ok.

Settlement under Service load:

• Go to Display -> Show deformed shape.
• Select the combination as service.
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• Maximum deflection is 0.957mm. It is safe.

Flexural reinforcement design for footing slab:

• Click Display -> Show slab design.
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• Reinforcement details can be calculated from this.